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I have done major element analyses by electron microprobe, in-situ trace element analyses by laser ablation inductively coupled plasma mass spectrometry, and instrumental neutron activation analyses on bulk samples of olivine grains separated from main-group and Eagle-Station pallasites. Most main-group pallasite olivines have homogeneous Fe/Mg yet have varying Fe/Mn. Those few with anomalously ferroan olivine have Fe/Mn within the range of other main-group pallasites. High-temperature redox process coupled with diffusional exchange resulted in the homogeneous compositions of most main-group pallasites; simple diffusional exchange alone is insufficient. The Eagle-Station pallasites have Fe/Mn twice that of main-group pallasites with similar Fe/Mg, a result of having roughly half the Mn content. The Ni/Co ratio of main-group pallasite olivines is relatively constant and was imposed by the same high-temperature redox/diffusion process that established Fe/Mg-Fe/Mn relationships. Variability in trace lithophile element contents within individual pallasites and within individual olivine grains, coupled with very low contents for some that are inconsistent with formation from a magma, indicate that the current mm-sized olivine grains were recrystallized from a fragmental olivine breccia; grain fragments from different portions of an original dunitic mantle were juxtaposed in the breccia. Main-group pallasites are dimict breccias formed of fragmented and mixed monomict dunite breccia and metallic breccia that were formed in the walls and floor of a large basin that penetrated the mantle of their parent asteroid. Limited data indicate that Eagle-Station pallasites may have been formed by a similar process. Given that Eagle-Station and main-group pallasites were formed in distinct regions of the early Solar System, the pallasite forming process likely was common in early Solar System history. [ABSTRACT FROM AUTHOR]

Chondrules and Ca-Al-rich inclusions (CAIs) have been considered characteristic constituents of chondritic meteorites, although the outward transportation of CAIs has been theoretically pointed out. Stardust samples recovered by the Stardust mission from the 81P/Wild2 comet contained chondrule-like objects (CLOs) and refractory inclusions that include CAIs and amoeboid olivine aggregates (AOAs). However, it was not proven that the CLOs, AOAs, and CAIs coexist with fine-grained materials equivalent to chondritic porous interplanetary dust particles (CP IDPs) containing abundant glass with embedded metal and sulfides (GEMS). Here we report on two type II CLOs, containing <90 Mg# in ferromagnesian silicates, enclosed in GEMS-rich CP Antarctic micrometeorites (AMMs) (CP IDPs that reached the surface of the Earth) and one igneous object rich in kosmochloric (Ko-rich: NaCrSi 2 O 6 -rich) high-Ca pyroxene and Fe-bearing olivine (KOOL) that is enclosed in a CP IDP. KOOL grains have also been found in Stardust samples and CP IDPs. These three igneous objects are embedded in fine-grained matrices that do not show any evidence of aqueous alteration. The low Mg# and elevated Δ17O of olivine and pyroxene in these CLOs and the KOOL grain are consistent with previously studied CLOs from comet 81P/Wild 2 and a giant cluster IDP. These results support the view that CP IDP- and CP AMM-like materials constitute samples from comets or comet-like icy bodies. The CLOs were formed in oxidizing environment beyond the snow line and then transferred to the comet-forming region. In contrast, a spinel-hibonite (SHIB) fragment found in an AMM experienced aqueous alteration of its rim. The SHIB fragment contains ultrarefractory oxides and refractory metal nuggets and has a 26Mg excess like typical meteoritic CAIs. The mineralogy of the fine-grained matrix is very similar to CP IDPs and CP AMMs. However, because "GEMS" in the matrix of the SHIB fragment-bearing AMM lacks Fe-Ni metal and amorphous silicate in it contains Fe, it is clear that the matrix weakly experienced aqueous alteration. Olivine / (Olivine + low-Ca pyroxene) ratios in the matrices of the four samples range from 0.4 to 0.6, which are comparable with those of anhydrous CP IDPs and CP MMs (around 0.5), and those of P- and D-type asteroids and Jupiter-family comets (around 0.5). [ABSTRACT FROM AUTHOR]

Natural carbonatite is associated with various types of deposits, including rare earth elements (REEs) and high-field strength elements (e.g., Nb). Although the spatial relationship between fluorine enrichment and the formation of carbonatite-type deposits is well-established, the underlying mechanism remains unclear. In this study, we investigate the influence of fluorine on the partitioning of trace elements in carbonatite melts (CM) from the mantle to the crust. Elemental partitioning experiments were conducted at 3 GPa and 1100–1400 °C, both with and without fluorine. We determined trace element partition coefficients for the mineral-carbonatitic melt pairs involving the following minerals: clinopyroxene, olivine, spinel, perovskite, magnetite, calcite solid solution, magnesio wüstite. The addition of fluorine influences the types and stability of Ti-rich oxides. Fluorine-bearing systems crystallize magnetite, while fluorine-free systems crystallize perovskite. Compared to fluorine-free compositions, systems with 1.55–2.40 wt% fluorine in the melt exhibited significantly lower (clinopyroxene, spinel, olivine)-melt partition coefficients for REEs and HFSEs by half to one order of magnitude. The decreased uptake of these elements by clinopyroxene, spinel, and olivine may be attributed to changes in their speciation within the melt phase, consistent with the formation of REE-F complexes. Using our partition coefficients, we simulated the differences in trace element characteristics between carbonatite melts derived from the mantle under fluorine-bearing and fluorine-free conditions. The fluorine-bearing carbonatitic melt extracted from mantle peridotite displayed significantly higher trace element compositions compared to fluorine-free carbonatitic melt, with trace element concentrations being 5–10 times higher. We simulated the trace element characteristics of carbonatitic melts released from subducted sediments and ocean crusts and found that these ocean crusts or sediments can release fluorine-rich carbonatite melts that are naturally enriched in REE due to their high fluorine and REE contents. Therefore, the subduction of sediments or the recycling of ocean crusts is highly favorable for the formation of carbonatite-hosted REE deposits. This can explain why the majority of carbonatite-hosted REE deposits are typically found in continental margins with subducted materials signals in source region. Furthermore, we modeled the impact of different mineral crystallization and fractionation in the shallow crust on the trace element compositions of carbonatitic melts. Late-stage magma crystallization and differentiation of silicate and carbonate minerals are favorable for the further enrichment of ore-forming materials (e.g., REEs). [ABSTRACT FROM AUTHOR]

Platinum-group elements (PGEs) have a strong affinity for sulfur and tend to accumulate in the deep continental crust, either concentrated within magmatic Cu-Ni-PGE deposits or dispersed throughout disseminated sulfides. However, PGE enrichment in shallow magmatic-hydrothermal systems implies an obscure link to deep sulfide destabilization, which releases PGEs into ore-forming fluids. To bridge this gap, our study investigates the PGE composition of magmatic sulfides with oxidative textures in dacitic rocks from the southwestern Okinawa Trough. We identified three groups of magmatic sulfides, primarily precipitated as Cu-poor monosulfide solid solution (MSS), which formed at distinct stages of magma evolution from deep to shallow crustal levels. Group A sulfides manifest as small-sized inclusions (<30 μm) within most high-Mg olivines, whereas Group B and C sulfides are larger (50–500 μm) and occur within cognate xenoliths of mafic cumulate rocks and the groundmass of host dacites. Group B and C sulfides exhibit distinct oxidative textures and newly-formed mineral assemblages, including magnetite, hematite, goethite, and magnetite ± pyrite, alongside hydrothermal silicate minerals, respectively. We attribute the oxidation process to the infiltration of orthomagmatic fluids exsolved from mafic magma that had underplated the sulfide-bearing felsic magma reservoir, which was nearly solidified. By comparing the chemical compositions between pristine sulfides and their oxidative remnants, we observed extensive mobilization of Pd, Pt, Cu, Ag, Ni, and Co from the altered sulfides of Group B, while Au enrichment occurred as nanoparticles under high oxidation states. In contrast, Au was extracted along with other mobile metals from the altered sulfides of Group C, with Pt remaining in place under more reduced conditions. These distinct scenarios may lead to the formation of PGE-rich and Au-rich fluids, respectively. The formation of deep crustal MSS and subsequent hydrothermal oxidation under varying redox conditions thus provides a viable mechanism for trans -crustal PGE mobilization and inter-element fractionation, typical of PGE-rich magmatic-hydrothermal deposits. [ABSTRACT FROM AUTHOR]

Shergottite meteorites, classified as depleted, intermediate, or enriched based on incompatible trace elements and specific radiogenic isotope compositions (Sr, Nd, and Hf isotope ratios), point to multiple Martian mantle source regions. The oxygen fugacity (f O 2) of these mantle regions, determined from early crystallizing minerals using the olivine-pyroxene-spinel oxybarometer, appears to correlate with incompatible trace element enrichment and isotope compositions. However, values derived from the vanadium-in-olivine oxybarometer challenge this correlation, hinting at potential biases in oxybarometry or complexities in the redox conditions of the Martian mantle. By analyzing the intermediate shergottite Northwest Africa (NWA) 11043 with various oxybarometers, this study deduced its origin from a reduced mantle source, with an average f O 2 value of −0.77 ± 0.35 relative to the iron-wüstite (IW) buffer. Notably, these values coincide with those of depleted shergottites, which represent the depleted Martian mantle region. This redox similarity between intermediate and depleted shergottites contrasts with earlier notions that postulated intermediate shergottites as a mix of depleted and enriched mantle derivatives. Moreover, intermediate shergottites such as NWA 11043, Elephant Moraine (EETA) 79001A, and Allan Hills (ALH) 77005 display 176Hf/177Hf values akin to those of depleted shergottites, suggesting that intermediate mantle components can be separated from the depleted mantle source at approximately 2.2 Ga based on model age calculations. Therefore, there presents a consistent redox state between mantle magma sources of both intermediate and depleted shergottites since the Hesperian period, while enriched shergottites lean toward more oxidized conditions past source formation. This study prompts a reassessment of conventional theories, emphasizing the nuanced redox evolution of the Martian mantle across distinct mantle source regions and underscoring the complexity of the redox evolution of the Martian mantle. The emergence of chemically diverse mantle reservoirs might predominantly arise from early magma ocean differentiation processes, albeit with inherent oxidation nuances. The differences in f O 2 observed between intermediate and depleted shergottites underscore the need for more in-depth studies to decipher Martian mantle differentiation and evolution. [ABSTRACT FROM AUTHOR]

Samples collected from the surface/subsurface of C-type asteroid 162173 Ryugu by the Hayabusa2 mission were nondestructively analyzed in three dimensions (3D). Seventy-three small particles (approximately 10–180 µm in size) were observed using X-ray nanotomography, with an effective spatial resolution of approximately 200 nm. Detailed descriptions of these samples in terms of mineralogy, petrology, and variations among particles were reported. The 57 most common particles consisted of a phyllosilicate matrix containing mineral grains, mainly magnetite, pyrrhotite, dolomite and apatite. The remaining particles were mostly monomineralic particles (pyrrhotite, dolomite, breunnerite, apatite, and Mg-Na phosphate) with two unique particles (calcite in a Al 2 Si 2 O 5 (OH) 4 matrix, and CaCO 3 , phyllosilicate, and tochilinite-chronstedtite inclusions in a carbonaceous material matrix). The results confirmed that the samples correspond to Ivuna-type carbonaceous chondrites (CI chondrites) or related materials. Many small inclusions of voids and carbonaceous materials were detected in pyrrhotite, dolomite, breunnerite, and apatite. However, no fluid inclusions were observed, except for those in pyrrhotite that have already been reported. Magnetite exhibited a wide variety of morphologies, from irregular shapes (spherulites, framboids, plaquettes, and whiskers) to euhedral shapes (equants, rods, and cubes), along with transitional shapes. In contrast, the other minerals exhibit predominantly euhedral shapes (pyrrhotite: pseudo-hexagonal plates, dolomite: flattened rhombohedrons, breunnerite: largely flattened rhombohedrons, and apatite: hexagonal prisms) or aggregates of faceted crystals, except for Mg-Na phosphate. The matrices were heterogeneous with variable phyllosilicate particle sizes, Mg/Fe ratios, density (1.7 ± 0.2 g/cm3), nanoporosities (36 ± 9 %), and abundances of nanograins of Fe(-Ni) sulfides. The macroporosity of the particles was estimated as 12 ± 4 %. The observed textural relationships among the minerals suggest a precipitation sequence of: magnetite (spherulite → plaquette/framboid → rod/equant) → pyrrhotite (pentlandite → pyrrhotite) → apatite → dolomite → breunnerite → coarse phyllosilicates. Fe-bearing olivine (or low-Ca pyroxene) might have precipitated later than dolomite, indicating a high Mg activity in the aqueous solution. This precipitation sequence corresponds to a transition from irregular crystal forms (as seen in some magnetite) to regular forms of euhedral crystals (observed in some magnetite and other minerals). Based on the precipitation sequence and mineral morphologies, together with previously reported observations, a model for aqueous alteration in the Ryugu parent body was proposed as follows: CO 2 -H 2 O ice, amorphous silicates (GEMS-like material), and some minerals (mostly metal, sulfides, and anhydrous silicates) accumulated to form the parent body of Ryugu. Amorphous silicates and Fe-Ni metal quickly dissolved into the melted ice to form a highly supersaturated aqueous solution. Poorly-crystalized phyllosilicate and spherulitic magnetite precipitated first, followed by plaquette/framboidal magnetites with decreasing degree of supersaturation due to precipitation. Pseudo-hexagonal pyrrhotite plates were formed by dissolution and reprecipitation under relatively low supersaturation. Subsequently, apatite, dolomite, and breunnerite precipitated in this order in response to decreasing supersaturation. [ABSTRACT FROM AUTHOR]

