ALBERT

Description: Cavities are considered plausible and favorable habitats for life on early Earth. In such microenvironments, organisms may have found an adequate protection against the intense ultraviolet radiation that characterized the Archean ozone-free atmosphere. However, while there is clear evidence that benthic life existed in the Paleoarchean, the oldest traces of cavity-dwelling microbes (coelobionts) have been found in Neoarchean rocks. Here we present the results of a detailed investigation of early silicified cavities occurring in the oldest well-preserved siliciclastic tidal deposits, the 3.22 Ga Moodies Group of the Barberton Greenstone Belt (South Africa). Downward-growing microstromatolitic columns composed of kerogenous laminae are commonly present in planar, bedding-parallel, now silica-filled cavities that formed in sediments of the peritidal zone. In-situ 13 C PDB (PDB—Peedee belemnite) measurements of the kerogen range from –32.3 to –21.3 and are consistent with a biogenic origin. Scanning electron microscopy analysis of the silicified cavities shows well-preserved chains of cell-sized molds that are interpreted as fossil filamentous microorganisms. The geological context, the morphology of the microstromatolites, the 13 C composition of the kerogen, and the presence of microfossils all suggest that a microbial community inhabited the cavities. These results extend the geological record of coelobionts by ~500 m.y., supporting the view that cavities were among the first ecological niches to have been occupied by early microorganisms.

Description: A key parameter in the study of magma evolution is the time scale on which magmatic processes occur. Using nanoscale secondary ion mass spectrometry (NanoSIMS), SIMS, and cathodoluminescence (CL) analyses, we have measured titanium (Ti) diffusion profiles in quartz phenocrysts from a Jurassic rhyolite of the El Quemado Complex (Patagonia, Argentina), providing new insights into the time scales of the associated volcanic processes. CL imaging of quartz phenocrysts reveals oscillatory magmatic zoning. We determined Ti concentrations with SIMS and acquired multiple NanoSIMS profiles across growth zones from core to rim. All transects show sharp changes in the 48 Ti/ 29 Si ratio, which correlate reasonably well with changes in CL intensity. Diffusion modeling of Ti in quartz yields a surprisingly short time scale for quartz crystallization of 5.6 ± 2.2 yr and a rapid crystal growth rate of 2.3 x 10 –12 m/s. Based on the observed quartz textures, we suggest that the rhyolite erupted shortly after initial onset of crystallization, followed by decompression-driven quartz dissolution during fast magma ascent. We further argue that the observed oscillatory zoning and the variation of the Ti concentration of the quartz phenocryst does not reflect temperature, pressure, or titanium activity (a Ti ) changes of the magmatic system, but rather is the result of growth kinetics, which has important implications for the Ti-in-quartz thermometry.

Description: The thermal evolution of a contact aureole is strongly dependent on the emplacement mode of the intrusion that is causing the thermal anomaly. Inferences on the emplacement mode can be made if solid temperature estimates are available. The contact aureole of the Western Adamello Tonalite provides a unique combination of magma ascent close to the host rock and suitable rock chemistry. We use three mineral reactions at different distances in the aureole in combination with phase petrology to estimate temperatures. These temperatures are not reproducible by thermal models considering the emplacement of the intrusion as a single batch. The external zone of the intrusion represents a feeder conduit in which magma was transported. We investigated the thermal effect of the conduit on the host rocks using thermal models. Different thermal profiles for the aureole are obtained by varying flow/no-flow times, conduit thickness and magma temperature. We show that only a few combinations match the temperature constraints in the aureole, and that the amount of magma transported through the conduit can be calculated. A comparison with time-averaged fluxes of recent volcanoes and the presence of reworked volcanic sediments in the surroundings of the Adamello batholith indicate that the calculated rates and volumes are plausible. Supplementary material: Tables comparing sample compositions with literature experimental data are available at http://www.geolsoc.org.uk/SUP18847

