ALBERT

Description: The Molucca Sea Collision Zone in eastern Indonesia is the site of an orthogonal collision between two active subduction systems. Both the Halmahera subduction zone, to the east, and the Sangihe subduction zone, to the west, have subducted oceanic lithosphere of the Molucca Sea Plate, which has now been completely consumed. Both volcanic arcs were active since the Neogene and provide a means of probing the element fluxes through the two systems. The geochemistry of Neogene and Quaternary lavas from each volcanic arc is compared to constrain changes in the mass fluxes through the systems and the processes controlling these fluxes at different times during their history. Both arcs show increased evidence for sediment recycling as the collision progressed, but for contrasting reasons. In Halmahera this may represent an increased sediment flux through the arc front, while in Sangihe it may simply reflect a greater opportunity for melting of sediment-fluxed portions of the mantle wedge. In both cases the change in arc geochemistry can be related to the evolving architecture of the particular subduction zone. The Halmahera lavas also record a temporal change in the chemistry of the mantle component that resulted from induced convection above the falling Molucca Sea Plate drawing compositionally distinct peridotite into the mantle wege.

Description: Recycling of oceanic plate back into the Earth's interior at subduction zones is one of the key processes in Earth evolution. Volcanic arcs, which form above subduction zones, are the most visible manifestations of plate tectonics, the convection mechanism by which the Earth loses excess heat They are probably also the main location where new continental crust is formed, the so-called subduction factoiy' About 400f modern subduction zones on Earth are intra-oceanic. These subduction systems are generally simpler than those at continental margins as they commonly have a shorter history of subduction and their magmas are not contaminated by ancient sialic crust. They are therefore the optimum locations for studies of mantle processes and magmatic addition to the crust in subduction zones. This volume contains a collection of papers that exploit the relative simplicity of intra-oceanic subduction systems to provide insights into the tectonic, magmatic and hydrothermal processes associated with subduction.

Description: The margin between Cote d'Ivoire and the Niger Delta is a region with a common structural history, this being reflected in similarities in the stratigraphic response and play fairways identified across the region. There has been significant exploration on the narrow shelf characterizing the margin, resulting in a series of modest oil and gas discoveries. It is shown in this paper that many of the aspects of the plays in the unexplored deep-water regions of the margin are considerably more favourable to the development of giant fields than those on the shelf. This play-fairway review is based on the integration of existing publications with focused studies of multiclient 3-D seismic data over a number of areas. Play fairways are classified by seismic sequence and trap type, with an analysis of each undertaken. The most attractive deep-water play types are: (1) anticlinal traps involving late syn-transform (Apto-Albian) and early post-transform (Late Cretaceous) reservoirs, (2) combination traps involving ponded turbidites on the shoreward flanks of these highs, and (3) stratigraphic traps associated with large Late Cretaceous submarine fan complexes. The anticlinal play is associated with the terminations of the St Paul and Romanche fracture zones, with the more recent structuring generally associated with the latter. 3-D imaging and amplitude mapping is critical to prospect delineation, particularly for the combination and stratigraphic plays. Active kitchens are evidenced involving Early and Late Cretaceous source rocks in the Cote d'Ivoire and western Ghana to Nigeria segments of the region, which are consequently upgraded. Considerable volumetric potential is indicated that promises to make the region one of significant new exploration activity in coming years.

Description: Tectonic activity, on a range of scales, is a fundamental control on sedimentary activity. The range of structural deformation within a region extends from the plate tectonic scale, governing, for example, rift initiation, to the basin scale, with the formation of basinbounding faults. Internal basin configuration is also strongly influenced by tectonic activity. However, the relationship between tectonic activity and sedimentation is a complex one, given the many additional factors which can also influence sedimentary activity, including erosion, sediment transport, source area lithology, groundwater chemistry, range of depositional environments, climate, eustasy, and the relative location of an area and its distality to marine influences. In this paper we provide a selective overview of the issues associated with the interlinked themes of tectonics and sedimentation, examining the main basin types forming in both extensional and compressional plate settings. We then review the various models of sedimentation in the selected basins, both on a local and a basinal scale. Finally, we look to the future -- providing a series of possible research areas, almost exclusively multidisciplinary, which would help to improve existing models of interlinked sediment-tectonics systems.

Description: The Oligocene to Miocene Tertiary Piedmont Basin (TPB) is located in the NW part of Italy at the junction between the Apennine and the Alpine thrust belts. The position of the TPB on top of the Alpine/Apennine Orogen poses fundamental questions as to the tectonics of the basin subsidence. Having undergone little deformation, the TPB sediments provide an insight into the stress regime and rotations in the kinematically very complex area surrounding the basin itself. In this study we integrate subsidence and structural analysis with measurements of magnetic susceptibility anisotropy (AMS) and natural remanent magnetization (NRM) in order to better constrain the tectonic kinematics of the basin evolution. A major important period of subsidence occurred in the Middle Miocene involving the whole basin. During this period the TPB experienced NE-SW-directed compression and limited shortening. Some NW-SE-directed compressional features have been identified and they were probably active during post Tortonian times. Structures associated with north-south tension are quite common, but the amount of strain that they accommodate is minor. In addition this research provides new preliminary data suggesting counterclockwise rotation in the TPB by c. 20{degrees} which has taken place during Middle Miocene time.

Description: As frontier exploration in Africa moves into ever-deeper water and new basins, there is an increasing need to mitigate the high costs by reducing risk. One way of doing this is by drawing on knowledge from other areas. The book includes regional papers ranging from syntheses on a continental scale to details of the salt tectonics of the West African margin. It also addresses the need to continue using technologies to their limits by including papers on topics as diverse as gravity interpretation and fluid inclusion studies. The book provides essential reading for those requiring further insight into the origins of Africa's diverse petroleum systems, and will be key to geologists and geophysicists involved in the exploration for and exploitation of oil and gas throughout Africa. Of particular note is the supplementary CD-ROM containing maps of Africa's petroleum basins classified by age of target reservoir system.

Description: The study of sediments and sedimentary basins in terms of their tectonic environment requires a multidisciplinary approach and has increasingly drawn both techniques and objectives from fields outside sedimentology. The application of different theoretical, experimental and empirical resources provided by structural geology, geochemistry, geophysics, scale modelling, and field geology, complement sedimentological methods, with the combined aim of achieving a deeper understanding of the origins, evolution and significance of sedimentary sequences in terms of their tectonic history. Studies presented in this volume range across a wide spectrum from the analysis of sedimentary sequence architecture at basin scale down to the chemical properties of individual grains, and include studies from a range of tectonic settings. The volume will be of interest to those involved with, or contemplating, studies involving the linkages between tectonics and sedimentation, as well as a wider audience to whom the results of such studies may provide fresh insight.

Description: The deposition of the late Eocene to early Oligocene Pechelbronn Beds in the northern Upper Rhine Graben was controlled by changes in accommodation space, sediment supply and basin physiography, imposed by the syn-rift tectonic framework. Base-level cycles, defined by variations of the ratio of accommodation space to sediment supply (A/S ratio), allow untangling of the depositional history in this complex structural setting. A transfer zone divided the northern part of the Upper Rhine Graben into a southern and a northern sub-basin and created major depositional gradients. The low A/S ratio in the transfer zone led to sediment bypassing and cannibalisation. Only asymmetric cycles of fluvial and alluvial fan deposits developed, as the sediment was transported to the sub-basins. The higher A/S ratio on the major gradient of the southern sub-basin, which increased from the transfer zone to the south, allowed the formation of symmetric delta/shoreface and lacustrine cycles. At times starvation occurred in the transfer-zone-distal parts of the sub-basin. On subordinate scale, within the southern sub-basin, tilt-blocks bounded by growth faults created halfgrabens with inferior depositional gradients. On the footwall crest, due to low A/S ratio, bypassing and erosion occurred. Here asymmetric cycles of coarse-grained channel fill deposits were preserved. On the hangingwall, close to the normal fault, high A/S conditions were present and symmetric cycles developed. The creation of accommodation space kept pace and even outpaced the footwall-derived sediment supply, which created thick shallow water deposits.

Description: The Belluno syncline is a structure located in the Venetian fold-and-thrust belt in the Italian Southern Alps. This SSE-vergent chain mainly developed during the Neogene as a back-thrust belt of the Alps deforming the northern margin of the Adria plate. The syncline is asymmetric in shape, with a steep backlimb, as the forelimb of the paired Monte Coppolo Anticline; it is the result of fault-propagation folding during the development of the underlying Belluno thrust. In plan view the Belluno thrust shows a prominent curvature interpreted as reactivation of inherited Mesozoic extensional structures. The hinge of the Belluno syncline presents a similar curvature. Rocks cropping out in the syncline core are Upper Chattian to Langhian fine-grained molassic sediments, mainly siltstones and marls. Most fractures on the Belluno syncline have formed at high angles to bedding. The dominant fracture trends are subparallel and orthogonal to the fold hinge. Sometimes these fractures show plumose structures revealing extension (Mode I) fracture mechanism and therefore they are referred to as longitudinal (bc) joints and cross (ac) joints respectively. Other fracture trends oblique to the hinge line are found in measurement stations located on limbs distant from the fold hinge. They are referred to as oblique (shear) fractures. Throughout the western part of the Belluno syncline the fracture network maintains a roughly symmetrical distribution with respect to the fold hinge, suggesting a development in association with folding.

Description: Intraplate strike-slip deformation belts are typically steeply-dipping structures that develop in both oceanic and continental lithosphere where they form some of the largest and most spectacular discontinuities found on Earth. In both modern and ancient continental settings, intraplate strike slip deformation belts are of major importance in accommodating horizontal displacements where they additionally form very persistent zones of weakness that substantially influence the rheological behaviour of the lithosphere over very long time periods (up to 1 Ga or more). These deformation zones provide a fundamental geometric, kinematic and dynamic link between the more rigid plate-dominated tectonics of the oceans and the non-rigid, complex behaviour of the continents. During convergence, they help to transfer major displacements deep into the plate interiors. During divergence, they act as transfer zones that segment rifts, passive continental margins and, ultimately, oceanic spreading ridges. Such belts are also of great economic importance, controlling the location of many destructive earthquakes, offshore and onshore hydrocarbon deposits and metalliferous ore deposits. In the oceans, intraplate strike-slip movements are relatively minor along transform-related fracture zones, but there are an increasing number of documented examples that may reflect spatial and temporal variations in spreading rate along individual active ridge segments.

Description: The Mongolian Altai is a Late Cenozoic intraplate strike-slip deformation belt which formed as a distant strain response to the Indo-Eurasian collision over 2000 km to the south. We report results from 5 weeks of detailed fieldwork carried out during summer 2000 in northwestern Mongolia investigating the crustal architecture of the Altai at latitude 48{degrees}N. The region can be divided into discrete Cenozoic structural domains each dominated by a major dextral strike-slip fault system or range-bounding thrust fault. Gentle bends along the major strike-slip faults are marked by transpressional uplifts including asymmetric thrust ridges, restraining bends, and triangular thrust-bounded massifs. These transpressional uplifts (Tsambagarav Massif, Altun Huhey Uul, Sair Uul, Hoh Serhiyn Nuruu, Omno Hayrhan Uula, Mengildyk Nuruu) comprise the highest mountains in the Mongolian Altai and are structural and metamorphic culminations exposing polydeformed greenschist-amphibolite grade basement recording at least two phases of Palaeozoic ductile deformation overprinted by Cenozoic brittle structures. Cenozoic thrust faults with the greatest amounts of displacement bound the W and SW sides of ranges throughout the region and consistently verge WSW. Each major range is essentially a NE-tilted block and this is reflected by asymmetric internal drainage patterns. Many faults are considered active because they deform surficial deposits, form prominent scarps, and define range fronts with low sinuosity where active alluvial fan deposition takes place. Reactivation of the prevailing NW-striking, NE-dipping Palaeozoic basement anisotropy is a regionally important control on the orientation and kinematics of Cenozoic faults. At first order, the Altai is spatially partitioned into a low-angle thrust belt that overthrusts the Junggar Basin on the Chinese side and a high-angle SW-vergent dextral transpressional belt on the Mongolian side. The mechanically rigid Hangay craton and Junggar basement block which bound the Altai on either side have played a major role in focusing Late Cenozoic deformation along their boundaries and within the Altai. The geometric relationship between rigid block boundaries, Palaeozoic basement structural anisotropy, and the dominantly NE SHmax (derived from India's continued NE indentation) has dictated the kinematics of Late Cenozoic deformation in the Altai, Gobi Altai, and Sayan regions.

Description: Provenance analysis of siliciclastic sedimentary rocks gives indications of the tectonic evolution and setting of source regions and the rocks contained in them. The composition of sedimentary rocks ideally reflects the nature of these regions, and only indirectly the tectonic setting of the basin where the erosional debris is deposited. This makes it possible to interpret Late Devonian to Early Carboniferous metasedimentary basement rocks of the Andes in southernmost Chile as having been deposited at a passive margin, despite geochemical indications of an active margin setting for the source rocks, and the position of the metasediments in the backstop of an accretionary wedge. Major and trace elements point to felsic source rocks from an active margin environment. The Nd model ages of 1170-1490 Ma indicate that the source rocks were part of an old continental crust in the Late Palaeozoic. The {varepsilon}Nd(T) values range between -7 and -2. These characteristics, in combination with the regional geology, suggest that the geochemical signal is dominated by rocks formed at an active margin, which later acted as feeders for the sediments deposited in a passive-margin environment. If corroborated by research in progress this emphasizes the problem of deducing the tectonic setting of a depositional basin from provenance data.

Description: Three-dimensional fracture networks, combined from seismic scale to wellbore scale, can greatly enhance the knowledge of fracture contribution to hydrocarbon storage and flow inside the reservoir. This paper presents some techniques used at ENI-Agip for fracture network simulation at wellbore scale. First, new imaging techniques adopted for enhancing and facilitating the data acquisition from orientated cores will be shown. We will discuss how the fracture data, acquired from the combined analysis of orientated core and wellbore image logs, have to be processed in order to calculate the geometrical parameters of each feature (dip direction, dip, size, terminations) and to classify them according to their filling (oil, water, shale, calcite, etc.). Second, we will illustrate how these kind of data are processed in order to extract the fracture representative parameters needed for stochastic simulation of fracture network at the wellbore scale: spatial distribution along the cored/logged interval, number of fracture sets, representative orientation and statistical distribution of each set, distribution laws of the fracture length and relevant minimum radii and fracture aperture estimate. Fracture porosity evaluation, matrix block size, fracture network connectivity at wellbore scale constitute the outputs of such simulations: they are used to better characterize a fractured reservoir and to describe its behaviour. The synthetic results of the application of such a methodology on a real case (tight packstone/wackestone of carbonate platform from a southern Apennine italian oil reservoir) complete the paper.