Olivine-rich lavas in the Margi area, Cyprus, are part of the boninitic Upper Pillow Lava series of the Troodos ophiolite. Re-Os isotope compositions of both whole rock and olivine phenocrysts of these lavas have been measured to determine the initial 187Os/188Os isotope compositions of the primary boninitic magma. The Margi lavas have high and variable whole rock Re-Os isotope compositions (187Re/188Os: 1.060–8.613; 187Os/188Os: 0.1307–0.1599), with (87Sr/88Sr) 90Ma (0.705936 ± 0.001021), ε Nd (90 Ma) (5.6 ± 2.5), and δ18O (8.59 ± 0.38 ‰). In contrast, olivine phenocrysts, which host spinel and melt inclusions, have low and constant 187Os/188Os from 0.1301 to 0.1336 and low 187Re/188Os from 0.1350 to 1.557. Initial 187Os/188Os of these olivine phenocrysts (0.1300 ± 0.0041) is lower than that of whole rock (0.1373 ± 0.0135). These facts indicate that the olivine phenocrysts faithfully record initial 187Os/188Os of the primary boninitic magmas, which are comparable to the 187Os/188Os of primitive mantle, whereas whole rock Re-Os isotopes have been affected by secondary processes after olivine crystallization. Further geochemical modeling indicates that, although Os can be mobilized during slab dehydration, the contribution of crustal Os is insufficient to significantly raise the 187Os/188Os of the mantle wedge in a nascent subduction zone. Overall, our findings provide new insights about Os recycling in the early stages of subduction. [ABSTRACT FROM AUTHOR]

Petrogenetic models of martian magmas reflect the compositional, thermal, and geodynamical evolution of its interior. Most models for shergottite meteorites, which sampled relatively young martian basalts, invoke partial melting in a mantle plume head and filtering of the magma through the crust. Olivine-phyric, poikilitic, and olivine-gabbroic shergottites contain large, Mg-rich olivine crystals that record their parent magma history and are generally regarded to form initially in slow cooling, near-equilibrium environments in the lower crust. Here we report the unexpected common occurrence of Mg-rich olivine crystals with chained, mantled dendritic, and skeletal morphologies that reflect rapid growth, requiring relatively large degrees of undercooling and possibly moderate to fast cooling rates. Novel 3D analysis of crystal textures and core Fe-Mg content demonstrates that chained and mantled dendritic olivines in enriched shergottites are among the earliest-formed crystals, indicating rapid crystal growth during initial magma pooling in the lower crust, a process that has not been documented in Earth basalts. In geochemically depleted shergottites, skeletal olivines appear later in the crystallization sequence, but before the final eruption and solidification. Geochemically intermediate samples do not contain rapid-growth olivine, indicating extended equilibrium growth. The frequent occurrence of chained, mantled dendritic, and skeletal olivines in enriched and depleted shergottites suggests that the conditions needed for rapid growth are common for mafic magmatism on Mars, and that these magmas experience complex thermal histories characterized by discrete episodes of large undercooling at depth. [ABSTRACT FROM AUTHOR]

Fifteen experiments at 1 atm pressure and 1400 °C have been conducted to determine partition coefficients between olivine and silicate melt ( D Ol - m e l t ) of the first-row transition elements (FRTEs, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn), Ga and Ge in the system FeO-CaO-MgO-Al 2 O 3 -SiO 2 (FCMAS). Bulk iron contents are varied between 0 and 10 wt.% FeO, and oxygen fugacity ranges from 2 log units below the iron-wüstite buffer (IW-2) to 2 log units above the quartz-fayalite-magnetite buffer (QFM + 2), covering a range of igneous processes involving olivine in terrestrial and lunar conditions. Results show that multi-valent Fe and V are redox-sensitive and more incompatible at oxidizing conditions, consistent with previous studies. The moderately volatile elements (Cu, Zn, Ga and Ge) become more volatile at reducing conditions. No correlation between partition coefficients and oxygen fugacity is observed for other multi-valent (Ti, Cr, Mn) and for homo-valent elements (Sc, Co and Ni). Most D Ol - m e l t show no sensitivity to bulk system iron contents, but D Cr Ol - m e l t is significantly higher in our experiments compared to D Cr Ol - m e l t derived from olivine-melt inclusion pairs in lunar samples with much higher FeO contents. D Ni Ol - m e l t values are nearly constant at a range of oxygen fugacities above the IW buffer, but abruptly decrease when the system is very reducing (below the IW buffer). As a result, D Co Ol - m e l t / D Ni Ol - m e l t ratios that are constant (∼0.3) at or above the IW buffer increase significantly (0.72–0.99) at IW-2. Using the newly derived partition coefficients, we re-assess two aspects of lunar basalt generation. First, we conclude that the Cr-rich nature of the olivines in lunar basalts compared to terrestrial basalts must be attributed to the Cr-nature of cumulate mantle source of lunar basalts, linked to the early crystallization of Cr-poor minerals olivine and orthopyroxene in the lunar magma ocean resulting in a shallow Cr-rich cumulates. Second, the higher Co/Ni ratios in olivine in high-titanium lunar basalts compared to olivine in low-titanium lunar basalts suggest the former were formed at more reducing conditions (below the IW buffer). [ABSTRACT FROM AUTHOR]

The partition coefficients of Sn (tin) between minerals and silicate melts (D Sn min / m e l t) are vital for understanding the Sn enrichment during magmatic processes related to tin-granites. However, experimentally determined D Sn min / m e l t values remain scarce due to the difficulty in avoiding severe Sn loss to noble metal capsules. In this study, we performed mineral/melt Sn partitioning experiments at 0.5–1.0 GPa, 850–1000 °C, and f O 2 of FMQ + 8 to ∼ FMQ − 1. The f O 2 s were imposed by solid buffers of Ru–RuO 2 , Re–ReO 2 , Ni–NiO, Co–CoO and graphite using three improved capsule designs: 1) a single sample capsule (Pt) for the Ru–RuO 2 buffered runs, 2) double capsules with Pt 95 Rh 05 as outer capsule and Re as inner sample capsule for the graphite and Re–ReO 2 buffered runs, and 3) triple capsules for the Co–CoO and Ni–NiO buffered runs, with Pt 95 Rh 05 (or Au) as outer capsule, Re lined Pt as inner sample capsule. These new capsule designs avoided significant Sn loss and enabled us to obtain accurate D Sn min / m e l t at a controlled f O 2. The experimental results show that D Sn min / m e l t values are 0.08–12.66 for amphibole, 0.01–5.55 for biotite, 0.09–10.39 for clinopyroxene, 0.004–0.97 for orthopyroxene, < 0.01 for olivine, 1.34–108.43 for Ti-magnetite, 0.04–4.17 for spinel and 0.10–0.64 for ilmenite. The large variation of D Sn for each mineral was mainly ascribed to the effect of f O 2 , which results in an arresting decrease of D Sn with decreasing f O 2. Under the reducing conditions (f O 2 < FMQ), Sn is highly incompatible (D Sn < 0.1) in almost all the minerals. Modeling results indicate that partial melting at low f O 2 conditions results in Sn enrichment in the derived magma, while subsequent high degree of fractional crystallization is also important for the Sn enrichment in the residual melt. The experimental and modeling results thus explain why major primary tin deposits are always related to reducing and highly fractionated granites. [ABSTRACT FROM AUTHOR]

In order to better constrain the alteration history of the Ryugu parent body, we performed a multi-analytical study combining scanning electron microscopy, transmission electron microscopy and infrared spectroscopy on sections extracted from the three fragments A0064-FO019, A0064-FO021 and C0002-FO019 returned from Ryugu by the Hayabusa2 space mission. The three sections show large differences in terms of structure, mineralogy and infrared signature. Section A0064-FO019 resembles the major Ryugu lithology with the presence of both fine-grained phyllosilicates (fg-phyllos) with embedded nanosulfides and coarse-grained phyllosilicates (cg-phyllos), whereas section C0002-FO019 belongs to the group of the less altered lithologies with the presence of anhydrous minerals embedded in a partially amorphous matrix. Section A0064-FO021 also belongs to this group but shows two different lithologies, a compact amorphous one and a more porous and very fractured one showing the presence of Na-rich phosphate, calcite and olivine. The two less altered lithologies (sections A0064-FO021 and C0002-FO019) show the presence of numerous mineralogical features similar to those observed in cometary interplanetary dust particles, ultra-carbonaceous Antarctic micrometeorites or in the CM Paris meteorite, i.e. amorphous and partially crystallized matrix with GEMS-like ghosts objects, whisker olivine, phosphide, or FeNi metal. This supports an outer solar system origin common with that of cometary material for the Ryugu parent body. Combined with the results of Nakamura et al. (2022b) reporting the presence of a lithology showing the presence of GEMS-like objects, we propose that section C0002-FO019 represents the onset of aqueous alteration of such primitive materials. The cg-phyllos and fg-phyllos of section A0064-FO019, i.e. of the major Ryugu lithology, representing the advanced stage of alteration, exhibit distinctive IR signatures with a higher abundance of oxygen-rich functional groups in the organic matter (OM) from the cg-phyllos. We thus suggest the following chronology of formation and evolution for Ryugu: (1) accretion of highly porous aggregate of GEMS-like units with fine-grained high-temperature anhydrous silicates, (2) onset of alteration with the dissolution of primary nanosulfides and development of amorphous/partially crystallized material in the pores, (3) crystallization of fg-phyllos with a second generation of sulfides, (4) later formation of cg-phyllos devoid of nanosulfides and their associated oxygen-rich OM in a more water-rich environment. [ABSTRACT FROM AUTHOR]

Petrologic investigations of martian meteorite Northwest Africa (NWA) 13227 indicate it is an olivine-gabbroic shergottite, a relatively new shergottite group, which differs from previously described gabbroic shergottites due to relatively high quantities of olivine. NWA 13227 is comprised of phenocrystic, oscillatory-zoned pyroxene and olivine, set in a matrix of maskelynite, Fe-Cr-Ti oxides, phosphates, and sulfides. It displays gabbroic and poikilitic textures in 2D from back-scattered electron images, and in 3D from X-ray Computed Tomography (XCT) imaging, suggesting affinities to both poikilitic and gabbroic shergottites. Measured εHf and εNd values of bulk rock (-19.7 and −5.9, respectively) and its chondrite-normalized La/Yb ratio of 1.13 indicate the specimen is derived from a mantle reservoir relatively enriched in incompatible trace elements and is similar to that which produced most 'enriched shergottites.' Based on the Ti/Al ratio of pyroxene, phosphorous zoning in olivine, and minor components in phosphates and oxides, we infer that NWA 13227 began crystallizing under reducing conditions of QFM–2.6 and temperatures of ∼ 1100 °C, consistent with conditions in Mars' lower crust/upper mantle. The sample finished crystallizing at or near the surface under redox conditions between QFM–0.5 to QFM–0.1 and temperatures of ∼ 850 °C. The volatile element compositions in apatite indicate that NWA 13227 experienced degassing during the last stages of crystallization. The timing of crystallization is estimated at 225 Ma ± 50 Ma using a Lu-Hf and Sm-Nd source versus age model. [ABSTRACT FROM AUTHOR]