Description: The shallow crustal Torres del Paine Intrusive Complex in southern Patagonia offers an opportunity to understand the chemical evolution and timing of crystallization processes in shallow plutonic rocks. It is characterized by hornblende-gabbros, gabbronorites, monzodiorites and granitic plutonic rocks. The exceptional exposure of the intrusion permits the identification of two structurally and petrographically different zones. Layered gabbronorite, olivine-bearing pyroxene–hornblende gabbronorite and monzodiorite forming vertical sheets and stocks in the west are referred to here as the feeder zone. These mafic rocks are in vertical contact with younger granitic rocks on their eastern border. The eastern part is a laccolith complex. It is characterized by three major units (I, II, III) of granitic rocks of over 1000 m vertical thickness; these are underlain in places by a sequence of hornblende-gabbro sills intermingled with evolved monzodiorite granite. Chilled, crenulated margins as well as flame structures between gabbroic rocks and monzodiorites suggest that the mafic sill complex remained partially molten during most of its construction. Bulk-rock major and trace element data indicate that the Paine mafic rocks follow a high-K calc-alkaline to shoshonitic differentiation trend. The parental magmas were basaltic trachyandesite liquids, with variable H 2 O and alkali contents. The majority of the feeder zone gabbronorites have high Al 2 O 3 contents and positive Eu and Sr anomalies, consistent with accumulation of plagioclase and efficient extraction of intercumulus melt. The mafic sill complex largely lacks these cumulate signatures. Comparisons of the intercumulus groundmass in the hornblende-gabbros with intra-sill dioritic stocks and pods reveal similar rare earth element patterns and trace element ratios indicating incomplete extraction of evolved interstitial liquids. The Sr, Nd and Pb isotopic compositions of the mafic and granitic rocks exhibit ranges of 87 Sr/ 86 Sr of 0·704–0·708, Nd +3·8 to –1·2, 206 Pb/ 204 Pb 18·61–18·77, 207 Pb/ 204 Pb 15·67–15·67 and 208 Pb/ 204 Pb 38·56–38·77. Crystal fractionation and assimilation–fractional crystallization modelling, combined with high-precision U–Pb dating of zircons, indicates that the western feeder zone gabbronorites are linked to the uppermost Paine granite (granite I), whereas the mafic sill complex is younger and not directly related to the voluminous granite units II and III. These results are interpreted to indicate that crystal–liquid separation is facilitated in subvertical, dynamic feeder systems whereas subhorizontal sill complexes are inefficient in separating large volumes of mafic cumulates and complementary felsic rocks.

Description: Albitization is a common process during which hydrothermal fluids convert plagioclase and/or K-feldspar into nearly pure albite; however, its specific mechanism in granitoids is not well understood. The c . 1700 Ma A-type metaluminous ferroan granites in the Khetri complex of Rajasthan, NW India, have been albitized to a large extent by two metasomatic fronts, an initial transformation of oligoclase to nearly pure albite and a subsequent replacement of microcline by albite, with sharp contacts between the microcline-bearing and microcline-free zones. Albitization has bleached the original pinkish grey granite and turned it white. The mineralogical changes include transformation of oligoclase (~An 12 ) and microcline (~Or 95 ) to almost pure albite (~An 0 · 5 – 2 ), amphibole from potassian ferropargasite ( X Fe 0·84–0·86) to potassic hastingsite ( X Fe 0·88–0·97) and actinolite ( X Fe 0·32–0·67), and biotite from annite ( X Fe 0·71–0·74) to annite ( X Fe 0·90–0·91). Whole-rock isocon diagrams show that, during albitization, the granites experienced major hydration, slight gain in Si and major gain in Na, whereas K, Mg, Fe and Ca were lost along with Rb, Ba, Sr, Zn, light rare earth elements and U. Whole-rock Sm–Nd isotope data plot on an apparent isochron of 1419 ± 98 Ma and reveal significant disturbance and at least partial resetting of the intrusion age. Severe scatter in the whole-rock Rb–Sr isochron plot reflects the extreme Rb loss in the completely albitized samples, effectively freezing 87 Sr/ 86 Sr ratios in the albite granites at very high values (0·725–0·735). This indicates either infiltration of highly radiogenic Sr from the country rock or, more likely, radiogenic ingrowth during a considerable time lag (estimated to be at least 300 Myr) between original intrusion and albitization. The albitization took place at ~350–400°C. It was caused by the infiltration of an ascending hydrothermal fluid that had acquired high Na/K and Na/Ca ratios during migration through metamorphic rocks at even lower temperatures in the periphery of the plutons. Oxygen isotope ratios increase from 18 O = 7 in the original granite to values of 9–10 in completely albitized samples, suggesting that the fluid had equilibrated with surrounding metamorphosed crust. A metasomatic model, using chromatographic theory of fluid infiltration, explains the process for generating the observed zonation in terms of a leading metasomatic front where oligoclase of the original granite is converted to albite, and a second, trailing front where microcline is also converted to albite. The temperature gradients driving the fluid infiltration may have been produced by the high heat production of the granites themselves. The confinement of the albitized granites along the NE–SW-trending Khetri lineament and the pervasive nature of the albitization suggest that the albitizing fluids possibly originated during reactivation of the lineament. More generally, steady-state temperature gradients induced by the high internal heat production of A-type granites may provide the driving force for similar metasomatic and ore-forming processes in other highly enriched granitoid bodies.