Description: Ground penetrating radar (GPR) surveys were used to identify three distinct sea-level highstands within the oxygen isotope stage (OIS) 5-equivalent sediments of the Quaternary Cape May Formation in southern New Jersey. The three units, QCM1a, QCM1b, and QCM1c, are designated as submembers of the Cape May 1 member. The transgressive ravinement surfaces (TRS) forming the bounding unconformities of these units are recognized as laterally extensive reflections on the radar records and have been corroborated as gravel lag deposits and associated lithological discontinuities in continuous vibracores and hand-augered boreholes. GPR and lithological data have been used to interpret these highstand deposits in a sequence-stratigraphic framework. These units are thin (3 m or less), lithologically similar and present limited geomorphic expression of their individual existence beneath a low terrace (+6 m to mean sea level/MSL) on the northern margin of Delaware Bay. In the upper two units, QCM1b and QCM1a, radar surveys across subtle, bay-margin parallel, sublinear topographic highs of 3 m relief or less in the Jones Island area reveal internal structures consistent with geomorphic interpretation as stranded estuarine shorelines. The newly identified shoreline complexes are located well bayward of, and lower in elevation than, the commonly accepted +5 m to +6 m MSL OIS 5e shoreline. This older shoreline abuts the Cedarville Scarp and is attributed to the sea-level highstand associated with QCM1a. GPR was also used to accurately place vibracores for gathering samples of the three units for optically stimulated luminescence dating. Together, these three highstand deposits record sea-level fluctuations within OIS 5, at time scales shorter than those commonly recognized in the mid-Atlantic region.

Description: Skeithararsandur in southeastern Iceland, with an area of 〉 1000 km2, is the world's largest active proglacial outwash plain. In July-August 2000, a total of over 10 km of ground penetrating radar (GPR) profile data (at 50 MHz and 100 MHz) was collected from a variety of proglacial outwash sediments across the Gigjukvisl channel region of the Skeithararsandur plain. GPR-profile results and their corresponding facies interpretations are presented for the flood deposits of a single supraglacial outwash fan and its associated source-proximal ice-walled canyon created entirely by the November 1996 jokulhlaup event. By combining the GPR data with ground surveying, photogrammetry and detailed sedimentary outcrop evidence, this study adds a new perspective to the large-scale analysis of single, high-magnitude flood events and the sedimentary record of former, ice-proximal outwash plains. The GPR derived architectures point to a higher degree of sediment reworking than predicted by previous sedimentary models and may provide a useful analogue for the study of sedimentation within similar bedrock fluvial and alluvial fan feeder systems.

Description: The internal structure of a lobate rock glacier located in the San Juan Mountains of southwest Colorado was investigated using ground penetrating radar (GPR). A 440 m, 25 MHz longitudinal profile oriented along the central axis of the rock glacier shows moderate to strongly coherent reflection horizons or layers that can be recognized clearly to a depth of 30-35 m. The layers are interpreted as representing ice-supersaturated sediments and coarse, blocky rockslide debris that are the result of flow, perhaps generated by seasonal snow pack covered by episodic debris flows or high-magnitude discharges of talus from the cirque headwall. Profiles collected at 50 MHz indicate that, in the upper 20 m thickness of the rock glacier, many of these layers are laterally continuous. The total depth of penetration ([~]40 m at 25 MHz) was sufficient to detect the rock glacier-cirque-floor contact, which is composed of underlying moraine. Several prominent reflection events that subdivide the profile into broad 10-15 m-thick layers represent contacts between major depositional units. These units are believed to be individual flow lobes that were initiated at various cirque-headwall locations. We interpret this rock glacier to be a composite feature that formed by a process involving the development and subsequent overlap of discrete flow lobes that have over-ridden older glacial moraine and protalus rampart materials. The latter materials have been incorporated into the present flow structure of the rock glacier.

Description: Vertical radar profiling (VRP) is an application of ground penetrating radar (GPR) technologies that can extract important subsurface information from suitable outcrops. Using standard GPR equipment, a site-specific time-depth calibration can be obtained, along with correlation of observed sedimentological horizons exposed on cliff-faces. These horizons may then be correlated with subsurface reflection events imaged on fixed-offset profiles. Summaries of six GPR study sites, where the VRP technique was used, are detailed. Where possible, CMP and VRP velocities have been compared, and show good correlations. Geochemical analysis of selected sedimentary rocks shows that increasing grain size and quartz mineral percentages generally lead to increased GPR velocities. Reflection events tend to be associated with sandstone/shale boundaries.

Description: Ground penetrating radar (GPR) images depend strongly on the relative geometry between the transmitter, the receiver and the subsurface reflectors. This dependence results from the directional properties of the antennae and from the sensitivity of the reflector to the polarization of the incident electromagnetic wave. In the present study, GPR data sets have been recorded using several 100 MHz antennae configurations, including transverse electric (TE) or transverse magnetic (TM) modes that have been used both parallel and perpendicular to the recording line. The images obtained show a high complementarity that provides more detailed information of the studied geological reflectors compared to the image derived from a single conventional acquisition configuration. Consequently, we compute pseudoprofiles that gather information derived from all acquisition modes, thus improving the GPR images. These pseudo-profiles are characterized by either a low directional degree of the antennae or a neutral acquisition mode. In addition, we have conducted a polarization analysis by surveying the studied profile twice with parallel and perpendicular antennae, in both modes (TE and TM). The comparison of these data sets provided three-dimensional (3-D) information on reflectors, particularly their depolarization properties. These studies clearly emphasize how multiconfiguration antennae surveys have the potential to improve GPR imaging and interpretation. Furthermore, polarimetric surveys have been carried out in order to explore further whether depolarization phenomena could explain observations of phase inversion and amplitude decrease on TE common mid-point gathers that would induce perturbations of the GPR images in the stacking procedure for multi-offset surveys.

Description: Petroleum reservoir models are currently built from two-dimensional (2-D) information. An understanding of both the large-scale and internal three-dimensional (3-D) architecture of turbidite channel deposits is important for both hydrocarbon exploration and production. A ground penetrating radar (GPR) survey was undertaken on a study site exposing Upper Carboniferous Ross Formation deposits in western Ireland. Both channel margins and intrachannel fill were imaged in 3-D. Constant-offset, 2-D reflection sections were calibrated by vertical radar profiles. GPR data were integrated with sedimentary and survey data to produce a 3-D model of the study site.

Description: We report the results of a geochemical survey of fumaroles, thermal springs, and gas discharges from areas in and around the active crater lake of Poas volcano (Costa Rica) from February 1998 to February 2001. The springs are highly acidic-sulphate waters with temperatures approaching boiling point, whereas gas chemistry is characterized by typical magmatic species, such as SO2, HF, HCl, H2, and CO. From February 1998 new fumarolic fields formed inside the southern part of the crater. They moved anticlockwise from the S to the NE inner walls of the crater, while those located in the southern part of the crater and close to the pyroclastic cone south of the crater lake diminished or disappeared altogether, during 1999 and 2000. This shift was also characterized by chemical variation of the magmatic gas species. In spite of the chemical changes of fumaroles, the composition of the lake changed little during this time. This fact, together with the chemical profile with depth of the lake, suggests that the lake is a very efficient condenser of magmatic fluids. An apparent chemical stratification of the lake suggests that dilution with meteoric water is not complete, due to the presence of liquid sulphur at the lake bottom and/or due to the continuous influx of new magmatic components.

Description: Between 1989 and 1997 United Kingdom Nirex Limited (Nirex) studied in detail the geology and hydrogeology of a rock volume near Sellafield in Cumbria, NW England. The aim of the study was to determine the suitability, or otherwise, of the site as the location for a deep repository for intermediate-level and certain low-level radioactive wastes. An important factor in determining site suitability was the nature of groundwater flow in the potential repository host rock, the Borrowdale Volcanic Group. In the host rock, interpretation of borehole core, wireline logs and hydrogeological pumping test data indicated that groundwater flow was predominantly through a limited subset of discontinuities, mainly fractures, parts of which form networks of connected channels. Within this overall understanding of the nature of groundwater flow, there is a wide range of possible geometrical descriptions for the flow channels. Determination of one or more appropriate conceptualizations of the flow system must be soundly based on site characterization data as a prerequisite for any numerical modelling study. In the first part of this paper, details of the site characterization studies that were used to identify the location, orientation and mineralogical characteristics of discontinuities, and in particular the set of discontinuities referred to as Potential Flowing Features (PFFs), are provided. These features have either demonstrable present-day open porosity, or display evidence of geologically recent groundwater flow as part of the evolution of the current groundwater system. It is inferred that the PFFs observed in boreholes correspond to flowing features and that the borehole data can be used to infer the distribution and characteristics of flowing features that are present in the unobserved rock mass. On this basis, knowledge of the distribution, orientation and permeability associated with the PFFs provided the framework for developing conceptual models for groundwater flow. Numerical models were constructed to represent the flow. It is not computationally practicable to undertake regional scale groundwater flow and transport calculations in which small-scale variability is explicitly represented. Therefore upscaled effective parameters need to be derived as a precursor to running large-scale numerical model simulations. A summary of the Nirex upscaling procedure applied to the flowing feature network in the Borrowdale Volcanic Group is provided.

Description: Fractures are ubiquitous in crystalline rocks and control the strength and geophysical and fluid transport characteristics of the Earth's upper crust. A quantitative description of fracture attributes may constrain models of fracture formation and evolution. In this study, fracture attributes collected from one-dimensional samples across exposures of typical crystalline rocks show comparable variability in fracture size and spacing to sedimentary rocks. Vein thickness and fracture aperture data show predominately power-law distributions. Vein and fracture spacing data are best described by exponential distributions with negative slopes and appear to vary with composition in intrusive rocks. The fracture systems exhibit a range of anti-clustered to clustered patterns, and densities are an order of magnitude higher for joints compared to veins. Fracture clustering data can be used in conjunction with the spatial distributions to provide information on the controlling processes of fracture spacing. We suggest that exponential spacing distribution is produced as a sampling effect for both periodic-spaced and clustered fracture sets. In the examples given here, thermal stress-related joint patterns are distinguishable from tectonic-related fractures in plutonic rocks and fracture density and clustering is increased towards a major reactivated basement fault.

Description: We present an analytical model that predicts some of the mechanical effects associated with the intrusion and subsequent cooling of a rectangular intrusion emplaced at a uniform temperature into elastic continental crust. Assuming an idealized geometry and initial conditions, we recover the temperature field and subsequent strain field as a function of both position and time. The strain field is particularly relevant as it provides information on the primary (cooling-related) fracture formation pattern and direction within and immediately surrounding the pluton. We find a large strain jump across the pluton-country rock contact, implying that fracture formation should be maximized at the edges and corners of the intrusion. The direction of the fractures is predominantly vertical within the pluton centre, but becomes progressively more inclined towards the pluton margin and into the adjacent country rock. Fracture orientation may depend critically on the geometry of the intrusion, in particular the ratio of the longest to shortest dimension L1/L2.

Description: A new genetic model is proposed for the formation of the St Austell kaolin deposits, incorporating geological, isotopic, paragenetic and microthermometric evidence from kaolin-quartz veins combined with a reconstruction of the thermal evolution of the Cornubian pluton during the Mesozoic. Fluid inclusions in quartz, paragenetically associated with kaolin, document that the kaolinization took place at temperatures between 50{degrees}C and 100{degrees}C, indicating that the kaolinization is a low-temperature hydrothermal event coincident with the oil generation window. Kaolinization occurred prior to the unroofing of the pluton, during the Late Jurassic to Early Cretaceous. The kaolinization is thus contemporary with the major Early Cretaceous uplift that affected the Cornubian massif as a consequence of rifting in the offshore Western Approaches. Geological, isotopic and geochemical considerations argue strongly against the involvement of unmodified meteoric waters in the kaolinization process. The most plausible fluid types for the kaolinization are either basinal brines expelled from Permo-Triassic sediments of the adjacent offshore Plymouth Basin, or highly evolved meteoric waters that circulated through the sediments enclosing the pluton. The kaolinization process converted large volumes of fractured granite to a porous quartz-kaolin rock matrix.

Description: Microfocus X-ray computed tomography ({micro}CT) was used as a tool to determine the apertures of a fracture in a cylindrical sample of crinoidal limestone. After scanning, artefacts were removed from the images. Phantom objects were used to establish a calibration relationship between real fracture apertures and fracture aperture measurements on the {micro}CT images. The performance of different procedures for quantitative fracture determination was examined. The calibration relationship was then used to determine the fracture apertures in a naturally fractured sample. A comparison of the {micro}CT technique and a microscope technique shows a good agreement between their results.

Description: The porosity and transfer properties of a very low porosity material (granite) are measured. A new procedure is defined using a capillary test and X-ray computed tomography (CT) scanning. Injected volumes are very low, i.e. a few cm3 for a sample volume of 1 dm3, using a fluid/rock ratio lower than 0.1%. This technique allows monitoring of the anisotropy of fluid flow during the test. Flow along the injection direction is higher than along the perpendicular direction. Saturation depends on the specific saturation of each mineral zone. Multiscale analysis allows defining the flow conditions as being controlled at both the mineral and the sample scale. Results indicate the specific role for various constituting parts of the material. High speed flow occurs in the crack network of K-feldspar, while the storage function is localized in the reaction zone forms by quartz and muscovite.

Description: Semivariograms of samples of Culebra Dolomite have been determined at two different resolutions for gamma ray computed tomography images. By fitting models to semivariograms, small-scale and large-scale correlation lengths are determined for four samples. Different semivariogram parameters were found for adjacent cores at both resolutions. Relative elementary volume (REV) concepts are related to the stationarity of the sample. A scale disparity factor is defined and is used to determine sample size required for ergodic stationarity with a specified correlation length. This allows for comparison of geostatistical measures and representative elementary volumes. The modifiable areal unit problem is also addressed and used to determine resolution effects on correlation lengths. By changing resolution, a range of correlation lengths can be determined for the same sample. Comparison of voxel volume to the best-fit model correlation length of a single sample at different resolutions reveals a linear scaling effect. Using this relationship, the range of the point value semivariogram is determined. This is the range approached as the voxel size goes to zero. Finally, these results are compared to the regularization theory of point variables for borehole cores and are found to be a better fit for predicting the volume-averaged range.