Quantitative knowledge of inter-mineral and mineral–melt isotopic fractionation factors is of crucial importance to interpret variability of mass-dependent isotope ratios in terrestrial rocks and meteorites. The isotopic composition of Ni shows great promise as a tracer of planetary differentiation, but the lack of solid constraints on the sign and magnitude of equilibrium Ni isotope fractionation factors hampers its further development. We address this issue by studying Ni isotope fractionation between silicate minerals and melt using a combination of carefully selected natural samples and olivine crystallisation experiments. Mineral separates from xenoliths are used to determine the Ni isotope fractionation behaviour of pyroxenes, spinel, and garnet relative to olivine. Nickel isotope fractionation between olivine and orthopyroxene is negligible, whereas clinopyroxene shows large variability that does not reflect equilibrium. Spinel has a clear affinity for the heavier isotopes of Ni relative to olivine; garnet has the lightest Ni isotope composition at equilibrium. The key parameter that dictates the sign and magnitude of Ni isotope fractionation during partial melting and crystallisation, however, is the olivine–melt fractionation factor. We present data for a natural basalt that contains olivine phenocrysts in chemical equilibrium with the host glass, and furthermore employ olivine crystallisation experiments in a 1-atmopshere furnace. Although the experiments invariably suffer from Ni loss to the Pt wire used to suspend the experiments in the furnace, these effects were carefully considered and quantified. The experiments limit the olivine–melt Ni isotope fractionation factor to lie between zero and slightly negative, which agrees very well with the natural datum, and we derive a best estimate value for Δ60/58Ni Ol–melt of −0.142 ± 0.031 ‰ at 1000 K. Hence, olivine has a subtle but clearly resolvable affinity for the lighter isotopes of Ni relative to basaltic melt. The implication is that equilibrium partial melts of mantle peridotite should have a slightly heavier Ni isotope composition than their source. Available Ni isotope data for mid-ocean ridge basalts show some overlap with modelled partial melts but extend to notably lighter Ni isotope compositions, potentially reflecting source heterogeneity or diffusive fractionation during melt transport. [ABSTRACT FROM AUTHOR]

Angrites are a small group of basaltic meteorites and their origin is currently disputed. Among these, Northwest Africa (NWA) 12774 is a quenched angrite that was reported having an anomalously high bulk Cr 2 O 3 content (∼0.45 wt%). However, the reason behind this anomaly, which is critical for understanding the evolution of the angrite parent body, remains unknown. Here, we performed a detailed petrographic, mineralogical, and bulk oxygen and chromium isotopic composition study on this meteorite. NWA 12774 consists of porous olivine macrocrysts, phenocrysts of olivine and Al-Ti-rich augite, and spinel micro-phenocrysts with fine-grained groundmass. The olivine macrocrysts and the magnesian cores of olivine phenocrysts show compositional correlations distinctly different from typical olivine phenocrysts. The olivine macrocrysts contain small chromite/chrome-spinel inclusions which have the highest Cr 2 O 3 content (53.2 wt%) for chromite/spinel in angrites to date. Based on these textural and chemical features, the olivine macrocrysts and the magnesian cores of olivine phenocrysts are identified as xenocrysts. Some pyroxene phenocrysts contain regions with complexly zoned microtextures, which have much larger chemical variations compared with those with simply zoned microtextures. The regions with complexly zoned microtextures are likely to be of xenocrystic origin. The bulk Cr 2 O 3 content in NWA 12774 was estimated through two approaches, both of which show the bulk Cr 2 O 3 content to be around 0.3 wt% or possibly up to ∼ 0.45 wt%, which is consistent with previously measured values. The high Cr 2 O 3 content in NWA 12774 could be attributed to both the high abundance of spinel micro-phenocrysts and their high Cr 2 O 3 contents, rather than the presence of Cr-rich xenocrysts. The calculated melt rare earth element concentration in NWA 12774 equilibrated with the most Mg-rich augite is essentially identical to those in LEW 87051 and Asuka 881371, which however have Cr 2 O 3 contents much lower than NWA 12774. We suggest that the mantle source of NWA 12774 may not be as depleted in Cr and probably other volatile elements as other angrite sources. The various Cr 2 O 3 contents in different quenched angrites probably reflect that their mantle sources have not been homogenized. [ABSTRACT FROM AUTHOR]

Ferromagnesian chondrules present a remarkable dichotomy between reduced (type I) and oxidized (type II) varieties. How these formed, and how they may be related remains contentious. Many type II chondrules, especially in carbonaceous chondrites, contain forsteritic grains in disequilibrium with FeO-rich host olivine grains, which must be relicts of precursor material. In this study, we analyzed the oxygen isotopic composition of magnesian relict and host olivine grains in type II chondrules in CO and CR chondrites. The analyzed Mg-rich relicts are generally more 16O-rich than ferroan olivine (mostly host) grains and plot in the range (in term of chemistry and isotopic composition) of type I chondrules in carbonaceous chondrites. Remarkably, they tend to cluster around the dominant Δ17O peaks of the type I chondrules in their host chondrites, viz. –6 ‰ and –2 ‰ for CO and CR, respectively. With the occurrence of relatively intact type I chondrules within some type II chondrules, this corroborates that local type I chondrules were among the precursors of type II chondrules, and that chondrule formation occurred within the accretion reservoir of the eventual chondrites. This supports the nebular brand of chondrule-forming scenarios. Since not all previous generations of chondrules (or other precursor objects) have been recycled, chondrule formation events must also have been extremely localized. [ABSTRACT FROM AUTHOR]

Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10–15 % higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable μ182W values ranging from modern bulk silicate Earth-like to strongly negative values. The μ182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest μ182W value (−53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/R A). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative μ182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary. [ABSTRACT FROM AUTHOR]

The element release rates from naturally altered basalts were measured in batch experiments at 27 °C, and in mixed-flow experiments at 25 °C in reactive fluids initially consisting of pH 3 or 4 aqueous solutions. The basalts reacted in this study consist of 1) surface weathered basalts ranging in age up to 13 million years, and 2) hydrothermally altered basalts from zeolite to actinolite alteration zones (temperatures of ∼50 to 300 °C). The in situ pH in the batch experiments increased over time with the final pH of the fluids ranging from to 4 to 8. Most experiments exhibited a preferential release of Ca and Mg compared to Si from the dissolving basalts, both initially and over the long-term. This behavior is attributed to the combination of an initial rapid cation-proton exchange reaction and the relatively fast dissolution rates of Ca-bearing minerals. The BET surface area normalized Si release rates for all of the altered basalts are within one order of magnitude of each other, and from one to two orders of magnitude lower than corresponding rates from fresh basaltic glass and fresh crystalline basalts. This is interpreted to stem from 1) the fast removal of reactive basaltic glass and olivine from fresh basalts during the natural alteration of the rocks, 2) the similarity of the dissolution rates of the remaining albitic plagioclase, pyroxenes, epidote, and zeolites, and 3) the high surface areas of some less reactive secondary minerals such as clays. Overall, the results suggest that altered basalts will be effective in consuming CO 2 during both natural and engineered processes, though at a somewhat slower rate than unaltered basalts. [ABSTRACT FROM AUTHOR]

Carbon dioxide removal (CDR) technologies at a gigaton scale need to be developed and implemented within the next decades to keep global warming below 1.5 °C. Coastal enhanced silicate weathering is one of the proposed CDR techniques that aims to accelerate the natural process of CO 2 -sequestration during marine chemical weathering of silicate minerals. To this end, finely ground rock containing olivine (Mg x Fe 2− x SiO 4) could be dispersed in dynamic coastal environments, where local biotic and abiotic factors potentially enhance the weathering process. However, accurate predictions of the olivine dissolution rate and the associated CO 2 sequestration under in situ conditions are currently lacking and ecosystem impacts remain to be assessed. Previously, it has been hypothesized that in situ grain collisions, induced by bed load transport due to currents and waves, could accelerate the in situ chemical weathering of olivine particles. To examine this, we investigated the effects of continuous grain tumbling on olivine dissolution in natural seawater. A 70-day experiment was conducted in which forsterite olivine sand was continuously tumbled in filtered seawater at different rotation speeds, and dissolution rates were measured on a weekly basis. Results showed that continuously tumbled olivine dissolved 8 to 19 times faster compared to stagnant (no rotation) conditions. Olivine dissolution was complete and stoichiometric (except for Ni release), air-seawater CO 2 exchange was not significantly rate limiting, and minimal particle fragmentation and secondary mineral formation were observed. Hence, we infer that olivine weathering was mainly enhanced via advective pore water flushing, which limits saturation effects at the grain scale. Overall, this study provides evidence that ambient physical stresses in coastal environments could enhance marine silicate weathering, which has implications for both the natural silicon cycle as well as the use of enhanced coastal weathering of silicates as a CDR technique. [ABSTRACT FROM AUTHOR]

In order to better constrain the Mg isotopic composition of the mantle, we have analysed twenty-eight samples of both oceanic and continental peridotite using a high-precision, critical mixture double spiking approach. The unaltered samples show no variability δ26Mg in outside analytical uncertainty and yield a value of −0.236 ± 0.006‰ (2 s.e.) for the accessible mantle, substantiating its non-chondritic composition. We have also determined inter-mineral Mg isotopic fractionations for a sub-set of samples. We document small but significant differences in δ26Mg between olivine and pyroxenes, Δ26/24Mg ol/cpx = −0.118 ± 0.018‰ and Δ26/24Mg ol/opx = −0.056 ± 0.018‰, in excellent agreement with ab initio calculations for temperatures ∼1000 °C, as recorded by mineral thermometry in the peridotites. The differences in δ26Mg between olivine and spinel (Δ26/24Mg ol/sp) are more variable and generally higher than theoretical calculations at corresponding temperatures, likely due to incomplete Fe-Mg diffusive exchange during post-eruptive cooling of the xenoliths. Using these data, together with a recently determined olivine-melt fractionation factor for Mg isotopes, we show that partial melting has a negligible influence on the δ26Mg of residual peridotites. This helps account for the minimal variability of δ26Mg in fresh, mantle peridotites. However, the δ26Mg of primary mantle melts are predicted to be discernibly higher than their sources (Δ26Mg ∼ 0.06‰ and ∼0.123‰ for representative partial melts of peridotitic and pyroxenitic sources respectively) across a wide range of melting conditions. Such elevated δ26Mg values are not generally observed in the current dataset of mantle derived melts. We propose that this inconsistency is likely a consequence of diffusive fractionation during partial re-equilibration between low Mg/Fe melts migrating through high Mg/Fe mantle en route to the surface. [ABSTRACT FROM AUTHOR]

Mare basalts collected at the Apollo 15 landing site can be classified into two groups. Based on differing whole-rock major element chemistry, these groups are the quartz-normative basalt suite and the olivine-normative basalt suite. In this study we use modelling of Fe-Mg interdiffusion in zoned olivine crystals to investigate the magmatic environments in which the zonation was formed, be that within the lunar crust or during cooling within a surficial lava flow, helping to understand the thermal histories of the two basalt suites. Interdiffusion of Fe-Mg in olivine was modelled in 29 crystals in total, from six olivine-normative basalt thin sections and from three quartz-normative basalt thin sections. We used a dynamic diffusion model that includes terms for both crystal growth and intracrystalline diffusion during magma cooling. Calculated diffusion timescales range from 5 to 24 days for quartz-normative samples, and 6 to 91 days for olivine-normative samples. Similarities in diffusion timescales point to both suites experiencing similar thermal histories and eruptive processes. The diffusion timescales are short (between 5 and 91 days), and compositional zonation is dominated by crystal growth, which indicates that the diffusion most likely took place during cooling and solidification within lava flows at the lunar surface. We used a simple conductive cooling model to link our calculated diffusion timescales with possible lava flow thicknesses, and from this we estimate that Apollo 15 lava flows are a minimum of 3–6 m thick. This calculation is consistent with flow thickness estimates from photographs of lava flows exposed in the walls of Hadley Rille at the Apollo 15 landing site. Our study demonstrates that diffusion modelling is a valuable method of obtaining information about lunar magmatic environments recorded by individual crystals within mare basalt samples. [ABSTRACT FROM AUTHOR]

Metal-silicate segregation is one of the most fundamental mechanisms in planetary differentiation, with primitive achondrites offering important constraints on this process. Brachinites and brachinite-like achondrites (BLA) are olivine-dominated primitive achondrites that experienced up to ∼20% partial melt removal under relatively oxidized (ΔIW ∼ −1) conditions within an initially chondritic parent body and represent residues after inefficient metal-loss. We present bulk rock and in situ lithophile and highly siderophile element (HSE) abundance systematics as well as 187Re-187Os data for five olivine-rich primitive achondrites. These new data confirm classification of Reid 013 as a brachinite, three of the samples as BLA (Northwest Africa [NWA] 6874, NWA 7499, and Miller Range 090805), and the final sample as an ungrouped primitive olivine-rich achondrite (NWA 7680). An aliquot of MIL 090805 shows amongst the highest total HSE contents (>35 ppm) and the highest Pt content (∼23 ppm) of any primitive achondrite. Compiled HSE data for brachinites and BLA show correlations between total HSE abundance, Pt enrichment, and average olivine Fo. This correlation can be explained by variable melting (∼10–20%) of an H ordinary chondrite-like protolith, with retention of both Fe-metal and a Pt-rich alloy phase distinct from the observed Fe-metal phases in more depleted residues. Such a Pt-alloy phase is likely stabilized by elevated abundances of the HSE during chondrite melting and the low solubility of HSE in melts. Rhenium-Os isotope data in the studied samples has been modified by recent mobilization of Re during terrestrial weathering, with the limited range of measured 187Os/188Os in brachinites and BLA supporting minor fractionation of Re/Os during melting and an ancient (∼4.5 Ga) partial melting event to explain their compositions. These results indicate that models of planetary differentiation should consider the low solubility of Pt in chondrite melts and the potential for alloy formation to modify HSE abundances of silicate mantles. [ABSTRACT FROM AUTHOR]

