ALBERT

Description: A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed “restingolites” (after the close-by village of La Restinga), appeared as black volcanic “bombs” that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these “floating stones” has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The “restingolites” have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte),(iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative “restingolites” and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that “restingolites” are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resem- bling pumice in appearance, but are xenolithic in origin, we refer to these rocks as “xeno-pumice”. The El Hierro xeno- pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the Canary Islands as well as in similar Atlantic islands that rest on sediment-covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of “restingolites” indicates that crustal recycling is a relevant process in ocean islands, too, but does not herald the arrival of potentially explosive high-silica magma in the active plumbing system beneath El Hierro. results of our textural, mineralogical, elemental and isotopic analysis lead us to conclude that the early floating stones of El Hierro are vesiculated crustal xenoliths that originate from the substantial layer of sub-volcanic pre-island sedimentary rocks (layer 1 of the oceanic crust) that is present underneath the Canary archipelago.

Description: The Swedish and the German Science Foundations (VR and DFG), the ERC grant EVOKES and the Center for Natural Disaster Science (CNDS) Sweden

Description: Published

Description: 97-110

Description: N/A or not JCR

Description: open

Description: The Indonesian island of Sumatra, located in one of the most active zones of the Pacific Ring of Fire, is characterized by a chain of subduction-zone volcanoes which extend the entire length of the island. As a group of volcanic geochemists, we embarked upon a five-week sampling expedition to these exotic, remote, and in part explosive volcanoes (SAGE 2010; Sumatran Arc Geochemical Expedition). We set out to collect rock and gas samples from 17 volcanic centres from the Sumatran segment of the Sunda arc system, with the aim of obtaining a regionally significant sample set that will allow quantification of the respective roles of mantle versus crustal sources to magma genesis along the strike of the arc. Here we document our geological journey through Sumatra’s unpredictable terrain, including the many challenges faced when working on active volcanoes in pristine tropical climes.

Description: Swedish Science Foundation (VR), Scripps Institution of Oceanography, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Upp-sala University Centre for Natural Disaster Science (CNDS) and Otterborgska donationsfonden

Description: Published

Description: 64-70

Description: 2.3. TTC - Laboratori di chimica e fisica delle rocce

Description: N/A or not JCR

Description: restricted

Description: :  Petrophysical and geochemical properties of dolomites may exhibit a nested set of lateral patterns, typically at the scales of a few meters to ~ 20 m. This study assesses whether those lateral patterns are inherited from the limestone precursor or formed during dolomitization. Sampling was done across a preserved limestone-to-dolomite reaction front in grainstones of the Miocene Seroe Domi Formation, Bonaire, Netherlands Antilles. Post-dolomitization diagenetic overprinting in the two rock types is absent or minimal. Two ~ 65 m lateral transects were drilled at 30-cm spacing in two separate beds. Porosity, permeability, and geochemical ( 18 O, 13 C, Mn, Sr, and Na) analyses were performed on all 287 recovered samples, and 287 thin sections were point-counted for petrographic attributes. Variography shows that dolomite abundance and porosity contain three scales of lateral variability. Much of the signal is random sample-to-sample noise as ~ 65% of the total spatial variance occurs at 30-cm spacing. However, porosity and dolomite abundance also exhibit a short-scale correlation length of 3.7 m and a long-range oscillatory pattern (hole effect) at ~ 16 m, both of which are also periodic. Trace elements (Mn, Sr) and permeability exhibit short-range correlation to 5.8 m, but no long-range pattern. Mole %Mg and 13 C show increasing variance with distance, and all other dolomite attributes exhibit no spatial patterns. No limestone attributes show evidence for any short-range or long-range oscillatory patterns. This indicates that the lateral patterns in dolomite abundance, porosity, permeability, Sr, and Mn contents were not inherited from the limestone precursor. Dolomitization is interpreted to be the cause of the observed spatial patterns as that is the only geologic process that has affected the dolomite but not the limestone on the other side of the front. Geochemical self-organization during the dolomitization process is suggested to be the mechanism by which the spatial patterns in the dolomites emerge. A model for that self-organizing process is proposed that involves positive feedbacks between precipitation, dissolution, textural properties, reaction rates, carbon transport, and the evolving flow field. Relative to Mississippian, Permian, and Eocene dolomites that have been studied previously, the Seroe Domi dolomites have similar porosity variance at 30 cm spacing and hole effects with slightly greater wavelengths and much larger magnitudes. Any differences are interpreted to be related to diagenetic history. The older dolomites have been affected by burial diagenesis and/or weathering, which would likely increase sample-to-sample variability, therefore increasing short-scale randomness, decreasing the magnitude of hole effects, and shortening wavelengths of hole effects. The lateral patterning formed by dolomitization may thus not be as strongly expressed in ancient dolomites that have complex burial histories.