Description: Water plays a fundamental role in rock weathering processes. Its penetration and movement inside rocks greatly influences the nature and intensity of damage affecting building stones. X-ray computed tomography (CT) is a useful technique for non-destructive mapping of water penetration. For the reported study, CT was used to investigate the internal structure and water penetration patterns for the Piedramuelle Stone, a Cretaceous limestone used as building material in the Oviedo Cathedral (North of Spain). X-ray CT provides good images of the internal structure of the samples: the sedimentary layering due to differences in composition and porosity is clearly seen, as well as other textural features. The movement and penetration rate of water was monitored during standard free absorption water tests. The images that were obtained show a clear difference between dry and wet zones in the interior of the sample. Water movement is related to the petrographic characteristics of the rock, mainly to layering which controls the direction of water penetration. Hounsfield Unit numbers provide a quantitative approach for assessing the penetration rate of water.

Description: It is usually implied that void ratio of a soil specimen is homogeneous and unchanging during an undrained triaxial test. In this study, X-ray computed tomography was used to measure void ratio redistribution during undrained triaxial compression of a cohesionless soil sample. A specially designed triaxial apparatus was used to scan the same sample at different axial strain levels, while the axial loads and confining pressures were maintained. Significant variation in sample void ratio was observed at the end of consolidation and sample uniformity was found to increase with increasing axial strain. The process of void ratio redistribution can be effectively illustrated in void ratio-effective stress space.

Description: Faults profoundly affect patterns and rates of fluid flow and solute transport in the geological environment. They may act as conduits, barriers or combined conduit-barrier systems. In order to elucidate the relationship between fluid flow properties and deformation mechanisms of fault-related rocks, we applied X-ray CT during laboratory permeameter measurements for direct imaging of fluid flow during permeability testing. A KI solution, which has high X-ray attenuation values, was used as a contrast medium for the advection imaging. Three-dimensional fluid flow distributions were measured for the studied fault-related rocks. Fault zones characterized by independent particulate flow as deformation mechanism act as conduits for fluid flow, whereas cataclastic fault zones act as barriers.

Description: High-resolution and ultra-high-resolution X-ray computed tomography are rapid, non-destructive and extremely powerful techniques for three-dimensional examination and measurement of a great variety of geological materials and specimens with sizes from several millimetres to several decimetres. A review of recent applications in petrology, meteoritics and palaeontology, which utilized an instrument optimized for geological studies (High-Resolution X-ray Computed Tomography Facility of the University of Texas at Austin), documents an abundance of novel scientific results and illuminates the potential for still broader application of these techniques in the earth sciences.

Description: Distal Aptian-Albian deep water channelled massive sands of the Vocontian Basin (SE France) are often associated with sand injections. The Bevons and Rosans areas in the Vocontian domain present probably the most spectacular outcrops showing complex networks of clastic sills and dykes injected into a thick marly/limy succession. Most injections are found in the channel banks, fed laterally from sandy channels. The sills are up to 10 metres thick in the vicinity of the connection with the channel feeder; they thin out and die into marls 2 or 3 kilometres away from it. Most dykes are injected from the sills rather from the channel itself: a few small dykes can be found under the channel fill. They are most abundant within a few hundred metres of the channel. Today, injections extending downwards from sills have up to 275 metres vertical extent, whereas injections extending upwards from sills never reach the contemporaneous palaeo-sea floor. Ptygmatic folding of the dykes by mechanical compaction indicates the amount of local post-injection compaction of shale and clearly shows that sand injection occurred prior to burial. Outcrop mapping shows that channel bank fracturing is contemporaneous with channel infilling. This is evidence of early syn-depositional injection of the sandy material. Vocontian clastic injections provide good geometrical analogues to deep offshore clastic injectite networks and the opportunity to better understand genetic processes.

Description: Sedimentary facies in the subsurface are usually interpreted from a epositional/stratigraphical perspective: the depositional layering is generally considered to remain undisturbed, except in a few settings. But, there is growing evidence that subsurface sediment mobilization (SSM) is more widespread than previously thought, as new observations arise from the ever-increasing resolution of subsurface data. Many examples are from hydrocarbon provinces but studies elsewhere, for example in preparation for the underground storage of hazardous waste, have yielded unexpected examples. Although until now the different aspects of SSM, including soft sediment deformations, sand injections, shale diapirs, mud volcanoes, etc, have been separated, the new discoveries emphasize their inter-connection, regardless of scale, depth, location, grain size or trigger mechanism. This volume integrates the different aspects of sediment mobilization in the subsurface and their structural consequences, allowing a more generaland a more coherent view of the subject. ... This 250-word extract was created in the absence of an abstract.

Description: Acoustic facies interpretation, high-resolution velocity analysis and amplitude versus offset modelling have been performed on high resolution seismic data acquired on the West African margin offshore Angola, in water depths of about 2000 m. The area has a complex structural, thermal and fluid-flow setting, in which sediments are affected by salt diapirism and faulting associated with sediment compaction. A discontinuous bottom-simulating reflector (BSR) at a depth of about 200 m below sea floor could mark the base of the gas hydrate occurrence zone, which does not always coincide with the top of the free gas zone. Within the gas hydrate stability zone, a shallow bottom-simulating reflector is observed at a depth of about 75 m below seafloor. This shallow bottom simulating reflector, that is termed sheep back reflector' (SR), correspond to a small amount of gas being trapped in the sediments. It could mark the top of the gas hydrate occurrence zone, where gas hydrate dissociation may occur. A reversed polarity reflector (R1) is also observed about 25 m below the sea floor. This reflector could correspond to a limit between normally compacted and underconsolidated sediments, possibly related to a permeability change in the sediments. Thus, the occurrence of excess pore pressure generated during gas hydrate dissociation could explain some subsurface sediment mobilization processes.

Description: Cylindrical structures, cross-cutting stratification at right angles, occur in the Muth Formation, representing Lower Devonian barrier island arenites of the North Indian Gondwana coast. These structures are up to 1.5 m in height and 0.8 m in diameter, with an internal structure comprising concentric, cylindrical laminae. The pipes, which probably represent water conduits for laminar upward flow of ground water, initiate from relatively thin horizons, with upper terminations formed by spring pits. Thus, the structures in the Muth Formation represent a rarely observed combined occurrence of spring pits and their conduits below. Their formation is explained by rising ground water seepage in a coastal depositional environment that produced a relatively high hydrostatic head, resulting in the formation of springs. The rise in relative sea level might be related to tectonic subsidence caused by tectonic activity linked to the formation of conjugate deformation bands in the Muth Formation. This means, if tectonic activity was involved, it did not form the cylindrical structures by seismic liquefaction directly, but might be responsible indirectly through ground water seepage rise resulting from tectonic subsidence. Due to the little relief in this environment, the sea level rise affected a relatively large area and fluidization structures can be found widespread in distant sections.

Description: The small intracratonic Cheb (Eger) Basin in NW Bohemia (Central Europe) is characterized by swarm earthquakes, many mineral springs and mofettes with upper mantle CO2 degassing and by neotectonic graben and basin structures. Especially in non-lithified Upper Pliocene clay formations of the basin, a variety of deformation patterns is exposed. They include non-tectonic and tectonic activity and comprise faulting and folding from {micro}m- to km-scale. Previously unrecognized N-S- and ENE-striking faults are sites of mantle degassing and seismic activities. Confined-layer deformation and liquefaction structures hint to palaeoseismic events and gas escape activity. Cleavage-like arranged clay mineral plates represent the microfabric of clay within fault zones. For the first time the degassing channels of Upper Mantle fluids/gases through the Pliocene clay sediments can be documented: {micro}m-scale micro-tubes were produced by the opening of Riedel shear planes induced by fault movements.

Description: Polygonal fault systems are widely developed in fine-grained sedimentary successions and have been recognized in over 50 basins worldwide. They are normal faults with modest throw values (typically 10-100 m), organized with a characteristic plan form pattern that is crudely polygonal, but with considerable variation in specific planform patterns. They have been attributed to four genetic mechanisms: gravity collapse, density inversion, syneresis and compactional loading. Their strain characteristics allow them to be distinguished from tectonic normal faults. The strengths and weaknesses of the four genetic mechanisms are considered in the light of these strain characteristics. It is argued that syneresis offers the likeliest mode of genesis and best explains the local and global features of these extraordinary structures. The detailed physical mechanism driving syneresis remains poorly understood.

Description: Quantitative analysis of faults within a South Australian polygonal fault system, interpreted from a 3-D onshore seismic survey, provides a basis for establishing their growth and linkage histories. The geometric characteristics of faults are consistent with an origin arising from the gravitational instability of an underlying low-density, overpressured, mobile layer. Fault size populations have scale-bound, non-power-law properties reflecting the thicknesses of the faulted and mobile layers and the strongly connected nature of the system. The spatial distributions of faults reflect the localization of conjugate faults at the top of the mobile layer and the scale of fault-bounded polygons. Displacement variations on faults show marked decreases at or adjacent to the top of the mobile layer and attest to its active role in faulting. The wide range of fault strike directions provides numerous fault intersections with high intersection angles ([≥] 60{degrees}) forming triple-junctions at which fault linkage and capture occurred. Fault linkage and capture is attributed to a simple model in which continued movement on faults which share a mutual footwall is favoured and hanging wall faults are deactivated. The model involves thickening of the mobile layer within the footwalls of faults and thinning and eventual grounding of the overlying sequence, within their hanging walls.

Description: New multi-beam sonar and seismic data collected in Lake Superior document the wide-spread development of lake-floor rings in fine-grained lake-floor sediments. The multi-beam images reveal that the rings develop as connected clusters and that individual rings have an irregular polygonal appearance. High-resolution seismic data collected with a 28 kHz echo sounder reveal extensive fracturing and faulting in the glacio-lacustrine sediments below the lake-floor. Displacement on the faults is typically normal with throws of less than 50 cm. Three styles of faulting are recognized: (a) monoclinal flexure; (b) graben-like; and (c) conjugate. Zones of acoustic blanking below the faults may be associated with de-watering and mobilization of the sediments. Lateral thickness variation in some horizons suggests that fault and fracture development is linked with lateral movement of sediment. Piston cores collected near lake-floor rings show well-developed fractures and micro-faults, suggesting that fracturing and faulting occurs on a wide range of scales. The seismic and lithological characteristics of the glacio-lacustrine section are similar to those of sediments in which Polygonal Fault Systems (PFS) have been described. This suggests that the rings in Lake Superior may be the surface expression of PFS in the near-surface sediments.

Description: Deep-seated mud volcanoes are observed in a variety of geological settings, which has led to considerable debate on their origin. This paper summarizes the geological features common to mud volcanoes around the world and possible mechanisms of their extrusion. Field and laboratory data from Trinidad and Taiwan are discussed to assess the possible sources and causes of the volcanoes. A close association between mud volcanoes and compressional tectonics leads to the conclusion that tectonic activity plays an important part in mud volcano development. Experimental data are presented to explain the role of tectonic activity and the association between shear stresses and mud volcanoes. It is demonstrated that shear stresses applied to low permeability sediments can produce a dramatic increase in pore pressure and can cause sediment flow. This is proposed as one possible contributing cause of mud volcanoes.

Description: Authigenic carbonates and muds from six mud volcanoes in the eastern Mediterranean Sea were recovered during the French/Dutch MEDINAUT cruise utilizing the submersible Nautile in November 1998. The mud volcanoes are active seafloor vents in two areas at the plate boundary between the converging African and Eurasian Plates: the Mediterranean Ridge accretionary prism near Crete (Greece) and the Anaximander Mountains south of Turkey. B contents and {delta}11B signatures were measured with the aim of identifying the diagenetic processes and source depths of the material in the collision zone. B concentrations of the carbonate precipitates cover a range of 8-45 ppm and vary isotopically from +15.6 to +22.9{per thousand} (corresponding to a parent solution of 34.9-42.2{per thousand} at pH 7). Both the B-enrichment and a {delta}11B valve slightly lower than seawater suggest the mud domes originate from a moderately deep fluid source, with local admixture of seawater. B contents and {delta}11B of the mud show distinct differences between the areas: the Mediterranean Ridge mud domes have lower B contents and higher {delta}11B (average 3.9{per thousand}) compared to Anaximander Mountains mud volcanoes ({delta}11B average -0.6{per thousand}). These B results attest the release of structurally-bound B from clay mineral lattices, probably due to stronger deformation near Turkey. These mudstones, which had previously been affected by deep-seated thrusting beneath the Antalya Complex, may have been liquefied and remobilized in their present setting. By contrast, the mud on the Mediterranean Ridge represents offscraped clay-rich strata that was incorporated into the large accretionary wedge.

Description: Emissions of SO2 by volcanic eruptions have been shown to be important for short-term environmental and climate changes. Stratospheric sulphur mass-loading by explosive silicic eruption is commonly considered to be the principal forcing factor for these changes. The SO2 emissions from basaltic flood lava eruptions have not featured strongly in the discussions on volcano-climate interactions, notwithstanding the fact that basaltic magma is typically richer in sulphur (by a factor of two to four), than silicic magmas, as well as the evidence of widespread atmospheric impact associated with historical flood lava eruption. Fourteen Holocene flood lava eruptions are known from the Veidivotn, Grimsvotn, and Katla volcanic systems of the Eastern Volcanic Zone in South Iceland, which include the three largest of its kind in Iceland; the 1783-1784 Laki, 934-40 Eldgja, and c. 8600 years BP Thjorsa events. We present new data on the sulphur content in melt inclusions from the Veidivotn system and use this information, along with existing inclusion data from the Grimsvotn and Katla volcanic systems, to establish an empirical method for estimating the sulphur mass release from these basaltic flood lava eruptions. The results show that these eruptions released a total of c. 700 Mt SO2 into the atmosphere in four 600- to 850-year-long eruption periods. During each period, between 98 and 328 Mt SO2 were emitted into the atmosphere, and the mass loadings from individual eruptions ranged from 5 to 210 Mt SO2. These flood lava eruptions are likely to have resulted in widespread atmospheric perturbations and, by analogy with the 1783-1784 Laki eruption, the effects of the largest eruptions may have been felt on a hemispheric scale.

Description: Magma-wall-rock interaction contributes gases to evolving magmatic systems, and removes volatiles into the country rock. These processes happen at depth, far away from direct observation. Micro-analysis of particles collected from volcanic plumes can provide information about these processes. For Popocatepetl volcano, scanning electron microscope (SEM) and field emission gun SEM (FESEM/EDS) analysis of contact-metamorphosed particles from fallout ash reveal the presence of wollastonite, hercynite and glass of non-volcanic, contact-metamorphic origin. Condensates from the passively degassing plume show a wide variety of chemical elements and are rich in phosphorus, indicating a possible non-magmatic source for this element.