The rate of copper diffusion in olivine governs the preservation of copper in olivine and olivine-hosted melt inclusions, but Cu diffusivity in olivine has not been well determined especially regarding its anisotropy. In this study, Cu diffusion in single crystal olivine (Fo 91 Fa 09) cuboid, prepared to comply with the three principle crystallographic axes, was investigated at 1273–1673 K and 1–2.5 GPa in a piston cylinder apparatus, using a diffusion-in approach with Cu or Cu 2 O as the source. Copper concentration profiles were analyzed by LA-ICP-MS scanning on the recovered samples along multiple directions. We found Cu diffusion in olivine to be appreciably anisotropic, with Cu diffusivity along the c axis being greater than that along the a axis by nearly an order of magnitude (D c ≈ 3 D b ≈ 7.5 D a). The activation energy for Cu diffusion is ∼210 kJ/mol, with little dependence on crystallographic orientation. Cu diffusivity at 2.5 GPa is only slightly lower than that at 1 GPa, yielding a small activation volume of 2.3 cm3/mol. While not being as rapid as the migration of H and interstitial Li, Cu diffusion, especially along the c axis, is significantly faster than the major elements of olivine (Fe-Mg, Si and O). Diffusion modeling indicates that the timescale for re-equilibration of Cu between olivine-hosted melt inclusion and external melt is relatively short (<60 yr at 1000 °C and <3000 yr at 800 °C) compared to the typical lifetime of magma chambers. Cu concentrations in olivine-hosted melt inclusions may therefore have been significantly modified from the pristine values. For Cu profiles preserved in olivine phenocryst, our determined Cu diffusivity can be used to constrain its residence time in magma chamber, and the diffusion anisotropy can provide a cross-check. [ABSTRACT FROM AUTHOR]

In the past few years, zinc isotope systematics of basaltic magmas have been widely used as novel proxies for terrestrial mantle heterogeneity induced by recycled crustal materials and for planet formation and evolution. The influence of crystal-melt isotope disequilibrium during magma differentiation on the zinc isotopic composition of basaltic melts, nonetheless, has received little attention. In addition, no quantitative constraint has yet been given for the diffusion-driven Zn isotope fractionation between olivine crystals and melts. Here we present high-precision zinc isotope data (δ66Zn JMC-Lyon) for a series of olivine phenocrysts separated from intra-plate alkali basalts from the Jiaodong Peninsula in Eastern China, together with in-situ chemical analysis. Olivine phenocrysts have Zn isotopic compositions which are too light in comparison with the host basaltic melts (Δ66Zn ol-melt = −0.41‰ to −0.10‰; n = 16) to be explained by equilibrium isotope fractionation at magmatic temperatures. Instead, the decrease of δ66Zn values with decreasing Mg# (i.e., Mg/[Mg + Fe2+] × 100) and increasing Zn + Fe2+ contents in olivine phenocrysts suggests diffusion-driven kinetic fractionation during olivine crystallization. The data is well-fitted with a diffusion model in which Zn together with Fe2+ diffuse from surrounding melt into olivine crystals and approximately equal flux of Mg diffuses into melt due to the large chemical gradient, yielding a kinetic Zn isotope fractionation factor β Zn of 0.07. By utilizing the fractional crystallization model under equilibrium or disequilibrium conditions, it is suggested that diffusion-induced isotope disequilibrium during olivine crystallization may drive Zn isotopic composition of the residual melt toward heavier values. When using zinc isotope systematics of any basaltic magma to probe the source heterogeneity, this impact must be taken into account if the Zn isotope disequilibrium widely exists in olivine phenocrysts. Based on the time-related quantitative diffusion model, our results show that the magma residence time for strongly alkali basaltic lavas (∼20 days) is 10–60 times shorter than that for weakly alkali basaltic lavas (6 months to 4 years). This supports the generation of weakly alkali basalts via interaction between silica-undersaturated melts and the surrounding lithospheric mantle, implying a probable common mechanism in intra-plate basalts that could be comparable globally. Thus, Zn isotope disequilibrium between olivine crystals and melts could provide valuable information on the evolution history of intra-plate basaltic magmas. [ABSTRACT FROM AUTHOR]

Oxygen-isotope studies of carbonaceous chondrite chondrules are of pivotal importance for understanding of the evolution of the solar protoplanetary disk. It has been previously concluded that the observed variations in Δ17O (=δ17O – 80.52 × δ18O) of chondrule olivines and pyroxenes are intimately tied to their Mg# (=MgO/(MgO + FeO) × 100, mol%) and the inferred oxygen fugacity (f O 2) of gaseous reservoirs produced by evaporation of disk regions with different (silicate dust ± water ice)/gas ratios. Using high resolution cathodoluminescence and secondary ion mass spectrometry, we show, in contrast to host chondrule data, that Δ17O of chondrule olivines from the Yamato 81020 CO3.05 carbonaceous chondrite is independent of their Mg# and of the imposed f O 2. Instead, there is a Δ17O bimodal distribution of Mg-rich olivines that gradually turns into a unimodal distribution as Mg# decreases. We suggest that chondrules recorded an evolution of an isotopically heterogeneous vapor plume resulting from a high temperature mixing of the 16O-enriched (Δ17O ≈ −6 ± 2‰) and 16O-depleted (Δ17O ≈ −2.5 ± 1‰) reservoirs. Drop in the vapor plume temperature under unbuffered redox conditions favors the dissolution of Fe,Ni-metal of chondrules and the subsequent crystallization of FeO-rich olivines at saturation; the 16O-depleted signature being the most resilient at lower temperature. Chondrules are thus inferred to have formed in a same turbulent heterogeneous environment in which locally high temperatures and reducing conditions prevailed (Type I), adjacent in space and/or in time to areas submitted to lower temperatures and more oxidizing conditions (Type II). Subtracting of chondrules at different stages of the gaseous plume evolution by fast cooling rates give rise to the chemical and isotopic diversity of chondrules. No extrinsic oxidizing agents (e.g., water ice) are needed in this scenario. [ABSTRACT FROM AUTHOR]

Alkaline magmatism is an important chemical end-member of magmatic activity that typically occurs in response to small volume melting of asthenospheric- and/or lithospheric mantle material in intra-continental settings. Understanding trace element partitioning and phase equilibria during alkaline magmatism can therefore provide constraints on intra-continental geodynamic settings. However, the partitioning of trace elements between alkaline melts and their dominant equilibrium mineral phases remains poorly constrained. Feldspathoids in particular have received limited attention with regards to their trace element contents, hampering our ability to interpret geochemical trends in alkaline magmatic systems. In this study, we performed a series of 1 atmosphere experiments in a gas-mixing furnace using a variety of highly alkaline (Na 2 O + K 2 O = 4.15–14.97 wt%) and silica-undersaturated (SiO 2 = 36.73–45.96 wt%) lava compositions from Nyiragongo, Democratic Republic of Congo, in order to investigate the partitioning behaviour of trace elements in minerals from alkaline magmas. Experimental runs were performed with oxygen fugacity buffered at both QFM (quartz-fayalite-magnetite equilibrium) and QFM + 1 and cover a range of geologically-relevant temperatures (1025–1200 °C). The quenched products of these experiments contained leucite, nepheline, melilite, clinopyroxene, olivine, and rhönite crystals, of which glass-crystal pairs were analysed for rare earth elements, large-ion lithophile elements, and high-field-strength elements. Leucite and nepheline host considerable quantities of large-ion lithophile elements but take up negligible amounts of more highly charged cations. Åkermanitic melilite readily incorporates mono- to trivalent cations with a preference for light over heavy rare earth elements, but incorporates only select divalent cations. Rhönite and clinopyroxene have analogous partitioning behaviours, with a strong preference for heavy over light rare earth elements. Fractionation modelling using the reported partitioning behaviours reproduces the 2021 eruption products of Nyiragongo, with 48% fractionation from an olivine-melilitic parental melt composition. Crystallization of trace-element poor feldspathoid amplifies pre-existing high LREE/MREE ratios of the parental magma and progressively increase trace element abundances for all but monovalent cations. [ABSTRACT FROM AUTHOR]

We conducted a transmission electron microscopy (TEM) study of an unusual oxide-silicate composite presolar grain (F2-8) from the unequilibrated ordinary chondrite Semarkona (LL3.00). The presolar composite grain is relatively large (>1 µm), has an amoeboidal shape, and contains Mg-rich olivine (forsterite), Mg-Al spinel, and Ca-rich pyroxene. The shape and phase assemblage are reminiscent of amoeboid-olivine-aggregates (AOAs) and add to the growing number of TEM observations of presolar refractory inclusion-like (CAIs and AOAs) grains. In addition to the dominant components, F2-8 also contains multiple subgrains, including an alabandite-oldhamite composite grain within the olivine and several magnetite subgrains within the Mg-Al spinel. We argue that the olivine, Mg-Al spinel, and alabandite-oldhamite formed by equilibrium condensation, whereas the Ca-rich pyroxene formed by non-equilibrium condensation, all in an M-type AGB star envelope. On the other hand, the magnetite subgrains are likely the result of aqueous alteration on the Semarkona asteroidal parent body. Additional evidence of secondary processing includes Fe-enrichment in the Mg-Al spinel and olivine, elevated Al contents in the olivine, and beam sensitivity and a modulated structure for the olivine. Compound presolar grains, in particular oxide-silicate AOA-like grains such as F2-8, record condensation conditions over a wide range of temperatures. Additionally, the presence of several different presolar phases in a composite grain can impart information on the relative rates and effects of post-condensation processing in a range of environments, including the interstellar medium, solar nebula, and the host asteroid parent body. For example, the olivine and spinel in F2-8 show evidence of fluid infiltration, but each component reacted in different ways and to different extents. The TEM observations of F2-8 provide insights across the lifetime of the grain from its formation by condensation in an M-type AGB star envelope, its transit through the interstellar medium, and aqueous alteration during its residence on Semarkona's asteroidal parent body. [ABSTRACT FROM AUTHOR]

Natural olivine from the Etna Fall Stratified eruption (3930 BP) was dehydrated to determine hydrogen diffusivity. Dehydration was carried out in several time-steps, from 0.5 to 40 cumulative hours, in a 1 atm. furnace at 810 and 1000 °C. This starting material has high H 2 O concentration (∼50 ppm), high forsterite (Fo ∼ 90 %), and high fO 2 (∼NNO + 1.2), reflecting characteristics of its host magma. H 2 O concentration was measured using Fourier transform infrared spectroscopy after each heating step. Spectral peaks were fit with Gaussian curves to determine the contribution from each H+ defect. Zonation profiles along each crystallographic direction were modelled using the 'whole-block' method to determine bulk and defect-specific H+ diffusivity. Our modelling reveals a common apparent diffusivity for all defects, which is faster along ([1 0 0]) than b ([0 1 0]) or c ([0 0 1]) by more than an order of magnitude. A common defect-specific diffusivity has not been found in experiments on pure forsterite, so it appears that the presence of Fe enables H+ from all defects to diffuse via the proton-polaron mechanism (supported by the observed anisotropy). However, the rates that we observe are slower than the proton-polaron mechanism, which supports the recent proposal by Ferriss et al. (2018) that the apparent diffusivity is rate-limited by reactions between H+ bound in defect sites and Fe2+. A coupled reaction and diffusion process may explain the observed decrease in diffusivity along a over time, as well as an apparent non-zero H 2 O concentration at the crystal edge. Both of these effects need to be considered when modelling H+ diffusion profiles to determine decompression rate – fixing the edge concentration to zero will underestimate the diffusion timescale. Arrhenius laws describing diffusion parallel to a (at the start and end of the experiments), b , and c are developed from the diffusivity at 810 and 1000 °C: Da start = 10 - 2.94 e x p - 182000 RT , Da end = 10 - 4.63 e x p - 154000 RT , Db = 10 - 2.35 e x p - 243000 RT , Dc = 10 - 4.01 e x p - 187000 RT , where R is the gas constant 8.314 (J/mol K); T is the temperature (K), the units of the diffusivities (D) are in m2/s. A comparison of experimental dehydration studies on natural olivines reveals that the diffusivity during dehydration likely depends on how the olivine was experimentally hydrated. To avoid this experimental effect and understand the diffusivity operating during ascent in a volcanic conduit, crystals which are naturally hydrous (such as the Etna olivines in this study) are particularly useful. Despite some ambiguity in the literature owing to different experimental approaches, our results suggest that for the studied range of pressure and temperature, forsterite content is the major control on H+ diffusivity, while H 2 O concentration and the fO 2 at which the crystal last equilibrated have a smaller effect within the ranges studied for magmatic olivines. [ABSTRACT FROM AUTHOR]