Description: : Syndepositional fractures are common in Upper Permian strata of Dark Canyon, Guadalupe Mountains, New Mexico, USA, and are typically oriented parallel to the platform-margin trend with subvertical traces. Fracture apertures range from millimeter- to meter-scale with sediment, microbialite, breccia, and cement infill. Syndepositional normal faults are less frequent, and commonly are filled with fault-breccia and flanked by damage zones. Syndepositional fault offsets range from tens of centimeters up to 18 meters. In some instances, fault offsets were large enough to alter sedimentation patterns and stratal architecture of the Tansill shelf in the form of growth monoclines or small-scale grabens. Evidence for flow of dolomitizing fluids along syndepositional deformation features consists of narrow (centimeter- to meter-scale) dolomite halos along the fractures and faults, and dolomite bodies that extend laterally for several meters to tens of meters away from faults and fractures. Halos are interpreted to have formed where deformation features crosscut low-permeability strata. Dolomite bodies formed where deformation features intersected more permeable strata and dolomitizing fluids flowed laterally into the surrounding host rock. Isotopic data from individual fracture-controlled dolomite bodies indicate temporal variability in the dolomitizing fluids, suggesting that those fluids moved through different syndepositional fractures systems at different times.

Description: : Syndepositional faults and fractures are known to affect early fluid flow in carbonate platforms. Less clear is whether they are active fluid conduits throughout the entire history of the platform strata. Syndeformational fractures in Permian (late Guadalupian) carbonates exposed in Dark Canyon, Guadalupe Mountains, New Mexico, U.S.A., address this question. Transmitted-light and cathodoluminescent petrography, stable-isotope and fluid-inclusion analyses, and clumped-isotope thermometry show that there were multiple episodes of fracturing, dissolution, cementation, and replacement in the fractures. Dolomite cement or dolomitized marine cements line the walls of some fractures and indicate the syndepositional reflux of evaporated Permian seawaters through the fractures. Fine- to medium-crystalline, luminescently zoned calcite may overlie the dolomite and marine cements, line fracture walls where those phases are absent, or cement karst breccia on fracture walls. The 18 O values of this calcite (–8.8 to –14.0 VPDB) and clumped-isotope temperatures (16° to 32°C) indicate precipitation from meteoric fluids ( 18 O SMOW of –6.2 to –10.5) associated with episodic sea-level lowstands during the development of high-frequency depositional sequences. The early calcites can themselves be fractured, rotated, and recemented, indicating recurrent deformation and meteoric influx. Evaporite cements were once the dominant pore-filling phase in the fractures, forming both before and after the early meteoric cements. The earliest evaporites formed during deposition of Tansill limestone, probably from the same brines that formed dolomites. Evaporites that postdate the early calcite probably did not form until Permo-Triassic burial, when geomechanical analysis indicates that the syndepositional fractures were likely reactivated and brines could have been sourced from overlying bedded evaporite. All evaporite cements subsequently were calcitized, mainly by coarse-crystalline, inclusion-rich calcites that formed from warm (59° to 96°C) fluids. Calculated fluid isotopic compositions ( 18 O SMOW of –0.5 to –4.7) imply mixing of meteoric and oil-field brines. Carbon isotope values (+2 to –17) indicate microbial degradation of hydrocarbons in some of those fluids, but not all. Geomechanical analysis indicates the potential for syndepositional fractures to have failed (reactivated) during Basin and Range extension, and the warm basinal fluids are interpreted to have migrated upward through the fractures during that event. Platform-margin fractures (unassociated with faults) witnessed cooler fluids (59° to 65°C) than outer-shelf fractures (70° to 96°C) because outer-shelf faults tapped waters from greater depths. The most recent fluid flow through the fractures generated dissolution features and laminated speleogenetic calcites, which are interpreted to result from intrastratal karsting associated with exhumation and weathering. Diagenetic features in the syndepositional fractures are equivalent to those observed in the adjacent limestones, suggesting active fluid communication between matrix and fractures throughout the diagenetic history of the rocks. The complex fracture paragenesis also indicates that syndepositional fractures are not only conduits for early fluid-flow networks, but they can also impact a rock's entire diagenetic history if reactivated by changing stress fields.