Description: Since their first depolyment in November 1978, the Total Ozone Mapping Spectrometer (TOMS) instruments have provided a robust and near-continuous record of sulphur dioxide (SO2) and ash emissions from active volcanoes worldwide. Data from the four TOMS satellites that have flown to date have been analysed with the latest SO2/ash algorithms and incorporated into a TOMS volcanic emissions database that presently covers 22 years of SO2 and ash emissions. The 1978-2001 record comprises 102 eruptions from 61 volcanoes, resulting in 784 days of volcanic cloud observations. Regular eruptions of Nyamuragira (DR Congo) since 1978, accompanied by copious SO2 production, have contributed material on approximately 30% of the days on which clouds were observed. The latest SO2 retrieval results from Earth Probe (EP) TOMS document a period (1996-2001) lacking large explosive eruptions, and also dominated by SO2 emission from four eruptions of Nyamuragira. EP TOMS has detected the SO2 and ash produced during 23 eruptions from 15 volcanoes to date, with volcanic clouds observed on 158 days. The EP TOMS instrument began to degrade in 2001, but has now stabilized, although its planned successor (QuikTOMS) recently failed to achieve orbit. New SO2 algorithms are currently being developed for the Ozone Monitoring Instrument, which will continue the TOMS record of UV remote sensing of volcanic emissions from 2004 onwards.

Description: Previous cladistic studies of pterosaur relationships suffer from restricted numbers of taxa and characters, incomplete data sets and absence of information on characters, tree structure and the robustness of tress. Parsimony analysis of a new character data set (60 characters, 20 terminal taxa, 97.75% complete) yielded six trees. In the strict consensus tree Preondactylus is the most basal taxon followed, stepwise, by the Dimorphodontidae and the Anurognathidae. Beyond this basal group, more derived pterosaurs (Campylognathoididae (Rhamphorhynchidae + Pterodactyloidea)) share a suite of characters principally associated with elongation of the rostrum. The Pterodactyloidea consists of four major clades. The Ornithocheiroidea is the most basal taxon consisting, stepwise, of Istiodactylus, the Ornithocheiridae, Nyctosaurus and the Pteranodontidae. The remaining taxa, Ctenochasmatoidea, Dsungaripteroidea and Azhdarchoidea, are weakly united in a clade of non-ornithocheiroid pterodactyloids, but their inter-relationships are difficult to resolve. Cycnorhamphus is the basal-most ctenochasmatoid, while the remaining taxa (Pterodactylus, Lonchodectidae, Ctenochasmatidae) form an unresolved trichotomy. The Dsungaripteroidea (Germanodactylus+Dsungaripteridae) is strongly supported by features of the skull and dentition. The Azhdarchoidea (Tapejara [Tupuxuara + Azhdarchidae]) is united by cranial characters such as elevation of the antorbital region, and relative shortening of the wing finger. The pattern of pterosaur evolution suggested by the results of this analysis is broadly similar to traditional ideas, but has greater resolution, more complexity and reveals several previously unrecognized events'.

Description: A large azhdarchid pterosaur is described from the Late Maastrichtian phosphatic deposits of the Oulad Abdoun Basin, near Khouribga (central Morocco). The material consists of five closely associated cervical vertebrae of a single individual. The mid-series neck vertebrae closely resemble those of azhdarchids Quetzalcoatlus and Azhdarcho in that they are elongate, with vestigial neural spines, prezygapophysial tubercles, a pair of ventral sulci near the prezygapophyses, and without pneumatic foramina on the lateral surfaces of the centra. The Moroccan pterosaur is referred to a new genus and species of Azhdarchidae: Phosphatodraco mauritanicus gen. et sp.nov. It is mainly characterized by a very long cervical vertebra eight, bearing a prominent neural spine located very posteriorly. Based on comparisons with azhdarchid vertebrae, the estimated wing span of Phosphatodraco is close to 5 m. This discovery provides the first occurrence of Late Cretaceous azhdarchids in northern Africa. Phosphatodraco is one of the few azhdarchids known from a relatively complete neck and one of the latest-known pterosaurs, approximately contemporaneous with Quetzalcoatlus.

Description: Tectonically active remnants of Neo-Tethys represented by Mediterranean and western Pacific marginal seas are characterized by rapidly propagating backarc extension episodes. These appear to be triggered by random subduction nucleation events, commonly signalled by the appearance of refractory boninites in volcanic proto-arcs'. As backarc basins evolve, active arcs separate from their proto-arc' remnants and may split again if more than one basin-opening episode occurs. Accreting arc-forearc terranes are therefore likely to incorporate proto-arc, backarc, and (in some cases) inherited continental fragments, as evidenced by their structural complexity and lithological diversity. Forearc complexes typically show positive Bouguer gravity anomalies and significant age discrepancies within and between their crustal and mantle components. Where exposed, their lower stratigraphic horizons may include boninite-bearing assemblages along with tectonized fragments of mid-ocean ridge basalt (MORB) basement and hydrated refractory peridotite. These are typically intruded by sodic plagiogranite (adakite) and high-temperature Mn-, Fe-rich hydrothermal veins ( epidosites'), further indications of subduction nucleation at, or close to a pre-existing spreading axis. Where the arc-trench rollback process is terminated by collision with an approaching continent, or with another retreating forearc complex, MORB-like backarc lithosphere is rapidly reconsumed, in some cases following a change in subduction polarity. In contrast, given their preponderance of ultra-refractory serpentinized peridotite, forearc complexes are relatively buoyant, resist subduction, and are prone to entrapment during early stages of an orogeny. The associated interplay of extension and compression offers a compelling scenario for resolving the so-called ophiolite conundrum' and explaining the near-ubiquity of ophiolites in orogenic belts. We propose that rapid arc-trench rollback pulses are driven largely by collision-induced mantle flow in addition to commonly cited slab pull' effects. This is supported by the evidence of isotopic mantle flow tracers, seismic tomography, and the coupled kinematics of marginal basins and continental escape. Model applications to some well-known Tethyan ophiolites are developed in a companion paper.

Description: Results of a field study as well as petrological and geochemical data demonstrate that substantial portions of the lithospheric mantle, exhumed during opening of the Jurassic Piedmont Ligurian ocean, were infiltrated by and reacted with migrating melts. Intergranular flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene {+/-} spinel and precipitated orthopyroxene + plagioclase {+/-} olivine, forming orthopyroxene + plagioclase-rich perioditite. Migrating liquids became progressively saturated in clinopyroxene, and then precipitated microgranular aggregates of clinopyroxene-bearing gabbronorite. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in clinopyroxene and plagioclase, forming interstitial clinopyroxene at olivine triple points followed by clinopyroxene {+/-} plagioclase megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Subsequent cooling during continued exhumation was accompanied by intrusion of kilometre-scale gabbroic dykes evolving from troctolite to Mg-Al and Fe-Ti gabbros. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks, span a wide range of compositions from silica-rich single melt fractions to T- and N-MORB (mid-ocean ridge basalt), characteristic of the melting column beneath midocean ridges. Explanations for the progressive evolution of an igneous system from diffuse to focused porous flow and finally dyking include the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling by exhumation. Whether or not rift-related melt infiltration and heating is recorded by exhumed subcontinental lithospheric mantle along ocean-continent transitions and/or oceanic lithospheric mantle along slow-spreading ridges depends on the relative position to the underlying upwelling asthenosphere.

Description: Andesitic to dacitic dome-building volcanoes often present a problem for eruption forecasting because signs of impending activity can be minimal or ambiguous. Gas monitoring is one of a number of techniques used to assist in eruption forecasting. However, a variety of explanations have been offered for the large variations in gas release that are commonly reported from erupting volcanoes. Difficulties in interpretation can arise because gas-flux measurements are generally acquired at lower sampling rates than other geophysical observations. Here, we report SO2 flux measurements, by correlation spectroscopy, recorded semi-continuously during December 1999 to January 2000 at the Soufriere Hills Volcano, Montserrat. We compare these data to continuously recorded seismic records, and interpret the results in terms of conduit dynamics. We demonstrate two- to six-fold variations in gas flux over a few hours, and show that these variations can be systematic and directly correlated with long-period swarm seismicity. For the period of study, we find that the gas-flux peak lags several tens of minutes behind the peak in seismic energy release. These features are consistent with models of oscillating magma flow, where magma viscosity is dependent on melt volatile content. We propose that seismicity reflects conduit pressurization, and find that gas flux directly reflects magma flow rate. Although other volcanoes might behave differently, our results suggest that it can be possible to use continuous gas measurements to monitor conduit behaviour, perhaps providing short-term warnings of impending eruptions.

Description: Humans have long marvelled at (and feared) the odorous and colourful manifestations of volcanic emissions, and, in some cases, have harnessed them for their economic value. The degassing process responsible for these phenomena is now understood to be one of the key factors influencing the timing and nature of volcanic eruptions. Moreover the surface emissions of these volatiles can have profound effects on the atmospheric and terrestrial environment, and climate. Even more fundamental are the relationships between the history of planetary outgassing, differentiation of the Earth's interior, chemistry of the atmosphere and hydrosphere, and the origin and evolution of life. This book provides a compilation of 23 papers that investigate the behaviour of volatiles in magma, the feedbacks between degassing and magma dynamics, and the composition, flux, and environmental, atmospheric and climatic impacts of volcanic gas emissions.

Description: A global three-dimensional chemistry-transport model has been applied to study the tropospheric sulphur cycle, and in particular the volcanic component. The model is in general agreement with previous studies of the global S budget. We find that volcanic emissions constitute 10% of the present-day global SO2 source to the atmosphere, but form 26% of the SO2 burden, and 14% of the sulphate aerosol burden. Two previous modelling studies suggested that the volcanic fraction of sulphate was 18% and 35%, from sources representing 7% and 14%, respectively, of the global total SO2 emission. The results are dependent upon various assumptions about volcanic emissions (magnitude, geographical location, altitude), the global distribution of oxidants, and the physical processes of dry and wet deposition. Because of this dependence upon poorly constrained parameters, it is unclear which modelling study is closest to the truth.

Description: Degassing at Mount Erebus occurs as a plume from a persistent convecting anorthoclase phonolite lava lake, and by flank degassing through warm ground and fumarolic ice towers within the summit caldera. The fumarolic ice towers offer a unique and simple approach to quantifying the flank CO2 emissions. Carbon dioxide effluxes were determined at openings in the ice towers by measuring the CO2 concentration, air-flow velocity, and size of the exit orifice. Fluxes ranged from 〈0.0001 to 0.034 kg s-1 at 43 actively degassing ice towers. Small patches of steaming warm ground contributed 0.010 kg s-1. The {delta}13C isotopic compositions of the CO2 samples ranged from -2.1 to -4.7{per thousand}, suggesting a magmatic origin for the CO2. Fumarolic ice towers allow diffuse degassing to be visually identified, providing a strong advantage in determining the total flux rate of these passive emissions. The estimated output of flank CO2 degassing is 0.46 kg s-1 (40 Mg d-1). Compared with direct airborne measurements of the volcanic plume, passive flank emissions constitute less than 2% of the total volcanic CO2 budget emitted from Mount Erebus.

Description: Subduction zones are major sites of magmatism on the Earth. Dehydration processes and associated element transport, which take place in both the subducting lithosphere and the down-dragged hydrated peridotite layer at the base of the mantle wedge, are largely responsible for the following characteristics common to most subduction zones: (1) the presence of dual volcanic chains within a single volcanic arc; (2) the negative correlation between the volcanic arc width and the subduction angle; (3) selective enrichment of particular incompatible trace elements; and (4) systematic across-arc variations in incompatible trace element concentrations. The occurrence of two types of andesites, calcalkalic and tholeiitic, typifies magmatism in subduction zones. Examination of geochemical characteristics of those andesites in the NE Japan arc and bulk continental crust reveals marked compositional similarity between calc-alkalic andesites and continental crust. One of the principal mechanisms of generation of calc-alkalic andesites, at least those on the NE Japan arc, is the mixing of two magmas, having basaltic and felsic compositions and being derived from partial melting of the mantle and the overriding basaltic crust, respectively. It may be thus suggested that this process would also have contributed greatly to continental crust formation. If this is the case, then the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust into the mantle in order to form andesitic' crust compositions. These processes cause accumulation in the deep mantle of residual materials, such as delaminated crust materials and dehydrated, compositionally modified subducted oceanic crusts and sediments. Geochemical modelling suggests that such residual components have evolved to form enriched mantle reservoirs.

Description: The collision of the Luzon Arc with southern China represents the best example of arc-continent collision in the modern oceans, and compares closely with the Early Ordovician accretion of the Lough Nafooey arc of Connemara, Ireland, to the passive margin of Laurentia. We propose a general model for steady-state arc-continent collision in which arc crust is progressively added to a passive margin during a process of compression, metamorphism and magmatism lasting 3-10 Ma at any one location on the margin. Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion can be more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events. During the collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2-3 Ma) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab.

Description: Large-scale felsic volcanic systems are a common, but not ubiquitous, feature of volcanic arc systems in continental settings. However, in oceanic volcanic arcs the erupted materials are dominated by basalts and basaltic andesites, whereas intermediate compositions are rare and dacites and rhyolites relatively uncommon. The Kermadec arc is an intraoceanic convergent system in the SW Pacific. Volcanoes occur as a continuous arc that is mainly submarine. Despite its simple tectonic setting, felsic magmatism is widespread. In the Kermadec Islands, Macauley Volcano is a basaltic volcano that produced a large felsic eruption about 6000 years ago. A comparable pattern of magmatic evolution is seen on adjacent Raoul Volcano, where basaltic activity built the main edifice of the volcano and where activity during the last 3000 years has been characterized by felsic eruptions of varying size. Elsewhere in the Kermadec arc and in its northward extension, the Tonga arc, felsic eruptions are recorded from 11 of the 30 volcanoes for which petrographic information is available, and in many cases these are the most recent eruptions. Felsic europtions are a widespread recent feature of the arc, and the scale and extent of this magmatism appears to be unusual for a tectonically simple oceanic subduction system. One explanation of the origin of the felsic magmatism is prolonged fractional crystallization from a parental basalt composition, but modelling of the chemical compositions of the felsic rocks does not support this. A second explanation, albeit apparently at odds with the oceanic setting, is crustal anatexis. An important feature of the felsic eruptives from the Kermadec arc is that each tephra sequence or occurrence has a unique chemical composition, although all show the same generalized characteristics. We suggest that this feature supports a model of crustal anatexis rather than fractionation of a range of parental magmas. We also suggest that in the thermal evolution of an oceanic arc system the processes of underplating, together with the continuous magmatic (and thermal) flux, can generate a crustal thickness in which dehydration melting of underplated arc material generates felsic magmas. Further, this condition can represent a unique adolescent' stage in a developing oceanic arc, as once the felsic melts are extracted the lower crust becomes an infertile residue.