Description: Cumulate and crystal mush disruption and reactivation are difficult to recognize in coarse-grained, shallow plutonic rocks. Mafic minerals included in hornblende and zoned plagioclase provide snapshots of early crystallization and cumulate formation, but are difficult to interpret in terms of the dynamics of magma ascent and possible links between silicic and mafic rock emplacement. This study presents the field relations, the microtextures and the mineral chemistry of the Miocene mafic sill complex of the Torres del Paine intrusive complex (Patagonia, Chile) and its subvertical feeder zone. We summarize a number of observations that occur in structurally different, shallow, plutonic rocks, as follows. (1) The mafic sill complex was built up by a succession of braided sills of shoshonitic and high-K calc-alkaline porphyritic hornblende-gabbro and fine-grained monzodiorite sills. Local diapiric structures and felsic magma accumulation between sills indicate limited separation of intercumulus liquid from the mafic sills. Anhedral hornblende cores, with olivine + clinopyroxene ± plagioclase ± apatite inclusions, crystallized at temperatures 〉900°C and pressures of ~300 to ~400 MPa. The corresponding rims and monzodiorite matrix crystallized at 〈830°C, ~70 MPa. This abrupt compositional variation suggests stability and instability of hornblende during recycling of the mafic roots of the complex and subsequent decompression. (2) The near lack of intercumulus crystals in the subvertical feeder zone layered gabbronorite and pyroxene–hornblende gabbronorite stocks testifies that melt is more efficiently extracted than in sills, resulting in a cumulate signature in the feeding system. Granitic liquids were extracted at a higher temperature ( T 〉950°C) than estimated from the composition of the granite minimum. We show that hornblende–plagioclase thermobarometry is a useful monitor for the determination of the segregation conditions of granitic magmas from gabbroic crystal mushes, and for monitoring the evolution of shallow crustal magmatic crystallization, decompression and cooling .

Description: We report experimental data for Y, La, Lu and Hf diffusion in garnet, in which diffusant concentrations and silica activity have been systematically varied. Experiments were conducted at 950 and 1050 °C, at 1 atm. pressure and oxygen fugacity corresponding to the quartz-fayalite-magnetite buffer. At Y and REE concentrations below several hundred ppm we observe both slow and fast diffusion mechanisms, which operate simultaneously and correspond to relatively high and low concentrations, respectively. Diffusivity of Y and REEs is independent of silica activity over the studied range. General formulae for REE diffusion in garnet, incorporating data from this and previous studies, are: log10DREEfm2/s=-10.24±0.21-221057(±4284)2.303RT(K) for the ‘fast’ REE diffusion mechanism at 1 atm. pressure, and log10DREEsm2/s=-9.28±0.65-265200±38540+10800(±2600)·P(GPa)2.303RT(K) for the ‘slow’ REE diffusion mechanism. These slow and fast diffusion mechanisms are in agreement with previous, apparently conflicting, datasets for REE diffusion in garnet. Comparison with high-pressure experiments suggests that at high pressures (〉 ∼1 GPa minimum) the fast diffusion mechanism no longer operates to a significant degree. When Y and/or REE surface concentrations are greater than several hundred ppm, complex concentration profiles develop. These profiles are consistent with a multi-site diffusion-reaction model, whereby Y and REE cations diffuse through, and exchange between, different crystallographic sites. Diffusion profiles of Hf do not exhibit any of the complexities observed for Y and REE profiles, and can be modeled using a standard (i.e. single mechanism) solution to the diffusion equation. Hafnium diffusion in garnet shows a negative dependence on silica activity, and is described by log10DHfm2/s=-8.85±0.38-299344±15136+12500±900·PGPa2.303RTK-0.52(±0.09)·log10aSiO2 In many natural garnets, diffusion of both Lu and Hf would be sufficiently slow that the Lu-Hf system can be reliably used to date garnet growth. In cases in which significant Lu diffusion does occur, preferential retention of 176Hf/177Hf relative to 176Lu/177Hf will skew isochron relationships such that their apparent ages may not correspond to anything meaningful (e.g., garnet growth, peak temperature or the closure temperature of Lu or Hf). Late-stage reheating events are capable of causing larger degrees of preferential retention of 176Hf/177Hf relative to 176Lu/177Hf and partial to full resetting of the Sm-Nd system within garnet, thus increasing the separation between garnet Lu-Hf and Sm-Nd isochron dates, due to the fact that these systems are more significantly disturbed through diffusion as more radiogenic 176Hf and 143Nd have accumulated.