The origin of solar 11B/10B ratio ∼4 remains an open question. It has been thought that a significant portion of boron in the Solar System was derived from continuous spallation nucleosynthesis during interactions between Galactic Cosmic Rays and C N O nuclei in the interstellar medium. However, because GCR-produced boron is characterized by 11B/10B ∼2.5, an endmember with 11B/10B > 4 is required to account for the solar 11B/10B ratio. Two leading hypotheses for the sources of 11B-rich components include low energy spallation in the Sun's parental molecular cloud and the neutrino process during the supernova explosions. In this study, lithium and boron elemental and isotopic compositions of seven porphyritic olivine chondrules and one isolated olivine crystal from four meteorites (Allende, Yamato 81020, Asuka 12236, and QUE 97008) were determined in-situ to help constrain which of the two nucleosynthetic mechanisms has most likely supplied the forming Solar System with extra 11B. Apparent isotopic variations in Li/Si, B/Si, δ7Li and δ11B were found in chondrule olivine crystals, but only three chondrules exhibit statistically resolved δ11B heterogeneities. Using these three chondrules as a basis for discussion, we evaluated the processes that could potentially cause elemental and isotopic variations of Li and B. We argue that the data can be best understood in the context of condensation of lithium-boron-rich material onto chondrule precursors in (an) initially heterogeneous gaseous reservoir(s), which could be realized if the Solar System derived 11B-rich components from a supernova or supernovae. [ABSTRACT FROM AUTHOR]

The mechanisms responsible for the formation of Fe-rich, metasomatic features in eucrites and their debris in howardites are yet controversial. In this study, secondary phases found in polymict eucrite NWA (Northwest Africa) 11911, howardite NWA 1664, monomict eucrite NWA 8675 and unbrecciated basaltic eucrite NWA 13269 were investigated to elucidate their origin and history of formation. Among them, NWA 11911 is the only rock that displays widespread Fe-enrichment near fractures in unequilibrated and slightly equilibrated basaltic clasts and pyroxene fragments. Fayalitic olivine veinlets with various textures, thicknesses and compositions (Fa 64-82) are detected in unequilibrated pyroxenes in NWA 11911, NWA 1664 and NWA 8675. Neighbouring Fe-enriched pyroxene is also Al-depleted compared to magmatic pyroxene (Al 2 O 3 = ∼0.3 wt%). NWA 13269 contains Ca-rich plagioclase (An 97-98) veinlets associated with Cr-spinel, but apparently no Fe-enrichment in pyroxene nor ferroan olivine despite a careful search. Olivine veinlets have been also detected in equilibrated pyroxene in NWA 11911. Our observations support the fluid-assisted metasomatism scenario that involves the circulation of hydrothermal fluids throughout the fractures of eucritic material. [ABSTRACT FROM AUTHOR]

Past experiments and observations on natural samples have largely focused on the roles of olivine and chromite in controlling the behaviour of the platinum-group elements (PGE) during melting and solidification, whereas other phases, such as pyroxene, have gone largely uncharacterized. To address this, experiments have been done to measure the partitioning of Pd (with a subset of results for Ir), between orthopyroxene and silicate melt at 1340 °C, 0.1 MPa and log f O 2 of FMQ − 1 to FMQ + 6 (FMQ = Fayalite-Magnetite-Quartz). The X-ray Absorption Near-Edge Structure (XANES) was measured in a subset of experiment glasses. Glass concentrations of Pd (corrected to unit Pd activity) increase from ∼6 to ∼650 ug/g over the f O 2 range of experiments. The slope of the solubility- f O 2 relation is consistent with Pd1+ as the dominant oxidation state, with evidence for Pd0 and Pd2+ at the lowest and highest experiment f O 2 , respectively. Consistent with this result, the XANES reveal spectral features similar to Pd0 and Pd2+ spectral reference materials (specRM) at the most reduced and oxidized synthesis conditions, respectively. Other lines of evidence require the presence of a third melt species, here interpreted to be Pd1+. Values of orthopyroxene/melt partition coefficients for Pd (D Pd Opx/melt) are 0.0051 (+/−0.006) at log f O 2 < ΔFMQ + 3, increasing with f O 2 to a maximum of 0.013 at ∼FMQ + 6. Sodium partition coefficients, expected to be similar to Pd, range from 0.0061 (+/−0.00061) at FMQ + 3, increase to 0.007–0.009 at higher f O 2 , but with no clear systematic trend. A value for D Ir Opx-melt of ∼0.6 was measured at ∼FMQ + 4, indicating significantly more compatible behaviour for Ir relative to Pd. Partitioning results are interpreted in the context of the Blundy-Wood elastic strain model in which the variation in partitioning is related to ionic radius mismatch to an optimal crystallographic site size. Based on the trend in ionic radius with oxidation state, the estimated ionic radius of Pd1+ in octahedral coordination is similar to Na1+, and comparison to previous orthopyroxene-melt partitioning experiments suggests D Pd1+ opx/melt and D Na opx/melt should be nearly identical, consistent with the results of this study. The ionic radius of VI-fold Pd2+ is close to the optimal M2 site size, so an increased proportion of this species with f O 2 accounts for the larger values of D Pd opx/melt at the highest f O 2 investigated. The much larger partition coefficient for Ir is consistent with the presence of Ir2+, whose estimated ionic radius is close to Fe2+ and Mg2+, as well as predictions for the optimal M1 site size. With the assumption that D Pd opx/melt = D Na opx/melt, combined with a revised value for the Pd content of the primitive mantle, a melting model is presented that better reproduces the Pd concentration of high degree melts from sulfide-free mantle sources. [ABSTRACT FROM AUTHOR]

A general assumption about the geochemical behavior of phosphorus (P) is that it exists exclusively in the +5 oxidation as phosphate. However, in extremely reducing environments, other oxidation states of phosphorus such as +3 may also be stable. Such environments—if prevalent globally—may determine planetary habitability, which is in part governed by nutrient availability, including the availability of the element phosphorus. Here we show a route to P liberation from water-rock reactions that are thought to be common throughout the Solar System. We report the speciation of phosphorus in several serpentinite rocks and muds to include the ion phosphite (HPO 3 2− with P3+) and show that reduction of phosphate to phosphite may be predicted from thermodynamic models of serpentinization. Furthermore, the amount of phosphite exceeds the amounts predicted from thermodynamic models in three of nine samples analyzed. As a result, as olivine and other silicates in ultramafic rocks alter to serpentine minerals, phosphorus as the significantly more soluble and reactive phosphite ion should be released under low redox conditions, liberating this key nutrient for life. Thus, this element may be accessible to developing life where water is in direct contact with ultramafic rock, providing a source of this nutrient to potentially habitable worlds. [ABSTRACT FROM AUTHOR]

We discuss if the detection of aqueous alteration depends on the techniques that are used. We apply different methods to estimate the extent of aqueous alteration on four ungrouped carbonaceous chondrites showing limited aqueous alteration and thermal metamorphism: Chwichiya 002, El Médano (EM) 200, Northwest Africa (NWA) 12957 and NWA 11750, classified as C3 or C3.00-ung. The aim is to propose a reliable methodology to identify the most primitive chondrites. Chwichiya 002, NWA 11750 and NWA 12957 display very primitive matrices and could be amongst the most primitive chondrites currently known, similar to the least altered lithologies of the CM chondrites Paris (CM2.9) and Asuka (A) 12085 (CM2.8), A 12236 (CM2.9) and A 12169 (CM3.0). The structure of organic matter and Cr 2 O 3 in ferroan olivines show that the four meteorites have been less heated than the least metamorphosed standard/reference type 3 chondrite, Semarkona (LL3.00), with Chwichiya 002, NWA 12957 and NWA 11750 similar to the CO3.0 s, Acfer 094 (C 2-ung) and Paris meteorites. Chwichiya 002 and NWA 12957 show similar alteration phases and degree of alteration, with high abundances of amorphous material with embedded metal and sulfide, resembling Glass with Embedded Metal and Sulfide (GEMS)-like materials, and tochilinite-cronstedtite intergrowths (TCIs) as the major alteration phases. The matrix in NWA 11750 contains aggregates of nanoscale olivine crystals and abundant carbonates, observed as micrometer-sized carbonate veins surrounding chondrules, and as nanoscale carbonates mixed with the fine-grained materials. It also contains abundant grains of metal and a low abundance of phyllosilicates. El Medano 200 shows a high abundance of magnetite (∼10 vol%), nanoscale phyllosilicates, troilite, and organic matter. The variability of the secondary alteration phases in the meteorites suggests different alteration mechanisms, likely depending on both the starting composition of the meteorites and the composition of the fluids of alteration. Scanning and transmission electron microscopy (SEM and TEM) allow the identification of primitive phases and the composition and spatial distribution of the secondary phases. X-ray diffraction (XRD) can detect alteration products, including some amorphous phases, although this is limited by the small coherence domains of small TCIs and other phyllosilicates. Transmission infrared (IR) spectroscopy can detect phyllosilicate and carbonate, but is ineffective for the detection of amorphous phases, metal, or sulfide. Both matrix defocused electron microprobe analyses (EMPA) and thermogravimetric analysis (TGA) allow detection of hydrated minerals, such as phyllosilicates and carbonates, but are strongly influenced by the presence of organic matter and do not reflect the overall alteration state of a meteorite. We conclude that the assessment of the primitivity of a chondrite is highly technique dependent. We propose a combination of XRD and the Cr 2 O 3 in ferroan olivines or Raman spectroscopy for a rapid characterization of the alteration state of a chondrite and the detection of the most primitive meteorites. Finally, the combination of XRD and TEM allows for the detection of all primary and secondary phases and represents an ideal methodology for the characterization and detailed study of primitive chondrites and the different types of incipient aqueous alteration. [ABSTRACT FROM AUTHOR]

Serpentinization of olivine-rich ultramafic rocks is recognized to have been widespread across the solar system throughout its history, with substantial implications for the chemical and physical properties of planetary lithospheres, atmospheric compositions, and astrobiology. One especially significant product of serpentinization is molecular hydrogen (H 2), whose generation is closely linked to the oxidation of Fe as serpentinization proceeds. While numerous experimental simulations of serpentinization have been conducted over the years, these studies have been performed almost exclusively using reactant minerals that contain relatively high Mg and low Fe contents representative of terrestrial mantle rocks. In contrast, very few studies have been conducted with the more Fe-enriched mineral compositions that may predominate on other solar system bodies. In this study, an experiment was conducted to investigate mineral alteration and H 2 generation during serpentinization of Fe-rich olivine (hortonolite; Fo ∼62) at 230 °C and 35 MPa. After 3500 h of reaction, ∼55 % of the hortonolite reacted to secondary minerals composed of serpentine (chrysotile) and magnetite. Chrysotile contained proportionally less Fe than the original hortonolite, reflecting the partitioning of some Fe into magnetite; however, it contained substantially more Fe than serpentine precipitated from alteration of Mg-rich, Fe-poor terrestrial mantle olivine (Fo ∼90) under the same reaction conditions. Reaction of hortonolite also produced more than four times as much magnetite as Mg-rich olivine. Generation of H 2 occurred steadily throughout the experiment, with more than five times as much H 2 generated per mole of hortonolite reacted than observed for Fe-poor olivine at the same conditions. The results suggest that serpentinization of Fe-rich ultramafic rocks on Mars and other planetary bodies may have a substantially greater capacity to generate H 2 and to precipitate magnetite than their Fe-poor terrestrial counterparts, which would enhance their potential to support H 2 -based biological communities, contribute to atmospheric warming, and augment local magnetic signatures in planetary lithospheres. [ABSTRACT FROM AUTHOR]