Description: The first scientific description of a pterosaur was published in 1784 by Cosimo Alessandro Collini, a former secretary of Voltaire and at that time curator of the natural history cabinet of Karl Theodor, Elector of Palatinate and Bavaria. The specimen came from one of the main sources of such fossils, the Late Jurassic lithographic limestones of northern Bavaria, and Collini, after much deliberation, interpreted it as the skeleton of an unknown marine creature (Collini 1784). In 1801, Georges Cuvier, on the basis of Collini's description and figure, identified the mysterious animal as a flying reptile (Cuvier 1801), for which he later coined the name Ptero-Dactyle' (Cuvier 1809). Cuvier's basically correct interpretation of the winged finger' marked the beginning of the study of pterosaurs as an extinct group of flying reptiles. In the two centuries which have elapsed since those first efforts to understand what have been considered bizarre fossils, the study of pterosaurs has developed enormously. Some of the basic questions about them have long been solved: pterosaurs were neither birds, nor bats, as was suggested by various authors of the early nineteenth century, but a peculiar group of vertebrates which acquired the ability to fly in an original way, using a membrane attached to a single finger of the hand. From the few fossils from the Bavarian lithographic limestones known to Cuvier and his contemporaries, the number of pterosaur specimens has increased enormously, starting with the Early Jurassic specimens from Lyme Regis found by Mary Anning in the 1820s ... This 250-word extract was created in the absence of an abstract.

Description: Pterosaur remains from the Late Cretaceous of the Ha[t]eg Basin of western Romania were reported by Nopcsa as early as 1899. Recent discoveries from the Late Maastrichtian Densu[s]-Ciula Formation include the giant azhdarchid Hatzegopteryx thambema, the holotype of which, consisting of skull elements and a humerus from the V[A]lioara locality, is described in detail. A very large femur from the same formation at Tu[s]tea is also described. The systematic position of Hatzegopteryx is discussed. The wing span of H. thambema is estimated to be close to that of Quetzalcoatlus northropi ([≥]12 m), but its skull is especially robust and may have been remarkably long ([≥] 2.5 m). The skull bones of H. thambema consist of a very thin outer cortex enclosing an inner meshwork of extremely thin trabeculae surrounding very numerous small alveoli, an unusual structure reminiscent of expanded polystyrene. This peculiar structure, combining strength with lightness, can probably be considered as an adaptation to flight in a very large animal, through reduction of skull weight.

Description: Among Pterosauria there are three types of scapulocoracoid construction. In the ornithocheirid scapulocoracoid the scapula is oriented almost horizontally; it is shorter than the coracoid and the glenoid fossa is level with the ventral margin of the vertebral column. In the azhdarchid scapulocoracoid the scapula is curved ventrally and is as long as the coracoid. In this construction the glenoid fossa lies approximately in the mid-horizontal plane of the chest. In the tapejaroid construction, the scapula is about one-third longer than the coracoid, which is oriented subhorizontally, and the glenoid fossa is level with the dorsal rim of the sternal plate. Both latter conditions are hitherto unknown among flying vertebrates and result in an unstable, but manoeuvrable flight, probably powered with wing beats.

Description: The discovery of a left scapula and coracoid (MBLUZ P-911) representing the first evidence of a pterosaur from Venezuela is reported here. The material comes from the Lower Cretaceous (Aptian) Apon Formation, in the northwestern part of the country. In MBLUZ P-911 the scapula is significantly smaller than the coracoid, a synapomorphy of the Pteranodontoidea, according to Kellner. The coracoid of the Venezuelan specimen is more elongated and gracile than those of Istiodactylus and Pteranodon, and also lacks the ventromedial coracoidal flange present in the latter. Overall MBLUZ P-911 is very similar to the scapulocoracoid of the Anhangueridae, including the presence of a longitudinal ridge on the medial surface of the coracoid and a comparatively short scapula, and is therefore tentatively referred to this taxon. This occurrence extends the pterosaur record to the northern part of the South American portion of Gondwana.

Description: A new vertebrate tracksite in southeastern Coahuila, northern Mexico, known as the El Pelillal tracksite (Latest Campanian, Cerro del Pueblo Formation) has yielded an important ichnofauna that includes the tracks of turtles, crocodilians, a small theropod dinosaur, a mammal-like organism and the tracks of pterosaurs. The pterosaurian manus impressions share an extraordinary similarity, in size and morphology, with the Jurassic ichnogenus Pteraichnus Stokes 1957 and are referred to Pteraichnus sp.; however, the pes impressions differ in that they are rather very elongated. According to the geological features and ichnofauna the El Pelillal tracksite represents a shallow, freshwater or lacustrine deposit, which agrees with our knowledge about the habitat preference of Cretaceous pterosaurs. This newly discovered tracksite in southeastern Coahuila, Mexico, offers great potential for palaeoichnological research and is thus becoming important in the understanding of the palaeoecosystems during the Late Cretaceous in southern North America.

Description: This paper deals with the systematics of tetrapod ichnotaxa based on footprints. Beyond the consideration of the nomenclatural rules for ichnotaxa in the ICZN, this paper tries to determine how to establish an ichnotaxonomy that reflects the identity of the track-maker (the organism that has made the track) and how to include this ichnotaxonomy in the skeleton-based taxonomy. This leads to the establishment of several criteria, e.g. ichnospecies should be defined on the print morphology and the relative position of the prints (including the variability due to the track-maker's dynamics), two ichnospecies should represent different species, the ichnospecies and ichnogenus levels are sufficient to discriminate the ichnotaxa and link them to the skeleton-based taxonomy. These ichnotaxonomical criteria are applied to a revision of the ichnogenus Pteraichnus Stokes 1957 (Pterosauria, Pterodactyloidea). Only the type species Pteraichnus saltwashensis is considered as valid, the pterosaurian origin of Purbeckopus is questioned and Agadirichnus is highlighted because it could be a senior synonym of Pteraichnus. The result of this drastic revision underlines the importance of the proposed ichnotaxonomical principles to avoid the unfounded proliferation of tetrapod ichnotaxa.

Description: Thin sections of pterosaur bones have not been extensively studied so far. Until now mainly isolated bones were the subject of this type of analysis. Here we present preliminary results of the histological analysis of two pterodactyloid pterosaurs from the Romualdo and Crato Members of the Santana Formation. The first specimen (Museu Nacional, MN 4809-V) comprises both wings (from humerus to the third phalanx of the fourth digit) and one hindlimb (tibia, fibula and pes). The second (MN 6527-V) consists of a partial skeleton with the incomplete left wing (humerus, radius, ulna, carpals, pteroid, wing metacarpal, two phalanges of the fourth digit) and fragments of one hindlimb (complete tibia and proximal articulation of the femur). Thin sections of the diaphyseal portion of each bone were ground in transverse, longitudinal and oblique orientation and the histological structures were compared. The following features can vary in the first observed specimen (MN 4809-V): presence of primary and secondary tissues in the cortex; absence or presence and position of lines of arrested growth; resorption of primary structures. These variations in different bones of the same individual represent differential growth rates. The second specimen (MN 6527-V) is well vascularized and has fibro-lamellar bone indicative of rapid growth in the thin section of the radius, ulna and first phalanx of the fourth digit. This last feature is very similar to the condition observed in most living birds.

Description: Ophiolites record significant evidence for tectonic and magmatic processes from rift-drift through accrefionary and collisional stages of continental margin evolution in various tectonic settings. Structural, petrological and geochemical features of ophiolites and associated rock units provide essential information on mantle flow field effects, including plume activities, collision-induced aesthenospheric extrusion, crustal growth via magmatism and tectonic accretion in subductionaccretion cycles, changes in the structure and composition of the crust and mantle reservoirs through time, and evolution of global geochemical cycles and seawater compositions. Ophiolite studies over the years have played a major role in better understanding of mid-ocean ridge and subduction zone processes, mantle dynanlics and heterogeneity, magma chamber processes, fluid flow mechanisms and fluid-rock interactions in oceanic lithosphere, the evolution of deep biosphere, the role of plate tectonics and plume tectonics in crustal evolution during the Precambrian and the Phanerozoic, and mechanisms of continental growth in accretionary and coltisional mountain belts. Through multi-disciplinary investigations and comparative studies of ophiolites and modern oceanic crust and using advanced instrumentation and computational facilities, the international ophiolite community has gathered a wealth of new data and syntheses from ophiolites around the world during the last 10 years. The purpose of this book is to present the most recent data, observations and ideas on different aspects of 'ophiolite science' through case studies and to document the mode and nature of igneous, metamorphic, tectonic, sedimentological and/or biological processes associated with the evolution of oceanic crust in different tectonic settings in Earth's history.

Description: Most eruptive rocks in the Lesser Antilles arc are compositionally evolved. However, lavas with primitive characteristics do occur including, in the central part of the arc, a suite of rocks from Soufriere, St Vincent, and the Ilet a Ramiers basalt from Martinique. High-pressure experiments performed on a Soufriere basalt point to a spinel lherzolite source. Glass inclusion data and phase equilibria analysis suggest extraction of the Soufriere melt under relatively dry conditions (c. 2 wt% H2O in melt). Using estimates of the H2O content of mantle sources fluxed by an hydrous slab-derived component, H2O concentrations as high as 5 wt% are considered possible for primary mantle melts in the Lesser Antilles arc. Experiments at low pressures (4 kbar) simulate the evolution of primitive melts within the arc crust. For elevated melt H2O concentrations (6-8.5 wt%), derivative liquids ranging from low-MgO basalt to basaltic andesite are generated at 1050-1100{degrees}C. Their crystallization at 950-1000{degrees}C yield andesitic liquids similar to those erupting at active volcanic centres such as Mt Pelee, Martinique, and Soufriere Hills, Montserrat. Therefore, experimental data support the derivation of Lesser Antilles arc eruptives by different degrees of fractionation from primary mantle melts.

Description: Neoproterozoic ophiolites, ranging in age from c. 870 Ma to c. 627 Ma, occur in several discrete suture and/or fault zones within the Arabian Shield and display a record of riftdrift, sea-floor spreading and collision tectonics during the evolution of the East African Orogen. The ophiolites within the Yanbu and Bir Umq suture zones in the west are among the oldest (870-740 Ma) in the Shield, locally show a Penrose-tyep complete pseudostratigraphy, and have chemical compositions typical of modern forearc oceanic crust. They are spatially associated with coeval and younger volcanic arc assemblages and were incorporated into the Arabian Shield during a series of collisional events that amalgamated these ensimatic arc terranes. The ophiolites of the Hulayfah-Ruwah suture zone in the central Arabian Shield are coeval with and/or slightly younger (c. 843-821 Ma) than the ophiolites in the west and probably developed in a rifted ensimatic arc system that evolved as a volcanic archipelago near the Afif continental plate. Younger ophiolites (c. 694 Ma) of the Halaban and Al Amar suture zones in the eastern Arabian Shield were incorporated into a subduction-accretion complex that evolved at the Andean-type active margin along the eastern edge (in present coordinate system) of the Afif continental plate. The Halaban suture zone ophiolites represent forearc oceanic crust, whereas the Al Amar suture zone ophiolites are scraped-off fragments of Mozambique ocean floor, seamounts and/or ocean island(s); the Abt Schist between them corresponds to a Franciscan-type accretionary prism of the Halaban' subduction zone. The incorporation of these ophiolites and the continental plates (Afif and Ar Rayn) into the Arabian Shield during 640-620 Ma marks a major shift in the direction of convergence (from northerly to westerly) during the assembly of the Shield and distinct episodes of continental collisions during closure of the Mozambique Ocean. The ophiolites of the Nabitah-Hamdah fault zone within the Asir terrane are the youngest (c. 627 Ma) in the Shield, post-collisional in origin, display mid-ocean ridge basalt chemical affinity, and represent Ligurian-type oceanic crust developed in an intracontinental pararift zone. The ophiolite tectonics of the Arabian Shield indicates an eastward progression of continental growth through time as the East African Orogen was built during the late Neoproterozoic, following the breakup of Rodinia.

Description: Many of the world's largest ophiolite masses are now being interpreted as remnants of oceanic forearcs stranded on continental margins in the course of arc-continent collision. This interpretation implies a former association between the ophiolite and a volcanic arc at a distance of 100-200 km. The New Caledonia region of the SW Pacific is one of the few areas where there is good evidence for the presence of such an arc. On New Caledonia itself, the Grand Massif du Sud ophiolite has been thrust over the Norfolk Ridge continental fragment, while the coral islands of the Loyalty Group, c. 100 km to the NE, cap large sub-sea edifices with volcanic morphology. New Caledonia can be correlated geologically with New Guinea, despite the considerable width of open ocean that now separates the two. In central New Guinea, where ophiolites emplaced along the northern flank of the main mountain spine lie some 100 km south of exposures of arc-volcanic basement in the north coast ranges, the similarities include the relationship between New Caledonia and the Loyalty Islands. There is, however, no obvious comparable relationship in the case of the largest of the New Guinea ophiolites, the Papuan Ultramafic Belt of the eastern peninsula, which is backed to the north and east by the oceanic Solomon Sea. An associated volcanic arc can be recognized in this area only by assuming a complicated history of collision, post-collision arc-forearc separation and sea-floor spreading, followed by renewed contraction and a very recent and continuing second collision. The case for such a sequence of events can be made on a number of grounds. If the processes responsible are general in their nature, they could explain the apparent absence of arc-volcanic belts in association with many other supposedly forearc ophiolites.

Description: The South Sandwich Islands are one of the world's classic examples of an intraoceanic arc. Formed on recently generated back-arc crust, they represent the earliest stages of formation of arc crust, and are an excellent laboratory for investigating variations in magma chemistry resulting from mantle processes, and generation of silicic magmas in a dominantly basaltic environment. Two volcanoes are examined. Southern Thule in the south of the arc is a complex volcanic edifice with three calderas and compositions that range from mafic to silicic and tholeiitic to calc-alkaline. It is compared to the Candlemas-Vindication edifice in the north of the arc, which is low-K tholeiitic and strongly bimodal from mafic to silicic. Critically, Southern Thule lies along a cross-arc, wide-angle seismic section that reveals the velocity structure of the underlying arc crust. Trace element variations are used to argue that the variations in both mantle depletion and input of a subducted sediment component produced the diverse low-K tholeiite, tholeiite and calc-alkaline series. Primitive, mantle-derived melts fractionally crystallized by c. 36% to produce the most Mg-rich erupted basalts and a high-velocity cumulitic crustal keel. Plagioclase cumulation produced abundant high-Al basalts (especially in the tholeiitic series), and strongly influenced Sr abundances in the magmas. However, examination of volumetric and geochemical arguments indicates that the silicic rocks do not result from fractional crystallization, and are melts of amphibolitic arc crust instead.

