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Abstract
The Northwest Africa (NWA) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars 4.4Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7-1.4Ga, and a third that launched NWA 7475 from Mars similar to 5Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15GPa. The mild postshock-heating that resulted from these shock pressures would have been insufficient to sterilize this water-bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium-carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma-ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of similar to 5Ma and an apparent diameter of 6.9km, marks one possible launch site of NWA 7475.
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Abstract
Olivine-dominated (70-80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO-rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Delta O-17 value of -1.19 +/- 0.10 parts per thousand, and low bulk-rock Mg/(Mg+Fe) (0.39), similar to the FeO-rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk-rock major-, minor-, and trace-element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and Os-187/Os-188 (0.1262), implies a FeO-and volatile-rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe-Ni-S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal-rich partial melts from FeO-rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to << 5%). As such, LEW 88763 represents the least-modified FeO-rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe, Ni-FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO-rich meteorites, such as brachinites, brachinite-like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.
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Abstract
Fluids exert a key control on the mobility of elements at high pressure and temperature in the crust and mantle. However, the prediction of fluid composition and speciation in compositionally complex fluid-rock systems, typically present in subduction zones, has been hampered by multiple challenges. We develop a computational framework to study the role of phase equilibria and complex solid-solutions on aqueous fluid speciation in equilibrium with rocks to 900 degrees C and 3 GPa. This is accomplished by merging conventional phase-equilibrium modeling involving electrolyte-free molecular fluids, with an electrostatic approach to model solute-solute and solute-solvent interactions in the fluid phase. This framework is applied to constrain the activity ratios, composition of aqueous solutes, and pH of a fluid in equilibrium with a pelite lithology. Two solvent compositions are considered: pure H2O, and a COH fluid generated by equilibration of H2O and graphite. In both cases, we find that the pH is alkaline. Disparities between the predicted peralkalinity of our fluid ([Na]+[K])/[Al] similar to 6 to 12 and results from independent mineral solubility experiments (similar to 2) point to the presence of Na-K-Al-Si polymers representing ca. 60 to 85% of the total K and Al content of the fluid at 600 degrees C and 2.2 GPa, and to an important fraction of dissolved Ca and Mg not accounted for in present speciation models. The addition of graphite to the system reduces the relative permittivity by ca. 40% at elevated T and low P, triggers the formation of C-bearing anions, and brings the pH closer to neutrality by up to 0.6 units at low T. This ionic C pool represents up to 45 mol% of the fluid ligands at elevated P, and is dominant at low P despite the low ionic strength of the fluid (<0.05). The present study offers new possibilities for exploring redox-pH dependent processes that govern volatile, major and trace element partitioning between rocks and fluids in experimental or natural systems. (C) 2015 Elsevier B.V. All rights reserved.
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Abstract
Northwest Africa (NWA) 5232, an 18.5 kg polymict eucrite, comprises eucritic and exogenic CM carbonaceous chondrite clasts within a clastic matrix. Basaltic clasts are the most abundant eucritic clast type and show a range of textures and grain size, from subophitic to granoblastic. Other eucritic clast types present include cumulate (high-En pyroxene), pyroxene-lath, olivine rich with symplectite intergrowths as a break-down product of a quickly cooled Fe-rich metastable pyroxferroite, and breccia (fragments of a previously consolidated breccia) clasts. A variable cooling rate and degree of thermal metamorphism, followed by a complex brecciation history, can be inferred for the clasts based on clast rounding, crystallization (and recrystallization) textures, pyroxene major and minor element compositions, and pyroxene exsolution. The range in delta O-18 of clasts and matrix of NWA 5232 reflects its origin as a breccia of mixed clasts dominated by eucritic lithologies. The oxygen isotopic compositions of the carbonaceous chondrite clasts identify them as belonging to CM group and indicate that these clasts experienced a low degree of aqueous alteration while part of their parent body. The complex evolutionary history of NWA 5232 implies that large-scale impact excavation and mixing was an active process on the surface of the HED parent body, likely 4 Vesta.