Extreme conditions encountered in some geological contexts (deep serpentinization, interaction of Venus atmosphere with its basaltic surface, volcanic degassing) activate mechanisms and rates of silicate alteration that are poorly understood. In the present study, we investigate the mechanisms of mineral reactions in a natural geological system at high temperature, under conditions where the low solvation of cations by fluids likely promotes surface reactions such as surface diffusion and/or local recrystallization. We focus on vitreous glasses and olivine, reputed to be the most alterable phases in volcanic rocks, by reacting samples for one week in a Ni-based alloy experimental vessel. For the framework of our experimental study, we chose to apply the deep atmosphere conditions on Venus: 470 °C and 90 bar of reconstituted Venus-like gas. We also tested the effect of water (Early Venus or wet volcanic degassing) by adding water vapor at up to 320 bar total pressure. The mineral reactions affecting the samples were identified by a set of spectroscopic surface analyses of the altered samples: Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray Diffraction in grazing incidence mode, X-ray Photo electron Spectroscopy and Raman spectroscopy. Samples of obsidian and tholeiitic glasses are found to be sensitive to a threshold water pressure, depending on glass composition, below which the reaction is limited to some elemental mobility in the glass (alkali enrichment, calcium loss) leading to a possibly more stable surface layer of tens to hundreds of microns. Above this threshold water pressure (ca. 50 bar H 2 O for the obsidian but >250 bar H 2 O for the tholeiitic glass), water promotes the depolymerization of the glass and the crystallization of stable minerals. This crystalline rim is less protective that the chemically modified layer. Olivine samples react differently depending on whether the olivine is isolated or included in a basaltic rock. In the latter case only, iron coatings are formed, which are identified as hematite, suggesting that this phase is not fed by olivine itself but rather by surface diffusion from neighboring Fe-rich phases. This supports the conclusions from experimental studies and orbital observations on the short-term visibility of unaltered olivine in Venus lava flows: such a coating is enhanced when Fe-bearing minerals are in the proximity of olivine. Under high water vapor pressure, Fe-bearing talc (and not serpentine) forms by a likely topotactic reaction that also incorporates silica from the gas. This talc layer may form a protective layer, implying that serpentinization of ultramafic rocks at high temperature may not be as prevalent as one might think in a gas-dominated system like the Early Venus surface. [ABSTRACT FROM AUTHOR]

CI chondrites have nonvolatile chemical compositions closely resembling that of the Sun's photosphere and are thus considered to have the most primitive compositions of all known solar system materials. They have, however, experienced pervasive parent-body alteration processes that transformed their primary constituents, obscuring the nature and origin of primordial CI dust. We used in-situ quantitative microprobe and secondary ion mass spectrometry techniques to characterize the chemistry and oxygen isotopic compositions of anhydrous silicates in two sections of the CI chondrites Ivuna and Alais, which contain higher abundances of those than other CI samples. These silicates are Mg-rich olivine and low-Ca pyroxene crystals mostly occurring as aggregates within sub-mm Fe-rich clasts. Our data reveal mass-independent oxygen isotopic variations with Δ17O values ranging from −23.63 to −0.57‰, representing the first evidence of extremely 16O-rich (Δ17O < −20‰) olivine and pyroxene grains in CI chondrites. Two of these olivines are characterized by MnO/FeO ∼ 1, typical of low-iron, Mn-enriched silicates commonly observed in amoeboid olivine aggregates. Other anhydrous silicate grains have Δ17O values ranging from −6 to 0‰, probably representing chondrule fragments. Combined, these results indicate that chondrule and refractory inclusion material were incorporated into the CI parent body(ies). This conclusion is consistent with recent models showing that refractory inclusions could have formed and/or been transported at larger heliocentric distances than previously thought during the concomitant injection of material from the molecular cloud and outward extension of the disk by viscous spreading. The CI chondrules are presumably of local origin, with their isotopic systematics suggesting an affinity with the CR clan. [ABSTRACT FROM AUTHOR]

• Ca-siderite comprises between trace amounts and 4% of Lafayette sections. • Electron Microscopy and synchrotron analysis shows variable carbonate dissolution. • Ferric-(hydr)oxide nanoparticles are present on saponite and corroded siderite. • Serpentine-berthierine is re-described as odinite, a ferric low-temperature mineral. • Dissolution of carbonate can lead to methane on Mars via a Sabatier type reaction. New mineralogical studies of Lafayette reveal that it contains a notably variable abundance of martian carbonate. Four percent was identified in mesostasis (3.2%) and olivine-hosted (0.8%) fractures in one polished section, but only 0.2% of both textural types in another. The Lafayette carbonates are Mg 0.0-2.0 Cc 13.2-38.6 Sd 17.7-81.9 Rh 3.1-42.9. They have undergone variable but extensive amounts of dissolution and replacement as the nakhlite secondary fluid evolved, associated with the precipitation of ferric saponite in olivine fractures and a serpentine-like phyllosilicate in the mesostasis. The mesostasis carbonate has undergone the highest degree of corrosion and replacement. TEM analysis has shown the presence of Fe-(hydr)oxide (likely ferrihydrite) nanoparticles on olivine-hosted carbonates which can be linked to the cessation of more extensive carbonate dissolution at those sites. The mesostasis serpentine-like mineral has been described here on the basis of WDS and EDX analyses, HRTEM and Fe-K XANES, as odinite, a ferric, 0.7 nm d 001 -spacings phyllosilicate mineral with a characteristic 1:1 serpentine-like structure. The carbonate dissolution stage and then formation of Fe-(hydr)oxide nanoparticles occurred under circumneutral-alkaline conditions (7) < pH < 10. This range of pH is also where the general dissolution mechanism switched from a proton-promoted, to a water hydrolysis reaction associated with a reduction in the dissolution rates. As dissolution rates were reduced and the fluid had cooled to ≤50 °C, the precipitation of the ferric saponite and odinite, a phyllosilicate associated with temperatures of ∼25 °C, dominated over the carbonate dissolution. The extensive dissolution of such crustal carbonate across the upper martian crust, producing bicarbonate and carbon dioxide, and the coupled formation of ferric phyllosilicates, would lead to the formation of CH 4 in substantial amounts via a Fischer-Tropsch type reaction. The results of our study illustrate a process to explain the relatively low abundance of detected carbonate on Mars and a likely source for some of the methane on Mars. [ABSTRACT FROM AUTHOR]

Geochemical and cosmochemical reservoirs show Cu/Ag variations from 500 to 40,000. To understand magmatic Cu–Ag fractionation and origins of such large Cu/Ag variations, we have measured the partition coefficients of Cu and Ag between minerals and silicate melts (D Cu , A g Min / S M) relevant for partial melting of the silicate mantles of Earth, Mars, and the Moon. The experiments were conducted at 1–3 GPa, 1300–1600 °C, and oxygen fugacity ∼2–5 log units below the FMQ buffer. The results show that D Cu Min / S M are 0.034–0.109 for olivine, 0.011–0.044 for orthopyroxene, and 0.046–0.092 for clinopyroxene; D Ag Min / S M are 0.003–0.012 for orthopyroxene and 0.037–0.093 for clinopyroxene. These D Cu , A g Min / S M values increase mainly with increasing pressure and are oxygen fugacity-independent. D Ag Min / S M are 0.0005–0.0017 for olivine, which do not show measurable dependence on P – T , oxygen fugacity, or silicate composition. One pair of D Cu , A g Min / S M values are 0.0037 and 0.0004 for garnet, respectively; 0.15 and 0.005 for spinel, respectively. These results indicate that Ag is more incompatible than Cu in all mantle minerals. Our D Cu , A g Min / S M combined with previous sulfide–silicate melt partition coefficients of Cu and Ag imply that although sulfides host large fractions of planetary mantle Cu and Ag, silicate minerals can be also important Cu and Ag reservoirs, particularly when sulfides occur as monosulfide solid solutions. The application of our D Cu , A g Min / S M to various melting models demonstrates that Cu–Ag fractionation occurs during partial melting of the silicate mantles of Earth, Mars, and the Moon; however, the modeled Cu/Ag ratios in basalts agree with their mantle source Cu/Ag ratios within 50% relative. Therefore, the superchondritic but similar Cu/Ag ratios of ∼3000–4000 in mid-ocean ridge basalts, arc basalts, mantle plume-related oceanic island basalts and plateau basalts, and the bulk silicate Earth (BSE) reflect homogeneous distribution of Cu/Ag in Earth's mantle and limited Cu–Ag fractionation during slab dehydration/melting in subduction zones. However, components with chondritic Cu/Ag ratios (∼600–2300) may exist in the mantle sources of Hawaii oceanic island lavas. The Cu/Ag ratios (∼500–1100) in the Martian basalts and bulk silicate Mars (BSMars) are chondritic, but the Cu/Ag ratio (∼15,000–40,000) in the bulk silicate Moon (BSMoon) is strongly superchondritic. Such largely different Cu/Ag ratios in different planetary reservoirs cannot be explained by magmatic Cu–Ag fractionation, solar nebular process, or core-formation process. We propose that different degrees of planetary melting and evaporation provide a solution because Ag is more volatile than Cu during silicate melt evaporation. The chondritic Cu/Ag ratios in the BSMars could be explained by limited evaporation of planetesimals that delivered Mars' moderately volatile elements, and the superchondritic Cu/Ag ratios in the BSE may be due to Earth's accretion of planetesimals with partial evaporative loss of Cu and Ag. However, high degrees of evaporative Cu and Ag loss during the Moon-formation are required to explain the strongly superchondritic Cu/Ag ratios in the BSMoon. Accordingly, Cu/Ag ratios in the bulk silicate planets may provide new insights into the nature of their building blocks. [ABSTRACT FROM AUTHOR]

Reactive porous or focused melt flows are common in crystal mushes of mid-ocean ridge magma reservoirs. Although they exert significant control on mid-ocean ridge magmatic differentiation, their role in metal transport between the mantle and the ocean floor remains poorly constrained. Here we aim to improve such knowledge for oceanic crust formed at slow-spreading centers (approximately half of present-day oceanic crust), by focusing on specific igneous features where sulfides are concentrated. International Ocean Discovery Program (IODP) Expedition 360 drilled Hole U1473A 789 m into the lower crust of the Atlantis Bank oceanic core complex, located at the Southwest Indian Ridge. Coarse-grained (5–30 mm) olivine gabbro prevailed throughout the hole, ranging locally from fine- (<1 mm), to very coarse-grained (>30 mm). We studied three distinct intervals of igneous grain size layering at 109.5–110.8, 158.0–158.3, and 593.0–594.4 meters below seafloor to understand the distribution of sulfides. We found that the layer boundaries between the fine- and coarse-grained gabbro were enriched in sulfides and chalcophile elements. On average, sulfide grains throughout the layering were composed of pyrrhotite (81 vol.%; Fe 1-x S), chalcopyrite (16 vol.%; CuFeS 2), and pentlandite (3 vol.%; [Ni,Fe,Co] 9 S 8), which reflect paragenesis of magmatic origin. The sulfides were most commonly associated with Fe-Ti oxides (titanomagnetites and ilmenites), amphiboles, and apatites located at the interstitial positions between clinopyroxene, plagioclase, and olivine. Pentlandite exsolution textures in pyrrhotite indicate that the sulfides formed from high-temperature sulfide liquid separated from mafic magma that exsolved upon cooling. The relatively homogenous phase proportion within sulfides along with their chemical and isotopic compositions throughout the studied intervals further support the magmatic origin of sulfide enrichment at the layer boundaries. The studied magmatic layers were likely formed as a result of intrusion of more primitive magma (fine-grained gabbro) into the former crystal mush (coarse-grained gabbro). Sulfides from the coarse-grained gabbros are Ir-Platinum Group Element-rich (PGE; i.e., Ir, Os, Ru) but those from the fine-grained gabbros are Pd-PGE-rich (i.e., Pd, Pt, Rh). Notably, the sulfides from the layer boundaries are also enriched in Pd-PGEs, and therefore elevated sulfide contents at the boundaries were likely related to the new intruding melt. Because S concentration at sulfide saturation level is dependent on the Fe content of the melt, sulfide crystallization may have been caused by FeO loss, both via crystallization of late-precipitating oxides at the boundaries, and by exchange of Fe and Mg between melt and Fe-bearing silicates (olivine and clinopyroxene). The increased precipitation of sulfide grains at the layer boundaries might be widespread in the lower oceanic crust, as also observed in the Semail ophiolite and along the Mid-Atlantic Ridge. Therefore, this process might affect the metal budget of the global lower oceanic crust. We estimate that up to ∼20% of the Cu, ∼8% of the S, and ∼84% of the Pb of the oceanic crust inventory is accumulated at the layer boundaries only from the interaction between crystal mush and new magma. [ABSTRACT FROM AUTHOR]

Using high-resolution cathodoluminescence (HR-CL) panchromatic imaging for the location of high-precision oxygen three-isotope analyses by secondary ion mass-spectrometry (SIMS), this study is aimed at characterizing the oxygen-isotope variations in Mg-rich olivines (≥Fo 99) of selected type I chondrules from the Yamato (Y) -81020 CO3.05 (Ornans-type) carbonaceous chondrite. Cathodoluminescence being extremely sensitive to faint changes in CL activator/quencher concentrations (Al, Cr, Mn, Fe) allows us to describe various overlooked cycles of growth and dissolution in Mg-rich olivines, which strongly suggest an intimate relationship with their gaseous environment during their formation. The present study confirms significant Δ17O variations of ten ‰ in Mg-rich olivines but does not support the relationship previously found between Mg# [MgO/(MgO + FeO) × 100, mol%] and Δ17O among type I chondrules, nor the interpretation of redox changes that has been made of it. We instead show that Mg-rich olivines in Y-81020 chondrules exhibit a prominent 16O-enriched and 16O-depleted bimodal distribution, which is considered as the most primordial signature of type I chondrules from Y-81020 and very likely other carbonaceous chondrites. This signature is interpreted as a snapshot of the early stages of a mixing occurring between two clouds/environments in which chondrules formed and evolved by gas–melt interaction and mixed according to hydrodynamical instabilities imposed by the process responsible for the mixing. As far as this study allows, O-isotope variations of Mg-rich olivines seems to account for large scale dynamical instabilities while chemical variations highlighted by HR-CL (dissolution/growth) bear witness of smaller scale instabilities very likely occurring in the immediate vicinity of the chondrules. Without being able to decide on plausible astrophysical settings yet, we note however that processes like disruptive and vaporizing collisions between planetesimals offer a range of processes and physicochemical conditions, e.g., expansion, decompression, dynamical instabilities, that deserve to be explored in more detail, some of which resembling those highlighted in this study, e.g., gas–melt interaction, partial pressure fluctuations, heterogeneous materials, gas mixing. [ABSTRACT FROM AUTHOR]