Description: New specimens of pterosaurs with soft-part preservation from the Solnhofen Lithographic Limestone (S Germany) and the Crato Formation (northeastern Brazil) yield hitherto unknown and unexpected details of pterosaur anatomy: the presence and internal anatomy of softtissue crests, the internal anatomy of the brachiopatagium, including a blood vessel system and structural details of foot and hand. Some consequences for pterosaurian flight, thermoregulation and aspects of evolution are discussed.

Description: The tectonomagmatic history of ultramafic rocks in the Brezovica massif (Serbia) involved two separate magmatic stages, as inferred from the mineral and bulk-rock chemistry data and thermal history of the peridotites. In the first stage, a suite of spinel harzburgites was formed during partial melting of the mantle and segregation of tholeiitic melts. During the second stage, these spinel harzburgites were repeatedly heated and affected by percolating melt. This process formed dunites and refractory spinel harzburgies during melt-harzburgite interaction. The melt that segregated from these rocks during the second magmatic stage was of high-Ca boninite affinity. Both magmatic stages occurred in a suprasubduction geodynamic setting at a relatively deep level (25-28 km). In its present position the Brezovica massif has been interpreted as a relic of a suprasubduction-type oceanic lithosphere derived from the Central Dinaridic-Mirdita ocean basin. During eastward emplacement of the Brezovica massif over the underlying olistostrome, the ultramafic rocks were cooled to temperatures around 735 {+/-} 20{degrees}C.

Description: Integration of petrographic observations, mineral chemistry, garnet Sm-Nd isochrons, and MnNaCaKFMASH pseudosection phase equilibria models constructed with THERMOCALC provides quantitative metamorphic pressure-temperature-time (P-T-t) paths which allow determination of assemblage/reaction history for pelites. Examples are presented for the Cretaceous to Tertiary magmatic arc of the North American Coast Plutonic Complex. Metamorphism in the western Coast Plutonic Complex of southeastern Alaska and in the North Cascades of Washington resulted from at least three widespread events from 〉 100 Ma to c. 60 Ma, and in both areas partly resulted from crustal thickening, evidenced by local occurrences of kyanite after andalusite. Pressure-temperature pseudosections constructed from bulk rock compositions and the intersection of garnet core composition isopleths provide estimates for the pressure and temperature of garnet core growth. Intersections of these isopleths indicate garnet growth 18 to 85 {degrees}C above the predicted garnet-in reaction temperatures. Rim and near-rim garnet compositions and matrix mineral chemistry provide estimates for near-peak metamorphic conditions. Finite pressure-temperature-time paths of garnet zone metamorphism were determined from the combined core growth and pressure-temperature conditions determined from near-rim garnet and matrix mineral compositions. The western Coast Plutonic Complex near the Stikine River, southeastern Alaska, displays a complex pattern of regional metamorphism overprinted by contact metamorphic aureoles. Many of the c. 90 Ma aureoles contain andalusite, andalusite plus sillimanite, or andalusite plus kyanite with complex replacement textures. A pseudosection constructed for a contact metamorphic rock on Kadin Island (95GL11c), predicts that garnet grew c. 555 {+/-} 10 {degrees}C and 4.8 {+/-} 0.7 kbar, above the garnet-in line and the aluminium silicate triple-point pressure. These results suggest that andalusite in samples from this aureole likely grew prior to garnet and that the pressure may have increased by [≤] 1 kbar during metamorphism. The southern part of the North Cascades in Washington also contain complex aluminium silicate replacement textures with early andalusite and later kyanite and sillimanite. A sample (96NC67), collected near the andalusite-bearing aureole of the Mt Stuart batholith, contains sillimanite and c. 10 mm garnet crystals containing staurolite inclusions in their cores. Temperatures estimated from the garnet core of this sample are within the pseudosection staurolite stability field, compatible with initial garnet growth significantly above the garnetin line. The garnet rim thermometry estimate of c. 668 {+/-} 59 {degrees}C for this sample is c. 85 {degrees}C higher than the core growth temperature. The calculated P-T-t path provides important information for interpreting regional and contact metamorphism. An extensive region NE of the Mt Stuart batholith in the North Cascades underwent a significant pressure increase; however, the timing and nature of medium- to high-pressure metamorphism is controversial. Quantitative P-T-t paths constructed for garnet growth along the NE margin of the batholith indicate that 87-85 Ma garnet growth was younger than the nearby Mt Stuart batholith (93.5 {+/-} 1.4 Ma, U-Pb zircon). Garnet core and rim segments are isochronous indicating a short interval for garnet growth. P-T-t paths indicate that garnet growth occurred in the sillimanite stability field during a maximum pressure increase of 1 to 2 kbar, after rocks passed through the andalusite stability field (Mt Stuart contact metamorphism). Careful sampling, hand-picking, acid leaching, and isotopic analysis of garnet provide geologically consistent ages with uncertainties of [≤] 1.0 Ma. Thermodynamic modelling in the MnNaCaKFMASH system provide reasonable P-T predictions for pelite mineral stability that can be integrated with isotope ages to provide quantitative P-T-t paths. The P-T-t paths developed for both regional and contact metamorphic rocks allow critical evaluation of tectonic models and of interpretations for mineral textures.

Description: Monazite is the mineral of choice in pelitic rocks for providing time constraints on metamorphic rocks and metamorphic processes. However, unlike rock-forming mineral chronometers such as garnet, the petrogenesis of monazite is relatively poorly understood. Consequently, although it is possible to generate precise monazite ages, the significance of the age in metamorphic rocks is often uncertain. In this contribution, we show how the petrogenesis of monazite can be linked to pressure and temperature information. Four complementary approaches, each illustrated by examples, are discussed: (i) the textural relationships of accessory minerals are used to relate the petrogenesis of monazite to that of the rock-forming mineral assemblage, and through this to P-T; (ii) monazite composition, in particular Y content, is used to relate monazite to the rock-forming mineral assemblage, and thus, to P-T; (iii) the bulk compositional control on monazite stability has been empirically determined and this relationship allows the temperature of initial monazite growth to be estimated in a given bulk composition; (iv) monazite-xenotime thermometry is utilized to provide estimates of the temperature of monazite growth. Either individually or combined, these approaches successfully enable monazite age data to be placed in a P-T framework.

Description: Detailed analysis of marine magnetic profiles from the western part of the East Scotia Sea confirms continuous, organized back-arc spreading since at least 15 Ma ago. In the eastern part of the East Scotia Sea, the South Sandwich arc lies on crust that formed at the back-arc spreading centre since 10 Ma ago, so older back-arc crust forms the basement of the present inner forearc. Interpretations of two multichannel seismic reflection profiles reveal the main structural components of the arc at shallow depth, including evidence of trench-normal extension in the mid-forearc, and other features consistent with ongoing subduction erosion. The seismic profile interpretations have been used to constrain simple two-dimensional gravity models. The models were designed to provide constraints on the maximum possible thickness of the arc crust, and it is concluded that this is 20 and 19.2 km on the northern and southern lines, respectively. On the northern line the models indicate that the forearc crust cannot be much thicker than normal oceanic crust. Even with such thin crust, however, the magmatic growth rate implied by the cross-section of the arc crust is within the range recently estimated for two other arcs that have been built over a much longer interval.

Description: In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite {+/-} osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M2 metamorphic episode. Zircon intergrown with or included within M2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 {+/-} 7 Ma (2{sigma}, n = 20). The youngest observed results are found in zircon outside M2 minerals, some overgrown by M3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 {+/-} 9 Ma (2{sigma}, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of rules of thumb' in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

Description: Mantle melting and the production of magmas along the NE Japan arc may be controlled by hot regions in the mantle wedge (hot fingers) that move toward the volcanic front along upward-sloping trajectories. At depths equivalent to 1-2 GPa, where magmas are expected to segregate from mantle diapirs, the hot-finger structures result in a decreasing thermal gradient away from volcanic front. Low-alkali tholeiite is therefore formed by the greater degree of diapiric melting near the volcanic front; high-alumina basalt and alkali olivine basalt are produced by lesser degrees of diapiric melting to the west. The grouping of volcanoes at the volcanic front is interpreted as being controlled by thermal structure in the mantle wedge, and groups are concentrated above the tips of the hot fingers. Map-view variations of minimum 87Sr/86Sr of NE Japan volcanoes are interpreted as resulting from transport of fertile and high-87Sr/86Sr mantle material into the magma source region in the hot fingers. Given that mantle diapirs are formed in the lower part of the mantle wedge, a greater proportion of fertile material will be contained in the diapirs at the tips of the hot fingers, resulting in higher 87Sr/86Sr magmas along the volcanic front. Conveyor-like return flow carries the sheet-like remnants of the fingers to depth along the top of the subducting slab.

Description: In the Sm-Nd and Rb-Sr isotopic geochronology of metamorphic rocks, an important question is whether radiometric systems of mineral isochrons have achieved isotopic equilibrium during a given metamorphic event and preserved the equilibrium afterwards. An analogue to mineral chronometry is O isotope geothermometry. Because the rates of Sm-Nd, Sr and O diffusion in metamorphic minerals are comparable in many cases, the state of O isotope equilibrium between metamorphic minerals can provide a test for the validity of mineral Sm-Nd and Rb-Sr chronometers. In order to illustrate this applicability, O isotope geothermometry was carried out for Sm-Nd and Rb-Sr isochron minerals from ultrahigh-pressure (UHP) eclogites and gneisses at Shuanghe in the Dabie terrane of east-central China. Although the Sm-Nd isochrons give consistent Triassic ages of 213 to 238 Ma for UHP metamorphism, the Rb-Sr isochrons give Jurassic ages of 171 to 174 Ma for the same samples. O isotope geothermometry of the gneiss, eclogite and amphibolite minerals yields two sets of temperatures of 600 to 720 {degrees}C and 420 to 550 {degrees}C, respectively, corresponding to cessation of isotopic exchange by diffusion at about 225 {+/-} 5 Ma during high pressure eclogite-facies recrystallization and at about 175 {+/-} 5 Ma during amphibolite-facies retrogression. The preservation of Triassic Sm-Nd isochron ages, but the occurrence of Jurassic Rb-Sr isochron ages and the regular O isotope temperatures for the same samples, suggest that rates of Sr and O diffusion in such hydroxyl-bearing minerals as biotite and hornblende are faster than rates of Nd diffusion in garnet and Sr diffusion in phengite on the scale of a hand-specimen during the amphibolite-facies retrogression. While the mineral with slow diffusivity has exerted the primary control on the homogenization rate of initial isotope ratios among isochron minerals during retrograde metamorphism, the mineral with high parent/daughter ratio has exerted the principal control on the initiation of the mineral isochron clock in response to retrogression. Valid mineral isochrons can be expected to date the timing of metamorphic resetting only if the mineral with high parent/daughter ratio has a fast rate of radiogenic isotope diffusion during the metamorphic resetting.

Description: Drill core samples of garnet-clinopyroxene granulite at Tirschheim and a reference sample at Waldheim (Saxon Granulite Massif, Germany) endured the same P-T conditions, but developed variable mineral assemblages due to differences in bulk chemistry, reaction progress, deformation and retrogression. Titanite formed during peak-metamorphic conditions of 22-24 kbar and 1020-1050 {degrees}C. Dating titanite from the various samples should yield the same age for all. The observed age variation, which exceeds the duration of the entire metamorphic cycle, originates from the contrasting preservation of isotopic inheritance during peak metamorphism and from post-peak re-equilibration. (1) Pb inheritance observed in some peak-metamorphic titanite demonstrates that geochronologically relevant elements are redistributed among remaining reactants and reaction products during prograde metamorphism and that the sequence of metamorphic reactions does not result in isotopic homogenization. Instead, metamorphic minerals inherit the radiogenic signatures of the precursor minerals and may in extreme cases approach the age of the precursor mineral. (2) Titanite that formed at peak-metamorphic conditions is characterized by high A1 contents and XF {approx} 0.8-1. Texturally comparable titanite that re-equilibrated during cooling (reduced Al contents and XF) yields too young U-Pb ages. The age of such re-equilibrated titanite does not correspond to the age of the event indicated by the texture.

Description: The Sonnblick Dome forms a large antiformal structure within the Pennine of the SE Tauern Window. The structural and metamorphic evolution of this area is relatively well constrained. However, mineral age determinations of the Alpine event within the Zentralgneis granitoid basement remain ambiguous due to a lack of isotopic equilibration during the Alpine event. In this paper we use a newly developed technique, namely Rb-Sr microsampling, in an attempt to place more reliable age constraints upon Alpine deformation. The new age determinations range between c. 22 Ma and 28 Ma. The oldest ages are interpreted to result from shearing related to the earliest stages of dome formation while the youngest ages probably represent late fabric development during uplift of the Zentralgneis complex. A clustering of six fabric ages around 25.5 {+/-} 0.3 Ma is thought to represent the peak of deformation activity associated with formation of the dome. These new data can be fitted into a model for the metamorphic and structural evolution of the SE Tauern Window. In most of the rocks studied isotopic disequilibrium is apparent between and within minerals. It is clear that large feldspar augen did not attain isotopic equilibrium despite amphibolite facies metamorphism. In fact, even where chemical equilibrium was achieved isotopic homogenization did not always occur. The Rb-Sr data from white mica suggest that isotopic equilibration may occur during isochemical rotation of grains suggesting differing mechanisms for chemical and isotopic movement within grains. The data presented herein suggest that small-scale, in-situ sampling methods may provide the most reliable method for deformation chronology in basement gneisses.