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Abstract
Miller Range 07273 is a chondritic melt breccia that contains clasts of equilibrated ordinary chondrite set in a fine-grained (<5m), largely crystalline, igneous matrix. Data indicate that MIL was derived from the H chondrite parent asteroid, although it has an oxygen isotope composition that approaches but falls outside of the established H group. MIL also is distinctive in having low porosity, cone-like shapes for coarse metal grains, unusual internal textures and compositions for coarse metal, a matrix composed chiefly of clinoenstatite and omphacitic pigeonite, and troilite veining most common in coarse olivine and orthopyroxene. These features can be explained by a model involving impact into a porous target that produced brief but intense heating at high pressure, a sudden pressure drop, and a slower drop in temperature. Olivine and orthopyroxene in chondrule clasts were the least melted and the most deformed, whereas matrix and troilite melted completely and crystallized to nearly strain-free minerals. Coarse metal was largely but incompletely liquefied, and matrix silicates formed by the breakdown during melting of albitic feldspar and some olivine to form pyroxene at high pressure (>3GPa, possibly to similar to 15-19GPa) and temperature (>1350 degrees C, possibly to 2000 degrees C). The higher pressures and temperatures would have involved back-reaction of high-pressure polymorphs to pyroxene and olivine upon cooling. Silicates outside of melt matrix have compositions that were relatively unchanged owing to brief heating duration.
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Abstract
In order to establish the role and expression of silicate-metal fractionation in early planetesimal bodies, we have conducted a highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and Re-187-Os-187 study of acapulcoite-lodranite meteorites. These data are reported with new petrography, mineral chemistry, bulk-rock major and trace element geochemistry, and oxygen isotopes for Acapulco, Allan Hills (ALHA) 81187, Meteorite Hills (MET) 01195, Northwest Africa (NWA) 2871, NWA 4833, NWA 4875, NWA 7474 and two examples of transitional acapulcoite-lodranites, Elephant Moraine (EET) 84302 and Graves Nunataks (GRA) 95209. These data support previous studies that indicate that these meteorites are linked to the same parent body and exhibit limited degrees (< 2-7%) of silicate melt removal. New HSE and osmium isotope data demonstrate broadly chondritic relative and absolute abundances of these elements in acapulcoites, lower absolute abundances in lodranites and elevated (> 2 x CI chondrite) HSE abundances in transitional acapulcoite-lodranite meteorites (EET 84302, GRA 95209). All of the meteorites have chondritic Re/Os with measured Os-187/Os-188 ratios of 0.1271 +/- 0.0040 (2 St. Dev.). These geochemical characteristics imply that the precursor material of the acapulcoites and lodranites was broadly chondritic in composition, and were then heated and subject to melting of metal and sulfide in the Fe-Ni-S system. This resulted in metallic melt removal and accumulation to form lodranites and transitional acapulcoite-lodranites. There is considerable variation in the absolute abundances of the HSE, both among samples and between aliquots of the same sample, consistent with both inhomogeneous distribution of HSE-rich metal, and of heterogeneous melting and incomplete mixing of silicate material within the acapulcoite-lodranite parent body. Oxygen isotope data for acapulcoite-lodranites are also consistent with inhomogeneous melting and mixing of accreted components from different nebular sources, and do not form a welldefined mass-dependent fractionation line. Modeling of HSE inter-element fractionation suggests a continuum of melting in the Fe-Ni-S system and partitioning between solid metal and sulfur-bearing mineral melt, where lower S contents in the melt resulted in lower Pt/Os and Pd/Os ratios, as observed in lodranites. The transitional meteorites, EET 84302 and GRA 95209, exhibit the most elevated HSE abundances and do not follow modelled Pt/Os and Pd/Os solid metal-liquid metal partitioning trends. We interpret this to reflect metal melt pooling into domains that were sampled by these meteorites, suggesting that they may originate from deeper within the acapulcoite-lodranite parent body, perhaps close to a pooled metallic 'core' region. Petrographic examination of transitional samples reveals the most extensive melting, pooling and networking of metal among the acapulcoite-lodranite meteorites. Overall, our results show that solid metal-liquid metal partitioning in the Fe-Ni-S system in primitive achondrites follows a predictable sequence of limited partial melting and metal melt pooling that can lead to significant HSE inter-element fractionation effects in proto-planetary materials. (C) 2017 Elsevier Ltd. All rights reserved.