In order to understand the nature of the dust that accreted onto chondrules in the nebula and to unravel the conditions of formation of fine grained rims (FGRs), we studied three of the least altered chondrites from different chondrite groups (LL3.00 Semarkona, CO3.0 DOM 08006, CR2.8 QUE 99177) and compared the results with our previous work on the Paris CM chondrite (Zanetta et al., 2021). For each sample, we selected representative rimmed chondrules showing minimal traces of aqueous alteration. We performed high-resolution SEM X-ray chemical mapping to obtain relevant phase abundances and grain size distributions. Four FIB sections were then extracted from each meteorite, two in the rims and two in their adjacent matrix for quantitative TEM analysis. At the microscale, texture, modal abundances and grain size differ depending on the chondrite but also between FGRs and their adjacent matrix. At the nanoscale (i.e. TEM observations), matrices of the four chondrites consist mostly of domains of amorphous silicate associated with Fe-sulfides, Fe-Ni metal, Mg-rich anhydrous silicates and an abundant porosity. The related FGRs in Semarkona (LL) and DOM 08006 (CO) exhibit more compact textures with a lower porosity while FGRs in QUE99177 (CR) are similar to the matrix in terms of porosity. In the three chondrites, FGRs are made of smooth and chemically homogeneous amorphous (or nanocrystalline) silicate with no porosity that encloses domains of porous amorphous silicate bearing Mg-rich anhydrous silicates, Fe-sulfides, Fe-oxides and sometimes metal and Fe-rich olivines. The average compositions in major elements of the amorphous regions are similar for the FGRs and the matrix within a given chondrite (but differ between chondrites). The texture and the chemical homogeneity of the smooth silicate and the fact that it encloses domains of porous amorphous silicate bearing other mineral phases similar to matrix-like material suggests a formation by condensation. Areas that are enclosed in this smooth silicate exhibit Fe-rich olivine formed through Fe interdiffusion that also suggest a thermal modification of the dust accreted to form FGRs. These characteristics indicate a transformation process for the modification of the FGR material similar to the one proposed in our previous work on Paris. We conclude that matrix and FGRs accreted a similar type of dust but FGR material was affected by thermal modification and compaction contemporary with their accretion. For each chondrite, dust accreted onto chondrules under different conditions (dust density, temperature) which led to diverse degrees of compaction/thermal modification of the sub-domains and explain the textural differences observed in FGRs. They accreted on chondrules in a warm environment related to the chondrule formation episode, whereas matrix accreted later in a cooler environment. [ABSTRACT FROM AUTHOR]

Subducted oceanic crust can transform into eclogite in the upper mantle, and generate chemical heterogeneity of mantle plumes, as recorded by elemental and radiogenic isotopic variations in oceanic island basalts (OIBs). The secondary pyroxenite produced by the reaction between eclogite-derived melt and peridotite is increasingly regarded as a major source component of OIBs, as well as peridotite. However, it remains unclear whether eclogite can be a direct source component of OIBs. To test this possibility, we present high-precision whole-rock Fe isotopes and the chemical compositions of olivine phenocrysts from well-characterized EM1-type basalts from Pitcairn Island. The Pitcairn basalts are characterized by moderate 87Sr/86Sr, low 143Nd/144Nd and 206Pb/204Pb isotopic ratios, and lowest δ26Mg values among OIBs, suggesting a contribution from recycled ancient crustal components (oceanic crust plus sediment). For comparison, we also report the Fe isotope compositions of FOZO-type basalts (with low 87Sr/86Sr, moderately high 143Nd/144Nd and moderate 206Pb/204Pb ratios) from the Louisville hot spot track, which were suggested to be a typical peridotite-derived OIB. The Louisville basalts have MORB-like δ57Fe values (0.06‰–0.15‰), whereas the Pitcairn lavas have substantially heavier Fe isotopic compositions (δ57Fe = 0.17‰–0.31‰) than MORBs. Quantitative evaluations suggest that magmatic differentiation, partial melting, and elevated oxygen fugacity in the source are insufficient to generate the heavy Fe isotopic compositions of the Pitcairn basalts. A good correlation between δ57Fe and ε Nd(i) (or δ26Mg) values in the Pitcairn basalts indicates binary mixing, and the melts derived from the EM1 endmember have unusually heavy Fe and light Mg isotopic compositions. In order to explain the origin of the Fe and Mg isotopic compositions, we calculated the Fe, Mg, and Nd isotopic compositions of partial melts of eclogite, secondary pyroxenite and peridotite, respectively. The results indicate that eclogite is the only suitable candidate to generate melts with both heavy Fe and light Mg isotopes. This inference is strengthened by the major and minor elemental compositions of olivine phenocrysts from the Pitcairn basalts, which show low Fo (73.2–82.5) and Ni contents (620–1949 ppm), and low Mn/Fe (0.011–0.015) and Ni/(Mg/Fe) (524–1041) ratios. These characteristics are markedly different from those of olivines from MORBs and the Koolau lavas from Hawaii, which represent phenocrysts that equilibrated with peridotite-derived and secondary pyroxenite-derived melts, respectively. We therefore argue that eclogite is the source lithology of the EM1 endmember of the Pitcairn basalts. Binary mixing between our modelled eclogite- and peridotite-derived melts produced magmas with relatively low Mg# value, Mg/Fe ratios and moderate Ni contents characteristics, which are preserved in the low-Fo Pitcairn olivines. Our results highlight that, in addition to peridotite and secondary pyroxenite, eclogite may survive in mantle plumes at shallow depths and make a substantial contribution to the source of OIBs. [ABSTRACT FROM AUTHOR]

Spectral and mineralogical analyses were performed using nine naturally hydrated and dehydrated carbonaceous chondrite samples which were classified into heating stages (HS) from I to IV based on previous X-ray diffraction results. In-situ heating of samples at 120–400 °C was performed during spectral measurements and successfully removed absorbed water and part of rehydrated water from chondrite samples. Reflectance spectra of HS-I samples show the positive slope in visible (Vis)-infrared (IR) range and the significant 0.7- and 3-μm absorption bands. The 0.7-μm band appears in only HS-I sample spectra. With increasing temperature of heating, (1) Vis-IR slope decreases, (2) the 3-μm band becomes shallower, and (3) Christiansen feature (CF) and Reststrahlen bands (RB) shift toward longer wavelength. TEM-EDX analyses showed that the matrix of strongly-heated chondrites consists of tiny olivine, low-Ca pyroxene, and FeNi metallic particles mostly smaller than 100 nm in diameter, instead of Fe-rich serpentines and tochilinite observed in the HS-I chondrite. Therefore, in proportion to the heating degree, amorphization and dehydration of serpentine and tochilinite from HS-I to HS-II may cause the 0.7- and 3-μm band weakening, spectral bluing and darkening of chondrite spectra. In addition, formation of secondary anhydrous silicates and FeNi-rich metal grains at HS-IV would be responsible for the 3-μm band depth decrease, spectral reddening and brightening, CF peak shift, and RB changes of chondrite spectra. Those spectral changes in response to mineralogical alteration processes will be useful to interpret planetary surface composition by remote-sensing observations using ground-based or airborne/space telescopes or spacecraft missions. [ABSTRACT FROM AUTHOR]

We report the first zinc isotope data (expressed as δ66Zn relative to the JMC-Lyon standard) for chromian spinels (Cr-spinels) and coexisting olivines in oceanic peridotites. All spinel-olivine pairs fall on the 1:1 fractionation line in the diagram of δ66Zn spinel versus δ66Zn olivine , suggesting equilibrium isotope fractionation. Cr-spinels are always isotopically lighter than coexisting olivines (Δ66Zn spinel-olivine = δ66Zn spinel − δ66Zn olivine = −0.50‰ to −0.33‰; n = 13), which is opposite to the positive Zn isotope fractionation between Al-spinel and olivine observed in cratonic peridotites. The "inverse" Zn isotope fractionation between Cr-spinel and olivine is unlikely to have been caused by low-temperature alteration of the oceanic peridotites, given the lack of correlation between Δ66Zn spinel-olivine values and chemical indices of serpentinization and weathering (e.g., LOIs, MgO/SiO 2). Ionic model calculation indicates that even considering that Zn occurs at both tetrahedral and octahedral sites in the crystal lattice of spinel, the magnitude of equilibrium fractionation is still inconsistent with the observed Zn isotope fractionation between Cr-spinel and olivine. We suggest a "chemical effect" in which the Zn O bond length (tetrahedral site) in spinel increases when Cr substitutes Al in octahedral site, which is corroborated by the striking negative correlation of δ66Zn with Cr# [molar Cr3+/(Cr3+ + Al)] in natural spinels. Therefore, Zn isotopic compositions of natural spinels can be highly variable depending on their chemical compositions. During magma differentiation, zinc is moderately incompatible in silicate minerals (olivine and pyroxene) but highly compatible in Cr-spinels/chromites that have Zn contents tens of times higher than those of basaltic melts. Given its light Zn isotopic composition, chromite crystallization–if any–can evidently elevate δ66Zn and lower Zn contents of the residual melts. Lunar mare basalts are typically characterized by higher δ66Zn and lower Zn contents relative to terrestrial basalts, but modelling suggests that chromite crystallization during magmatic differentiation is unlikely to account for Zn isotopic and elemental data of lunar mare basalts. Global ocean island basalts (OIBs) and some intraplate alkali basalts have systematically higher δ66Zn and higher Zn contents than those of the normal mantle-derived melts (e.g., mid-ocean ridge basalts; MORBs), which contradicts with a chromite crystallization model. Instead, such signatures can reflect recycling of surface carbonates into the Earth's deep mantle, reinforcing the application of zinc isotopes as a tracer of deep carbonate recycling. [ABSTRACT FROM AUTHOR]

• We report the first mantle degassing transect across a continental convergent margin. • Three levels of mantle He degassing are recognized based on 3He/4He distribution. • Causes for possible decoupled He-Sr isotopes of Quaternary basalts are explored. • Volatile origin and outgassing are controlled by continental convergence processes. Mantle degassing transect across different tectonic units within a plate convergent setting has been well documented for oceanic convergent margins by systematic changes in geochemistry (e.g., 3He/4He, δ13C, and CO 2 /3He) of hydrothermal gases. However, little is known about spatial variations in volatile geochemistry across a continental convergent margin. In this study, we identify a mantle degassing transect in the southeastern Tibetan Plateau using He-CO 2 systematics of hydrothermal gases, which extends from India-Asia continental convergent margin to intra-continent extensional region. δ13C-CO 2 (−11.8‰ to −3.1‰) and CO 2 /3He (1.7 × 108 to 7.1 × 1011) values of hydrothermal gases show large variations that are consistent with modification by secondary physico-chemical processes, such as multi-component mixing, hydrothermal degassing, and calcite precipitation. Three levels of He degassing can be recognized based on 3He/4He dataset (0.01–5.87 R A) of the hydrothermal gas samples and their distances to volcanic centers. A magmatic level He degassing (35–74% mantle He) is found near active and/or Quaternary volcanoes fed by mantle-derived magmas. With increasing distance to volcanic centers, the outgassed magmatic volatiles are gradually diluted by crustal components (e.g., radiogenic 4He), defining a transitional level He degassing (13–33% mantle He). The 3He/4He values (8.16–8.48 R A) of olivine phenocrysts indicate a MORB (mid-ocean ridge basalts)-type mantle source for the magmatic and transitional levels of He degassing that are localized in Quaternary volcanic fields. In contrast, a background level He degassing (<12% mantle He) dominates the entire study area, and can be attributed to (i) degassing of sub-continental lithospheric mantle, and/or (ii) higher degrees of crustal contamination than those of the transitional level He degassing near volcanic centers. Combined with olivine 3He/4He data reported in this study and whole-rock 87Sr/86Sr data of host basalts from literature, source components of the mantle-derived magmas are suggested to include the MORB-type convective mantle, subducted Indian slab materials, and less degassed mantle materials, which can account for the possible decoupling between He and Sr isotope systematics. These findings delineate the origin and outgassing of mantle volatiles controlled by the India-Asia continental convergence, and would contribute to a better understanding of the deeply-sourced volatile emissions in these tectonic settings. [ABSTRACT FROM AUTHOR]

