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.

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.

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.

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.

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.

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.

Resolved measurements of the relative abundances of (CDH3)-C-13 and (CD2H2)-C-12 from a Taiwan mud volcano confirm that thermogenic CH4 with a formation temperature of 150 degrees C is bubbling from a pool of liquid mud. Analysis for both doubly-substituted molecules provides confirmation that isotopic exchange equilibrium was achieved between methane isotopologues. Methane extracted from the headspace of core samples taken from sediments deposited proximal to the bubbling pool of mud shows small departures from isotopic equilibrium.

The source of sulfur for sulfide mineralization is a major question for the origin of platinum group element deposits such as the Rustenburg Layered Suite (RLS) of the Bushveld Complex and the nearby Waterberg Project (WP; a large palladium-dominant deposit) in southern Africa. Both deposits are mafic-ultramafic intrusions associated with the ca. 2.06 Ga Bushveld magmatism but are hosted in distinct country rocks. This contrast allows a critical assessment of the contribution upper crustal assimilation provides to sulfide mineralization, and refinement of our understanding of sources of mass-independent fractionated sulfur (MIF-S) to these intrusions. The WP has a signature of anomalous sulfur (average Delta S-33 = 0.113 parts per thousand +/- 0.016 parts per thousand, 1 s.d.), similar to the RLS (avgerage Delta S-33 = 0.137 parts per thousand +/- 0.025 parts per thousand, 1 s.d.). There is no evidence for influence of host rock as a source of anomalous sulfur. The lack of a significant variation of Delta S-33 values within the WP stratigraphy, and the distinct upper continental crust into which the WP magmas would have been emplaced, shows that addition of upper crustal sulfur is not necessary for PGE formation. This suggests that contamination of WP and RLS magmas with a surface-derived component of Archean age occurred at depth, prior to emplacement.

Ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, and group IVB iron meteorites, are Ni-rich. Similarities include enrichments of 10-100 x CI for some refractory siderophile elements, and equivalent depletions in more volatile siderophile elements. Superimposed on the overall enrichment/depletion trend, certain siderophile elements (P, W, Fe, Mo) are depleted relative to elements of similar volatility. All three ungrouped irons derive from parent bodies formed in the early Solar System. Willow Grove and Chinga are characterized by cosmic ray exposure corrected W-182/W-184 consistent with metalsilicate segregation on their parent bodies within 1-3 Myr of Solar System formation, within the age range determined for segregation of magmatic iron meteorite parent bodies, including group IVB irons. Tishomingo is characterized by a younger model age 4-5 Myr subsequent to Solar System formation, reflecting either late stage melting resulting from Al-26 decay, or an impact resetting. The discovery of stishovite in Tishomingo, indicating exposure to a minimum shock pressure of 8-9 GPa, is consistent with the latter.