Description: By using a set of granitic whole rock samples, originating from the Regensburg Forest (Germany) and dated (approximately 350 Ma old) previously by means of thermal ionization mass spectrometry (TIMS), the capability of dynamic reaction cell (DRC) inductively coupled plasma mass spectrometry (ICPMS) for Rb-Sr age determination was demonstrated. With DRC-ICPMS, chemical separation of Sr from Rb during the sample pretreatment is no longer required as interference-free determination of the 87Sr/86Sr isotope ratio can be accomplished by monitoring the signals of the SrF+ adduct ions, formed as a result of the selective reaction between the Sr+ ions extracted from the ICP and the reaction gas CH3F. The mass discrimination was established to depend strongly on the matrix composition. This drawback could be overcome by using the United States Geological Survey reference material G-2 as an external standard. Results obtained by DRC-ICPMS (age and initial 87Sr/86Sr ratio) showed an excellent agreement with both (a) experimental values obtained by means of quadrupole-based and sector field ICPMS after isolation of Sr via cation exchange chromatography and (b) TIMS literature values. In addition, DRC-ICPMS offers a smaller combined uncertainty on the isotope ratio results as a result of (a) an improved internal isotope ratio precision (〈0.1% RSD when also using Ne as an additional non-reactive collision gas) and (b) the fact that, in contrast to quadrupole-based and sector field ICPMS, no correction for the remaining overlap between the signals of 87Sr+ and 87Rb+ is required.

Description: Quantitative constraints on the rates at which metamorphic reactions proceed in nature are now available from several sources. Most common are predictions made on the basis of laboratory kinetic data. However, the applicability of such laboratory-based predictions has long been questioned and many observations in the field now suggest much slower rates. Here, published quantitative field-based constraints on high temperature (〉400 {degrees}C) reaction rates are assembled from a variety of sources. Reaction rates attending regional metamorphism are four to seven orders of magnitude slower than most laboratory-based predictions. A general rate law for regional metamorphism has been derived which best describes these field-based data:[IMG]f1.gif" ALT="Formula" BORDER="0"〉 where Rnet is the net reaction rate (g/cm2/a) and T is temperature ({degrees}C). At the same time, natural reaction rates attending contact metamorphism differ from laboratory-based predictions by less than two orders of magnitude, and are in close agreement at higher temperatures. Thus, while existing laboratory-based kinetic data may be judiciously applied to some contact metamorphic systems, laboratory-based kinetic predictions clearly misrepresent regional metamorphism. To explain this kinetic discrepancy, regional metamorphic reaction rates may be limited by slow intergranular transport due to comparatively limited (or transient) availability of aqueous fluid in the intergranular medium. The general field-based rate law may be applied to regional metamorphic, and other environments (i.e. ultrahigh pressure or ultrahigh temperature metamorphism), if similar system characteristics (mainly, low aqueous fluid content) can be inferred.

Description: Fragments of the Ordovician sea floor preserved in the Solund-Stavfjord Ophiolite Complex in Western Norway serve as proxies for the {delta}18O of Ordovician seawater. The pillow basalt sections at Oldra and Strand are both enriched in 18O, recording their alteration by seawater at low temperature on the sea floor. In contrast, the sheeted dykes and gabbros generally are depleted of 18O, reflecting the modal proportion of secondary, low-18O chlorite and epidote formed from seawater at high temperature. These isotopic contrasts simply reflect the high water to rock ratio of sea-floor alteration and the temperature dependence of the 18O partitioning between minerals and water. Superposition of high-{delta}18O pillows on low-{delta}18O dykes and gabbros is a necessary consequence of alteration at both low and high temperatures by a fluid near 0{per thousand} and is easily recognized in well-preserved ophiolites. Also, the {delta}18O of seawater can be independently calculated from 18O fractionations among secondary minerals. Older, dismembered and highly metamorphosed segments of the oceanic crust may still retain the original seawater imprint because their subsequent obduction and metamorphism was relatively closed to external fluids. Suites of diamond-bearing nodules from kimberlites still have contrasting high- and low-{delta}18O eclogites, proving that even subduction into the mantle is not sufficient to erase the seawater fingerprint. Inspection of the sea-floor, ophiolite and eclogite data reveals no secular trend in {delta}18O, indicating that the {delta}18O of seawater has not changed with geological age. Because the {delta}18O of seawater itself is fixed by sea-floor-seawater exchange, the constancy of {delta}18O of seawater implies that the scale and style of sea-floor-seawater interactions has not changed over time.

Description: A belt of disrupted ophiolitic rocks occurs on the Boso Peninsula (Japan), currently located north of the oblique subduction boundary between the Philippine Sea and North American Plates, under which the Pacific Plate has been subducting westwards. This ophiolitic belt (Mineoka Belt) is composed of mafic-ultramafic rocks together with Tertiary chert and limestone and island-arc volcaniclastic rocks. Our detailed structural studies in and around the basaltic rock bodies within the ophiolite reveal three phases of deformation. The first phase is further divided into three stages, all related to oblique normal faulting associated with extensional tectonics at or near a spreading axis. Fluid pressures appear to have fluctuated in association with faulting and veining during this phase. The second phase of deformation is characterized by thrust-related shear zones with a significant strike-slip component and is probably related to the final emplacement of the ophiolite by oblique subduction-obduction processes. The third and final phase of deformation affected not only the ophiolite but also later terrigenous and island-arc pyroclastic rocks. This deformation involved large-scale transpressional dextral slip on forearc sliver faults, which are still active today.

Description: In this paper we summarize the present knowledge on bioalteration of basaltic glass from pillow lava rims of former oceanic crust, based on the study of four ophiolite complexes. These complexes range in age from Late Cretaceous to Mid-Proterozoic, and their metamorphic grades vary from non-metamorphosed to low greenschist- to low amphibolite-facies metamorphism. In the non-metamorphosed volcanic part of ophiolite complexes, in which undevitrified glass is still present in pillow lava rims and/or hyaloclastites, biogenerated textures are common. These textures (termed granular and tubular) are similar to those found in volcanic glass of recent to old (170 Ma) in situ ocean floor, and mimic microbes in terms of size and shape. In the metamorphic and completely recrystallized examples, the textural evidence of bioalteration is generally obliterated, although it may still be visible in little-deformed volcanic domains of low-grade greenschist-facies metamorphism. Where biogenerated textures are present, element mapping invariably reveals the presence of organic carbon, and sometimes nitrogen, sulphur and phosphorus. Carbon-isotope signatures ({delta}13C) in the pillow lava rims from all four investigated ophiolites show lower values than those of the adjacent crystalline parts. This phenomenon may be attributed to bio-induced fractionation of carbon isotopes during preferential bioalteration of the pillow lava glass, and may further represent a feature that seems to survive metamorphism and deformation.

Description: Recent advances in microscale 40Ar/39Ar geochronology have revealed argon concentration gradients in naturally deformed muscovite that are incompatible with volume diffusion uniquely, and have been interpreted to result from intragranular defect-enhanced diffusion. Defects and heterogeneously spaced stacking faults observed by transmission electron microscopy in such muscovites are evaluated as potential fast pathways for argon diffusion. Two-dimensional defects, such as stacking faults, are of particular interest for noble gas diffusion because of the net dilatation effect that a stacking fault is able to generate in minerals. In micas, partial dislocations (and the area between them known as stacking faults) within the interlayer displace the potassium atoms from a stable hexagonally centred position between opposing tetrahedral layers to an unstable position relative to one of the tetrahedral layers such that repulsive forces lead to a localized net dilatation effect within the interlayer. Such a dilatation effect may have direct consequences for argon retention in micas. Numerical modelling of the effects stacking faults have on argon diffusion was performed on the basis of the calculated interlayer spacing, measured isotope data, and observed linear stacking fault density. These calculations result in effective diffusivity ratios defined by volume diffusion to defect-enhanced diffusion of 106 to 107, which are comparable with diffusivity ratios in other materials (ceramics or metals). In the absence of defects causing physical grain size reduction (e.g. kink bands or subgrain boundaries), stacking faults are potentially the main defect in sheet silicates exerting a measurable influence on intragranular argon diffusion. Stacking-fault-enhanced argon diffusion differs from pipe diffusion, whose significance on bulk diffusion depends on high dislocation densities, by the small volume fraction of dislocations required to affect bulk diffusivities. In contrast to pipe diffusion, the linked occurrence of dislocations and stacking faults within mica interlayers represents a potentially significant volume fraction, even in samples that do not have high apparent dislocation densities.

Description: Sm-Nd garnet geochronology is often hampered by the presence of submicroscopic inclusions of rare earth element-rich phosphates, which lower age precision, lead to inaccurate ages or make dating impossible. We propose a single-step sulphuric acid leaching technique as a very efficient tool in eliminating phosphate inclusions, which helps to achieve more precise and more accurate Sm-Nd garnet dates. Examples from silimanite grade metapelites demonstrate the much higher efficiency of this method in comparison with previously proposed techniques based on HF and HCl. 147Sm/144Nd ratios obtained on garnets leached by sulphuric acid were twice as high as those obtained by HF and HCl leaching. This led to age precision better than 3% for Tertiary samples. Comparison of leached and unleached nearly inclusion-free garnets from high pressure granulites, demonstrates that there is no Sm/Nd fractionation induced by H2SO4 leaching. Our new technique eliminates phosphates, but does not attack garnet. This considerably reduces the necessity for hand-picking and lowers the amount of sample required for analysis making Sm-Nd garnet dating a more easily applied geochronometer.

Description: One of the key aims of geochronology, and the subject of the papers in this Special Publication, is the linkage of isotopic ages to petrological and textural information. A close link between the two types of information greatly improves the constraints available from geochronology on the nature and rates of lithospheric processes such as metamorphism and deformation. There have been several key advances in this area over the past 10-20 years, relating to increased precision and accuracy of isotopic ages but also, and crucially, to the spatial resolution available to geochronologists. This resolution now approaches that on which petrological, chemical and textural information is obtained. We also, in this introduction, identify the barriers that have impeded further progress, which relate both to technical issues as well as to problems of understanding. Finally we set the papers in this volume in the context of the preceding discussion and outline the key ways in which these papers point towards further progress in the future.

Description: Isotopic profiles through ophiolite complexes provide the necessary link between the study of global geochemical cycles and plate tectonics. The hydrothermal circulation that occurs beneath the sea floor is the primary mechanism for exchange between the mantle of the Earth and the hydrosphere. The subduction of the hydrothermally altered crust and overlying sediments is the primary mechanism for crustal recycling. Oxygen and strontium isotopes of seawater track the competition between continental weathering and mid-ocean ridge hydrothermal exchange to control the composition of the oceans. Information derived from ophiolite studies on the elemental fluxes and the depth of seawater penetration into the oceanic crust provides constraints on the important rate constants associated with these competing processes. The same tectonic rates that account for the Sr isotope evolution of seawater indicate that the oxygen isotopic composition of the ocean is constrained to vary within narrow limits (per mil level). Isotopic analysis of dredge samples and ophiolite complexes demonstrates that seawater-ocean crust interactions result in a oxygen isotopic zonation of the oceanic crust with masses (concentration times volume) centred on the initial isotopic composition of the crust. This requires that the oxygen isotopic composition of the ocean resides at near steady-state conditions over Earth history. The inferences from ophiolite complexes contrast strongly with the results of measurements on carbonates from epicontinental seaways, particularly for the Palaeozoic. Ophiolites and greenstone belts track exchange processes between the ocean and the igneous crust whereas most carbonate measurements track the surface ocean on continental shelves. For oxygen isotopes, the mass of epicontinental seaways and the rates of meteoric water input suggest a resolution to the controversy that accounts for both data sets.

Description: Rare earth element (REE) analysis of zircon coupled with high spatial resolution U-Pb geochronology and imaging is emerging as a useful petrogenetic tool capable of providing a link between dated zircon growth and contemporaneous magmatic conditions or metamorphic reactions. Notable amongst recent studies has been the link established between zircon growth in equilibrium with garnet and characteristic heavy REE depletion in zircon which has enabled the dating of specific pressure-temperature (P-T) evolution. Links to other REE-bearing minerals have yet to be established but have similar potential for dating P-T pathways. A brief review of recent applications in both igneous and metamorphic zircons is presented here to illustrate the range of investigations in this rapidly developing area. The method is then applied to two case studies from the late-Archaean Lewisian Composite Terrane of the Outer Hebrides, NW Scotland. In both cases, these new zircon REE analyses reveal previously unknown events in the early history of these samples, as well as clarifying the relationship of different zircon growth phases in relation to the geochronology.

Description: We present mineral chemistry data and petrological evidence from the Yarlung Zangbo suture zone ophiolites (Southern Tibet) suggesting that they represent a collage of heterogeneous massifs. Mantle sections in these ophiolites consist of harzburgite and lherzolite cut by several generations of gabbroic to diabasic intrusions, all affected by high-temperature deformation. Pyroxenitic bands are parallel to the mantle foliation. Crustal plutonic sections, consisting of dunite, wehrlite and gabbro, are thin or absent and have been observed only in the Dazhuqu massif. Plagioclase is an additional phase associated with crustal peridotites. The mineral chemistry of silicate minerals and spinel in the mantle and crustal rocks varies widely and is believed to reflect complex melt percolation and reaction. The massifs record polybaric exhumation steps from at least 50 km depth to the plagioclase stability field. Pyroxene has re-equilibrated compositions from 1200 {degrees}C down to medium-grade metamorphic conditions. The mantle peridotites are interpreted as the residues of 10-40% partial melting of a fertile lherzolitic source. High Cr number, low TiO2 content and relatively high Fe3+ number of spinels suggest that the ophiolitic massifs were generated in a suprasubduction zone (arc or back-arc) environment.

Description: The Neyriz ophiolite in SW Iran includes a NNW-SSE-trending, steeply dipping oceanic palaeo-transform fault zone that consists mainly of deformed and sheared gabbros and peridotites. Mylonitic rocks within the fossil palaeo-transform fault display C-, C'- and S-band shear cleavages containing green hornblende- and feldspar-rich bands. Deformed hornblendemantled porphyroclasts have symmetrical tails or {sigma}-type porphyroclast systems, which show clockwise stair-stepping rotation and oblique foliation with asymmetrical complex stripes and wings. These microstructures indicate dextral shearing on the palaeo-transform fault. The c-axes of hornblende in high-grade S-C mylonitic rocks exhibit strong lattice-preferred orientation (LPO) patterns with M- and G-type origin. These LPO patterns are asymmetrical with respect to the shear zone foliation and also indicate dextral shearing. The NNW-SSE trend of the palaeo-transform fault is perpendicular to the general NE strike of sheeted dykes and is parallel to the average harzburgite foliation. These observations suggest a noncoaxial shear orientation of mantle flow, which progressively rotated toward parallelism with the fossil fracture zone. This dextral transform fault is inferred to have connected ENE-trending oceanic spreading centre segments in a Neo-Tethyan ocean basin.