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Abstract
Compaction-driven fluid flow below the brittle-ductile transition may be a means of transporting fluids during metamorphism. In particular, when a decompaction weakening mechanism is introduced to account for the rock viscosity reduction due to fluid overpressures, channeling instabilities evolve into high-porosity/permeability fluid conduits that focus mass and energy transfer. In this study, we consider a crustal rheology that accounts simultaneously for upward-increasing viscosity and decompaction weakening to examine the nucleation and evolution of fluid channelization in two dimensions (2-D). The model shows that plume-shaped flow patterns can develop on time scales as short as 10(4) years, during which the plume tails act as fluid conduits and the plume heads act as fluid dispersion zones near the brittle-ductile transition. Collection of fluids into conduits is accomplished by a basal fluid catchment zone characterized by strong lateral fluid pressure gradients but low porosity/permeability. Relatively narrow ranges of viscous activation energy (approximate to 100 kJ mol(-1)) and decompaction weakening factor (approximate to 10(-4)) are constrained if the fluid conduits are of kilometer scale in width. Significant thermal excursions (approximate to 65 degrees C) can be induced if a high flow rate, potentially from rapid intermittent dehydration, is realized within channels. Moreover, if the focused fluids emanate from external anomalously hot sources (e.g., magma intrusion), thermal pulses (>100 degrees C), and steep lateral temperature gradients (>50 degrees C km(-1)) can be generated. Given the focusing efficiency estimated from our 2-D compaction model, simple 3-D modeling further shows that tubular conduits have the potential to cause thermal pulses >200 degrees C within 10(4) years.
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Abstract
A series of localized high-temperature granulite-facies domains ('hot spots') are present within the regional (10-100km(2) scale) amphibolite-facies rocks of the Central Maine Terrane in central New Hampshire (NH), USA. Based on the spatial coincidence of a thermal anomaly and contours of depressed delta O-18 values centered on networks of quartz-graphite and pegmatitic veins in the vicinity of Bristol, NH, it was proposed in an earlier study that large-scale ascending hot fluid focused through a vein network drove heating and introduced isotopically distinct fluids. The thermal anomaly could be preserved only if the timescales of heating were extremely short, otherwise conduction would smooth the field temperature gradient. Herein, we conduct a petrological test to estimate the peak temperatures and the durations of metamorphism across the Bristol region, using pseudosection analysis as well as forward modeling of garnet growth-diffusion-resorption profiles. This region attained granulite-facies conditions in the sillimanite-K-feldspar-cordierite zone over a larger area than previously mapped. Cordierite is variably present, which reflects bulk compositional controls on its stability as well as its destruction during retrogression. The forward modeling reveals protracted (5-8 Myr) granulite-facies conditions of 0.5-0.6 GPa and similar to 750-820 degrees C, and an overall counterclockwise P-T path. Furthermore, a short-lived thermal anomaly or 'spike' (> 100 degrees C, similar to 0.15 Myr) is superimposed on the granulite-facies core, reaching ultrahigh-temperature (UHT) conditions > 900 degrees C, much higher than previously recognized in the area. The short timescale is fully consistent with the localized radius of the thermal anomaly of similar to 1.5 km. Subsequently, the area underwent variably developed amphibolite-facies retrogression at similar to 650 degrees C and 0.4-0.5 GPa, accompanied by fluid infiltration, garnet breakdown, and muscovite growth. The transient thermal spike and the counterclockwise P-T path indicate that heat transfer could not have been solely the result of internal heating of overthickened crust. We posit that external heat fluxes driven by Acadian plutonism, in addition to heat generation in crust enriched in heat-producing elements, led to the granulite-facies metamorphism. Magmatic loading in the crust, potentially in response to an elevated basal heat flux during the Acadian orogeny, can account for the counterclockwise P-T path. Heat transported advectively by channelized flow of magma or magma evolving hydrothermal fluids is the most likely cause of the transient and local UHT thermal anomaly. The results show that UHT metamorphic events can be extremely brief.
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Abstract
Mud volcanoes provide an accessible channel through which deep subsurface environments can be observed. The manner in which deeply sourced materials shape biogeochemical processes and microbial communities in such geological features remains largely unknown. This study characterized redox transitions, biogeochemical fluxes and microbial communities for samples collected from a methane-rich mud volcano in southwestern Taiwan. Our results indicated that oxygen penetration was confined within the upper 4 mm of fluids/muds and counteracted by the oxidation of pyrite, dissolved sulfide, methane and organic matter at various degrees. Beneath the oxic zone, anaerobic sulfur oxidation, sulfate reduction, anaerobic methanotrophy and methanogenesis were compartmentalized into different depths in the pool periphery, forming a metabolic network that efficiently cycles methane and sulfur. Community members affiliated with various Proteobacteria capable of aerobic oxidation of sulfur, methane and methyl compounds were more abundant in the anoxic zone with diminished sulfate and high methane. These findings suggest either the requirement of alternative electron acceptors or a persistent population that once flourished in the oxic zone. Overall, this study demonstrates the distribution pattern for a suite of oxidative and reductive metabolic reactions along a steep redox gradient imposed by deep fluids in a mud volcano ecosystem.
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