It is well known that oxidation of ferrous to ferric iron by water can generate molecular hydrogen (H 2), with the most widely recognized natural manifestation being serpentinization of olivine and pyroxene in ultramafic rocks. A less known yet extremely important source of natural H 2 are peralkaline igneous intrusions, where spectacular enrichments of H 2 are documented from fluid inclusions and as free gas migrating through fractured rocks and soils. Of these occurrences, the best studied are those at Strange Lake in Canada, Lovozero and Khibiny in Russia, and Ilímaussaq in Greenland. Based on petrographic observations and fluid inclusions analysis, it has been proposed that the hydrothermal alteration of arfvedsonite, an FeII-bearing amphibole, is the source of H 2 in this context, although it is yet to be unequivocally demonstrated. To investigate the generation of H 2 during alteration of peralkaline granites, we performed hydrothermal experiments on pure arfvedsonite grains and arfvedsonite-bearing granite (10 wt% arfvedsonite) from the Strange Lake pluton (Canada). These materials, in the presence of aqueous solutions, were sealed inside gold capsules or placed within titanium autoclaves, which allowed monitoring H 2 generation in function of temperature (280–400 °C), chlorinity (0 and 3 m NaCl), pH, and starting mineral assemblage. Blank experiments were conducted to quantify the background amounts of H 2 generated from Au/Ti oxidation, diffusing through the reaction cells, release from fluid inclusions or otherwise occluded in minerals. Solids were characterized by XRD, SEM, TEM on FIB foils, and STXM-XANES. Outcomes of this study demonstrate the production of H 2 in agpaitic peralkaline rocks by the hydrothermal alteration of arfvedsonite. The rate of H 2 production, normalized to the specific surface area of arfvedsonite, increases with temperature from 1100 to 2200 pmol cm−2 day−1 between 280 and 400 °C, respectively. Chlorinity tends to have a negative impact on the reaction rate, while circumneutral to alkaline conditions clearly promote H 2 generation. Altering whole granite samples instead of arfvedsonite grains only also enhances H 2 production rate. The presence of aluminum, released from microcline and albite dissolution, may increase both the solubility and the dissolution rate of arfvedsonite by promoting precipitation of phyllosilicates. At least two different types of phyllosilicates were observed, chlorite and smectite. Magnetite and secondary zircon were also identified at the surface of reacted arfvedsonite (Zr content = 1200 ppm). The H 2 production rates reported here at 280–400 °C are comparable, and even faster than those documented for serpentinization of olivine and harzburgite. A major feature of arfvedsonite alteration in peralkaline plutons is the formation of aegirine as a replacing mineral. However, aegirine was never observed in the reacted solids from our experiments, under the condition tested (up to 400 °C and 400 bar). This may be an effect of pressure, oxygen fugacity, or reaction progress, parameters that remain to be investigated to better constrain the reaction mechanism of arfvedsonite alteration. Agpaitic peralkaline igneous intrusions thus represent a fertile geological setting for deep microbial subsurface ecosystems, abiotic synthesis of organic molecules, and natural H 2 exploration for an alternative source of energy. [ABSTRACT FROM AUTHOR]

Carbon is of fundamental interest for constraining the volatile element inventory of terrestrial planets. In some meteorites, like ureilites and enstatite chondrites, graphite is the major carbon-carrier. Here, we report the in-situ analyses of graphite in 19 ureilites, 11 enstatite chondrites, and 3 graphite-bearing clasts in ordinary chondrites by secondary ion mass spectrometry (SIMS). In coarse-grained ureilites the obtained carbon-compositions of graphite range from –9.2‰ to –0.1‰ (δ13C). The carbon-composition tends to be homogeneous within a sample and correlates with the Fa content in olivine. In contrast, fine-grained ureilites exhibit considerable intra-sample heterogeneity, and graphite tends towards 13C-enriched compositions (up to +10.4‰). Isotopic and petrographic differences are presumably a result of post-igneous shock processing, including annealing during impact smelting. Enstatite chondrites host a variety of graphite morphologies, occurring in two distinct assemblages: Silicate-associated graphite (SAG) and metal-associated graphite (MAG). These assemblages show diverging carbon-compositions: SAG consistently exhibits δ13C in a narrow range between –4‰ and +1‰, very similar to the bulk silicate Earth value. In contrast, diverse compositions from –19.7‰ to +13.7‰ were observed for MAG. These differences are likely pre-accretionary in origin and potentially point towards isotopically distinct precursors. If Earth accreted from enstatite-chondrite-like material, carbon potentially hosted by Earth's core may have an isotopically light signature when compared to the mantle. Although graphite-bearing clasts in unequilibrated ordinary chondrites (UOCs) are extraordinarily rare, these clasts are of particular interest as they might represent materials, not corresponding to known meteorites. Graphite from these clasts show coinciding carbon-compositions with a mean δ13C close to –1‰. Although the coinciding compositions might argue for a genetic relationship among the clasts, petrographic evidence suggests they have experienced distinct thermal histories. [ABSTRACT FROM AUTHOR]

Reduction process might play a critical role in the genesis of magmatic Ni-Cu sulfide deposits in convergent tectonic settings. To test this hypothesis, we have determined the oxygen fugacities (f O2) and C isotopes of several typical magmatic Ni-Cu sulfide deposits in convergent tectonic settings in western China. Two popular spinel-olivine-orthopyroxene (Spl-Ol-Opx) oxybarometers, one from Wood (1990) and the other from Ballhaus et al. (1991), are evaluated for their accuracy for this type of deposit by comparing the results with those from an independent method, the olivine-sulfide (Ol-Sul) oxybarometer of Barnes et al. (2013). We find that only the results from the Spl-Ol-Opx oxybarometer of Wood (1990) are consistent with those from the independent method. The f O2 of the sulfide-mineralized samples from the selected deposits, estimated using these two methods, are from QFM–0.56 to QFM+1.84 (quartz-fayalite-magnetite buffer). Using the Spl-Ol-Opx oxybarometer of Wood (1990), the estimated f O2 values of the sulfide-unmineralized ultramafic rocks from primitive arc cumulates in the same region, which have higher olivine Fo contents than the sulfide-mineralized rocks, are significantly higher (~QFM+2.42). The decrease of f O2 from the sulfide ore-barren to ore-bearing magmatic systems indicates that reduction process indeed played a critical role in the genesis of the studied magmatic sulfide deposits. The δ13C values of the sulfide-bearing ultramafic rocks are from –16.5 to –37.2‰ (n = 52), which are significantly lower than the primitive arc cumulates (−7.1 to −9.6‰) but similar to those of organic matter in sedimentary rocks worldwide (–15 to –35‰). The isotope data indicate assimilation of organic matter from the country rocks by the parental magmas for the studied magmatic Ni-Cu sulfide deposits, supporting the interpretation that sulfide saturation in the parental magmas were mainly induced by the reduction process associated with the addition of organic matter to the magmas. [ABSTRACT FROM AUTHOR]

Natural gem quality orthopyroxene crystals were experimentally dehydrated between 720 °C and 1020 °C, varying silica activity (buffered by olivine-orthopyroxene or orthopyroxene-quartz), oxygen fugacity (log f O2 relative to the quartz-fayalite-magnetite buffer (QFM) of around +1, −1 and −7) and composition (two different orthopyroxene types) to study these parameters' effects on both the diffusivity of hydrogen and the Fourier transform infrared (FTIR) spectra in orthopyroxene. Hydroxyl profiles were measured across partially dehydrated grains, from which H diffusion coefficients were extracted. Fitting these H diffusion coefficients to the isobaric Arrhenius relationship, log D = log D 0 – E a /(2.303RT), gives log D 0 of −5.11, −4.93 and −5.52 m2/s, and E a of 139, 149 and 125 kJ mol−1 for the [100], [010] and [001] axes, respectively, where D 0 is the pre-exponential factor (m2/s), E a is the activation energy (kJ mol−1), R is the universal gas constant (kJ mol−1 K−1) and T is the temperature (K). Hydrogen diffusivities at ~ΔQFM-1 and 700 °C to 1000 °C are between log D = −12.5 to −10.4 m2/s. This is similar to the fastest H diffusivity measured in olivine, suggesting diffusion primarily by a 'proton-polaron'-type mechanism, whereby the movement of protons through the crystal is associated with Fe redox, with the net result being that dehydration should be accompanied by Fe oxidation. The determined diffusion coefficients are ~0.3 log units faster than those determined in H-2H self-diffusion experiments (Stalder and Behrens, 2006). The effect of a SiO2 and ƒ O2 (between ~ΔQFM-1 to ~ΔQFM + 1) is only minor or within uncertainty. These data were applied to a natural sample from a xenolith from Las Cumbres in Chilean Patagonia, where orthopyroxene shows > 500 µm H loss profiles in length, and olivine shows 50–200 µm Fe-Mg, Y, Ti, V and Sc profiles on millimeter sized crystals. Modelled timescales calculated from a H profile in orthopyroxene, using our H diffusivities along with published values, are considerably shorter (minutes to days) compared to those derived from Fe-Mg inter-diffusion in olivine (weeks to years) from the same xenolith using Fe-Mg diffusivities in olivine. This discrepancy is likely due to H-loss in the natural orthopyroxene representing the timescale between the onset degassing of the host magma and emplacement, whereas Fe-Mg inter-diffusion records the time elapsed between xenolith entrainment and final emplacement. This supports previous assertions that H diffusion in orthopyroxene is sufficiently fast such that many xenolith-hosted orthopyroxene crystals may have suffered significant re-equilibration with the host magma H 2 O prior to later degassing-induced H-loss. [ABSTRACT FROM AUTHOR]

Hydrothermal fluids percolating through peridotite are highly enriched in abiotic CH 4 , which can fuel chemosynthetic microbial activity and potentially early life. In contrast to the paradigm favoring coupled abiotic methanogenesis and fluid circulation, recent studies have suggested that leaching of CH 4 included in peridotite can account for elevated levels of CH 4 in serpentinization fluids. As such, CH 4 venting at continental gas seepage hosted in orogenic peridotite should be derived mostly from CH 4 that originated within the host peridotite. However, the origin of CH 4 included in orogenic peridotite remains elusive, as the included CH 4 is reported to form either in situ during serpentinization within fluid inclusions, or to originate from reduced external fluids. Moreover, varying associations of step-daughter minerals documented in CH 4 -bearing fluid inclusions in orogenic peridotite demonstrate the diversity of fluid–mineral interactions within fluid inclusions. Here we present a detailed petrological investigation into olivine-hosted CH 4 -bearing fluid inclusions in ophiolitic harzburgite from the North Qilian orogen in northern Tibet, which reveals the occurrence of abiotic CH 4 synthesis during diverse serpentinization within multiple types of olivine-hosted fluid inclusions. Three types of CH 4 -bearing fluid inclusions are newly identified in the harzburgite. Type I fluid inclusions contain CH 4(g) + antigorite + brucite + magnetite + magnesite, which imply abiotic CH 4 synthesis during hydration of olivine directly into antigorite. This contradicts the previous proposal that suggests the inhibition of H 2 and CH 4 production during high-temperature serpentinization in the stability field of antigorite. Type II fluid inclusions consist of CH 4(g) + N 2(g) + lizardite + brucite ± magnetite or CH 4(g) + N 2(g) + lizardite + magnetite, with brucite-bearing inclusions yielding less CH 4 compared with brucite-free inclusions. The crystallization of brucite in type II fluid inclusions was probably controlled by the concentration of dissolved Si in the trapped fluids. Type III fluid inclusions are composed of CH 4(g) + antigorite + magnesite + graphite ± magnetite ± dolomite. High concentrations of oxidized inorganic carbon in type III fluid inclusions likely promoted the precipitation of carbonate and crystallization of antigorite. Moreover, the presence of relict lizardite in minor type III fluid inclusions suggests that antigorite growth accompanying carbonate saturation is probably a two-step process, with initial hydration of olivine into lizardite being followed by transformation of lizardite into antigorite. Above all, this study demonstrates that CH 4 -bearing fluid inclusions in orogenic peridotite can be very abundant, and may be a significant reservoir of abiotic CH 4 in serpentinite-hosted hydrothermal systems. Moreover, the multiple types of CH 4 -bearing fluid inclusions in orogenic peridotite presented in this study indicate that olivine-hosted CH 4 -bearing fluid inclusions can potentially be a novel window for studying abiotic CH 4 synthesis during serpentinization under different conditions. [ABSTRACT FROM AUTHOR]