Description: The Middle Unit of the central-northern Argolis Peninsula, in NE Peloponnesus (Greece), is composed of several tectonic slices, locally including intact sequences of mafic volcanic rocks topped by radiolarian cherts. Although some of these sequences are Jurassic in age, many of them display a Triassic age based on biostratigraphical evidence. The petrological studies presented in this paper indicate that the Triassic volcanic rocks were generated in a mid-ocean ridge setting, and that they represent the oldest remnants of the Pindos oceanic crust so far recognized in the Subpelagonian zone. On the basis of immobile trace element analyses, two chemically distinct groups of Triassic lavas can be recognized in the various volcanic sequences. One group is represented by transitional-type mid-ocean ridge basalts (T-MORBs) displaying moderate light rare earth element (LREE) enrichment, and incompatible element abundances very similar to those observed in present-day T-MORBs. The other group exhibits a range of characteristics typical of many normal-type MORBs: that is, variable LREE depletion and flat N-MORB normalized patterns of incompatible element abundance. Moreover, many geochemical characteristics indicate that the various N-MORB type volcanic sequences originated from chemically distinct (heterogeneous) sub-oceanic mantle sources. Analogous to similar basalts from ophiolitic melanges of the Dinaride-Hellenide belt, the T-MORBs from the Argolis Middle Unit are interpreted as having originated from a primitive mantle source variably enriched by an ocean-island basalt (OIB)-type component. In contrast, the contemporaneous occurrence of N-MORBs implies that, during the Mid-Late Triassic, oceanic spreading of the Pindos basin had already reached, at least in some sectors, a quasi-steady state involving only sub-oceanic mantle sources and their partial melt derivatives. Our model for the Triassic opening of the Pindos oceanic basin and its related tectonomagmatic evolution is largely supported by comparison with the Red Sea embryonic ocean, a modern analogous setting.

Description: Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models.

Description: The gabbroic crust of the Ordovician Bay of Islands ophiolite complex formed in an island-arc setting near the North American continental margin. Detailed structural studies on the North Arm Mountain massif provide us with a scheme of syn-oceanic deformation events recorded in the crust. During a first transtensional stage, which generated gabbroic rocks, sheeted dykes and lavas, the temperature of formation of amphiboles in the gabbroic unit fell with time in three steps from 880-745 {degrees}C, to 790-680 {degrees}C and to 550-500 {degrees}C. The Ti, Na and AlIV contents of amphiboles decreased, whereas the Si activity of the fluid increased with time. The first amphibole to form has typical mid-ocean ridge basalt {delta}18OVSMOW indicating equilibration with a magmatic fluid or evolved seawater at low fluid/rock ratio. Lower {delta}18O values for some amphiboles (0-2.5{per thousand}) indicate the circulation of large volumes of seawater. The lowest {delta}18O values are found in the inner part of the shear zones, which channelled deep infiltration of seawater into the gabbroic unit. During brittle deformation, infiltration of low-temperature seawater produced prehnite, carbonate and quartz veins, and plagioclase with high {delta}18O. This study documents that the hydration of ophiolitic crust in the Bay of Islands ophiolitic complex occurred mainly along pre-obduction oceanic structures in an intraoceanic setting.

Description: The rate of melt segregation in regional migmatite terrains can be estimated by various lines of research. Firstly, the segregation rate of a melt batch, with a volume below the melt percolation threshold, cannot exceed the melt production rate and is therefore limited by the heating rate derived from geothermometry and geochronology (method A). Other estimates come from physical models for melt percolation (method B) and from the degree of (dis)equilibrium reached between melt and source rocks (method C). The first method is restricted by the current time resolution of isotopic techniques. Results from the second and third approaches depend heavily on assumed values of melt viscosity and other parameters (B); on the correct recognition of (dis)equilibrium trace element distributions (C); and on the migmatization model used (B and C). The validity of method C is undermined by the mathematical equivalence of trace element models for five different scenarios: (1) disequilibrium melting (with or without melt escape) followed by in situ crystallization of non-segregated melt; (2) equilibrium melting, followed by equilibrium crystallization and major melt escape; (3) disequilibrium melting, followed by equilibrium crystallization and minor melt escape; (4) pervasive retrograde re-equilibration; and (5) subsolidus differentiation. Hence, trace element data in support of model 1 (implying fast melt segregation rates) are equally consistent with models 2 to 4 (implying slow melt segregation rates), and even with melt-absent model 5. The level of trace element saturation reached during accessory phase dissolution into melt may not provide better answers, as it is largely controlled by textural constraints, e.g. shielding of accessories by porphyroblasts. We conclude that the way forward is to directly couple microtextural and microgeochemical information with time constraints. This requires high-resolution (space and time) geochronology, possibly with more advanced methods than presently available.

Description: Rates of melt production and segregation in migmatites can be assessed using geochronology of accessory phases. Here we report on the distribution and growth patterns of accessory phases and their coupling with major phase microtextures and chemical zoning patterns in garnet. We propose that migmatization in SW Finland involved partial melting and subsequent moderate retrograde re-equilibration. The latter process has three major effects: (i) it partially obscures geochemical signatures formed during earlier equilibrium or disequilibrium melting; (ii) it excludes the possibility of very fast (〈 100 years per batch) melt segregation rates in migmatites where restite-melt back reaction has operated; (iii) trace element distributions between leucosome and host rock cannot be used to infer melt segregation rates. Garnet MnO patterns show flat cores, suggesting high-grade equilibration, and sharply increasing concentrations at the rims, attributed to garnet-melt back reaction. Trace element patterns (Zr, Y and heavy rare earth elements) also document retrogression at the rims, but in addition preserve earlier growth zoning related to incongruent melting. We report retrograde zircon and xenotime growth associated with garnet resorption, here related to restite-melt back reaction in leucosome, melanosome and mesosome. Hence, geochronological studies of migmatite terranes should take into account that the youngest zircon material is not restricted to leucosome, but can also form overgrowths on corroded grains in melanosome and mesosome.

Description: Kimberlites are extraordinary natural phenomena, ascending through the Earth's lithosphere, entraining xenoliths, to erupt at the surface within hours to days of their inception deep within the lithospheric mantle. With the realization that some Ar/Ar phlogopite grain core ages may be indicative of geological events, we have undertaken high spatial resolution Ar/Ar dating of phlogopites in xenoliths and megacrysts from Kimberley, Monastery and Letseng in southern Africa, and Malaita, in the Solomon Islands, to est whether other mantle phlogopite cores may yield meaningful ages. Modelling of Ar diffusive loss profiles from phlogopite grain boundaries to cores provides information on both the eruption age and the duration of outgassing within the kimberlite magma, and hence yields estimates on diatreme ascent rates. The ascent durations are very similar for all of the southern African pipes studied, yielding durations of 0.9-6.9 days, assuming an average kimberlite magma temperature of 1000 {degrees}C. These can be compared to estimates from phlogopite xenoliths from Siberian diamond-bearing kimberlites yielding ascent durations of 2-15 hours (assuming the same magma temperature).

Description: The extraordinarily well-preserved and well-exposed Semail ophiolite of northern Oman hosts several large plagiogranite intrusions in proximity to economic copper sulphide deposits of the Lasail mining district. A progression of isotopic, chemical and mineralogical transformations observed within the plagiogranites and high-level gabbros (HLG), and a comparison of these effects with those in the lowermost dykes of the immediately overlying sheeted dyke complex (SDC) tracks the evolution of hydrothermal fluids and the alteration of overlying dykes and pillow lavas during discharge of these fluids on the sea floor. The largest hydrothermal alteration aureoles, and the greatest extent of metamorphic veins and metasomatic replacement features, are found adjacent to the largest high-level plagiogranite bodies, beneath and adjacent to the major ore bodies in northern Oman. The ubiquitous presence of metamorphic actinolitic hornblende, sodic plagioclase, epidote and titanite in metabasalts within the high-temperature alteration zones points to the most likely mineralogical and structural controls on the development and evolution of the hydrothermal fluids. Depleted Cu contents of the adjacent crustal rocks and Cu enrichments above the plagiogranite intrusions demonstrate the redistribution of heavy metals adjacent to the complexes. Field relationships implicate the formation of both the epidosites and plagiogranites in the genesis of the ore deposits. An important process inferred from the field and geochemical data is the assimilation of previously hydrothermally altered basaltic and gabbroic country rocks by stoping into the magma chambers developed near the SDC-gabbro horizon in the ophiolite. We suggest that this process of combined assimilation-fractional crystallization, together with replenishment and recharge by injection and quenching of basaltic magma pillows' into these plagiogranite magma chambers (i.e. RAFC), plays a major role in the development of these composite intrusions.

Description: Samail ophiolite emplacement has become a type-example for ophiolite obduction. Despite this, problems and controversy remain with respect to the age of high-P metamorphism, the vergence of structures in deformed rocks beneath the ophiolite, the suprasubduction character of the ophiolite and the P-T conditions of the metamorphic sole. The presence of major, regional-scale NE-facing isoclinal folds and SW- and west-dipping shear zones with top-to-the-NE shear sense in Arabian margin rocks beneath the Samail Ophiolite nappe requires that the margin was not simply passively overridden during obduction of the ophiolite. The development of these folds at 72-76 Ma is at the time the ophiolite is emplaced finally onto the margin (80-70 Ma), accompanied by development of a major shear zone in Saih Hatat (the upper plate-lower plate discontinuity described by earlier workers) at c. 82-80 Ma. Structural scenarios that incorporate these folds and shear zones include lateral escape from a rising buoyant crustal slice along the former subduction interface, back-folding ( retrocharriage') associated with major oceanwards-directed underthrusting, or simple underthrusting of the margin by the oceanic realm. Previous models involving craton-directed overthrusting with domal culminations related to deep-seated footwall and lateral ramps are more applicable to the Tertiary structure and Tertiary evolution of the mountain range. Oman ophiolite obduction clearly involves ocean-vergent thrusting within the continental margin platform to slope facies sequences.

Description: Two end-member models have been proposed to account for the structure and metamorphism of rocks beneath the Semail ophiolite in the Oman mountains. Model A involves a single, continuous NE-directed subduction away from the continental margin during the late Cretaceous. The ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km SW onto the continental margin. Subduction of Triassic-Jurassic oceanic basalts to c. 10 kbar (c. 39 km depth) led to the accretion of amphibolite-facies rocks to the base of the ophiolite. Thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c. 20 kbar (c. 78 km depth), choking the subduction zone. Buoyancy forces caused the rapid exhumation of eclogites, blueschists and carpholite-grade HP rocks along the NE margin of the continental plate. During the later phase of foreland-propagating thin-skinned thrusting in the SW, NE-facing backfolding and backthrusting occurred in the hinterland, with the final exhumation of the HP rocks. Model B follows recent suggestions that a nascent SW-dipping subduction zone, dipping beneath the continental margin, existed between 130 and 95 Ma, prior to formation and emplacement of the ophiolite. A major NE-facing fold-nappe structure in the pre-Permian basement rocks of Saih Hatat is interpreted as reflecting subduction beneath the margin. Two high-pressure metamorphic events have been suggested, the first predating ophiolite emplacement, the second caused by ophiolite loading. This model is untenable, being based on a misinterpretation of the NE-facing structures in northern Saih Hatat, and on some dubious older 40Ar/39Ar cooling ages from the eclogite-facies rocks of As Sifah. We conclude that all structures in northern Oman and all the reliable geochronology point to a single emplacement-obduction event lasting from Cenomanian-Turonian time (c. 95 Ma) when amphibolites were accreted along the metamorphic sole of the ophiolite, to Campanian time, when the continental margin was subducted to the NE producing blueschists and eclogites, to the final thin-skinned emplacement of all thrust sheets, which ended before the Late Maastrichtian, at c. 68 Ma.

Description: The Mineoka ophiolite in the southern Boso Peninsula is situated in a unique tectonic setting in the collisional zone between the Izu and Honshu arcs in Japan. The ophiolitic rocks are composed mainly of tholeiitic pillow basalts and dolerites, alkali-basaltic sheet flows, and calc-alkaline dioritic to gabbroic rocks. The tholeiitic basalts show variable trace element compositions ranging from mid-ocean ridge basalt to island-arc basalt, whereas the alkali-basalts have a within-plate affinity. High-Fe and -Ti tholeiitic basalt and within-plate alkali-basalt have Ar/Ar ages of 49 {+/-} 13 Ma and 19.62 {+/-} 0.90 Ma, respectively. Three plutonic rocks have K-Ar ages of c. 25, 35 and 40 Ma. These ages are inconsistent with the known ages from the Pacific or Philippine Sea Plate. We infer that the Mineoka ophiolitic assemblage was part of another Tertiary oceanic plate, the Mineoka Plate', which underwent island-arc volcanism in the Miocene as a result of subduction initiation at a fracture zone or a transform fault system due to a change in the position of the Euler rotation pole of the Pacific Plate at c. 43 Ma. Eruption of within-plate type alkali basalts on the Mineoka Plate took place near the palaeo-Japan continental arc just before the emplacement of the Mineoka ophiolite into the Japanese continental margin.

Description: Ophiolites of the southern Uralides arc-continent collisional orogen include fertile mantle lherzolites and minor harzburgites in the Nurali and Mindyak massifs located along the Main Uralian Fault suture of the East European craton margin and the Magnitogorsk island arc. We present the first in situ analyses of pyroxene from Nurali spinel {+/-} plagioclase-bearing lherzolites and harzburgites and Mindyak spinel lherzolites and harzburgites. Based on the trace element signatures of pyroxene, the Nurali peridotites are divided into: Group 1, consisting of plagioclase-bearing spinel lherzolites with moderately to extremely light rare earth element (LREE)-depleted clinopyroxenes, consistent with [〈=]8% fractional melting followed by impregnation by incremental to mid-ocean ridge basalt (MORB)-like melts; Group 2, formed by a spinel peridotite with strongly LREE- to middle REE (MREE)-depleted to enriched clinopyroxenes that testify to re-equilibration with large volumes of melt of tholeiitic affinity; Group 3, consisting of amphibole-bearing spinel harzburgites that underwent pervasive percolation of alkali-enriched melts or fluids. Clinopyroxenes from the Mindyak peridotites are strongly depleted and re-equilibrated by reactive porous flow of infiltrating tholeiitic melts. Two alternative petrogenetic models are proposed. In Model 1 the peridotites derive from oceanic lithosphere generated by mid-ocean ridge processes and affected by refertilization via melt percolation. In Model 2 the peridotites were subcontinental lithospheric mantle that experienced deep-seated magmatism and sub-solidus re-equilibration prior to the opening of the Uralian Ocean, and interacted with melts generated in the asthenospheric mantle by extension-related decompression partial melting during the opening of the Uralian Ocean. In both models the final pre-orogenic events are related to the subduction of the Uralian oceanic lithosphere and to mantle wedge processes, notably intrusion of gabbro-diorite at c. 400 Ma into the Moho sections.