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The origin of diamonds in ureilite meteorites is a timely topic in planetary geology as recent studies have proposed their formation at static pressures >20 GPa in a large planetary body, like diamonds formed deep within Earth's mantle. We investigated fragments of three diamond-bearing ureilites (two from the Almahata Sitta polymict ureilite and one from the NWA 7983 main group ureilite). In NWA 7983 we found an intimate association of large monocrystalline diamonds (up to at least 100 µm), nanodiamonds, nanographite, and nanometric grains of metallic iron, cohenite, troilite, and likely schreibersite. The diamonds show a striking texture pseudomorphing inferred original graphite laths. The silicates in NWA 7983 record a high degree of shock metamorphism. The coexistence of large monocrystalline diamonds and nanodiamonds in a highly shocked ureilite can be explained by catalyzed transformation from graphite during an impact shock event characterized by peak pressures possibly as low as 15 GPa for relatively long duration (on the order of 4 to 5 s). The formation of "large" (as opposed to nano) diamond crystals could have been enhanced by the catalytic effect of metallic Fe-Ni-C liquid coexisting with graphite during this shock event. We found no evidence that formation of micrometer(s)-sized diamonds or associated Fe-S-P phases in ureilites require high static pressures and long growth times, which makes it unlikely that any of the diamonds in ureilites formed in bodies as large as Mars or Mercury. [ABSTRACT FROM AUTHOR]

We report the first natural occurrence and single-crystal X-ray diffraction qstudy of the Fe-analog of wadsleyite [a = 5.7485(4), b = 11.5761(9), c = 8.3630(7) Å, V = 556.52(7) A3; space group Imma], spinelloid-structured Fe2SiO4, a missing phase among the predicted high-pressure polymorphs of ferroan olivine, with the composition (Fe2+ 1.10Mg0.80Cr3+ 0.04Mn2+ 0.02Ca0.02Al0.02Na0.01)Σ2.01(Si0.97Al0.03)Σ1.00O4. The new mineral was approved by the International Mineralogical Association (No. 2018-102) and named asimowite in honor of Paul D. Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at the California Institute of Technology. It was discovered in rare shock-melted silicate droplets embedded in Fe,Ni-metal in both the Suizhou L6 chondrite and the Quebrada Chimborazo (QC) 001 CB3.0 chondrite. Asimowite is rare, but the shock-melted silicate droplets are very frequent in both meteorites, and most of them contain Fe-rich wadsleyite (Fa30 -- 45). Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)2SiO4 at high temperature (>1800 K) and pressure are clearly needed. This may also have a significant impact on the temperature and chemical estimates of the mantle's transition zone in Earth. [ABSTRACT FROM AUTHOR]

Expectations regarding structural deformation of asteroidal meteorites have typically revolved around impact-induced shock metamorphism or the gravity-driven axial compression of cumulates at the base of magma chambers. Recent structural analyses, however, of several olivine-rich diogenites (harzburgites) reveal solid-state plastic deformation not attributable to either scenario and propose dynamic mantle movements in the parent body, assumed to be Vesta. In this study we examine the microstructures of pyroxene and olivine in the olivine-rich diogenite NWA 5480. Coarse-grained, poikilitic texture, exsolution lamellae and plastic deformation attest to polyphase deformation and a re-heating event, followed by relatively slow cooling. Observations suggest that impact events alone are insufficient to generate and sustain the thermal and deformation conditions required to achieve all of the observed features. The proposed dynamic mantle movements in the Vestan interior may offer a means of heat transport to the system to provide a thermal environment inducive to slow cooling as well as generate the incremental stress fields required for the polyphase plastic deformation observed in the olivine. [ABSTRACT FROM AUTHOR]

Numerous petrologic and geochemical studies so far on the howardite, eucrite, and diogenite ( HED) meteorites have produced various crystallization scenarios for their parent body, believed to be the differentiated asteroid 4 Vesta. Structural analyses of diogenites can reveal important insights into postcrystallization deformation on the parent body. Recently published results (Tkalcec et al. ) of structural analysis on the olivine-rich diogenite NWA 5480 reveal that it underwent solid-state plastic deformation, although not at the base of a magma chamber. Dynamic mantle downwelling has been proposed as a plausible deformation mechanism (Tkalcec et al. ). The purpose of this study is to investigate whether the plastic deformation found in NWA 5480 is an isolated case. We expand the structural analysis on NWA 5480 and extend it to NWA 5784 and MIL 07001,6, two other samples of rare olivine-rich diogenites, using electron-backscattered-diffraction ( EBSD) techniques. Our EBSD results show that the diogenites analyzed in this study underwent solid-state plastic deformation, confirming that the observed deformation of NWA 5480 was not an isolated case on the diogenite parent body. The lattice-preferred orientations ( LPOs) of olivine in NWA 5784 and NWA 5480 are clearly distinct from that typical for cumulate rocks at the base of magma chambers, indicating a different stress environment and a different deformation mechanism. The LPO of olivine in MIL 07001 is less conclusive. The structural results of this study suggest that plastic deformation occurred on the diogenite parent body at high temperatures (1273 < T ≤ 1573 K) in the solid state, i.e., after crystallization of the diogenites themselves, in a dynamic environment with active stress fields. [ABSTRACT FROM AUTHOR]

We report on transmission electron microscopy (TEM) investigations of two mineralogically unusual stardust silicates to constrain their circumstellar condensation conditions. Both grains were identified by high spatial resolution nano secondary ion mass spectrometry (NanoSIMS) in the Acfer 094 meteorite, one of the most pristine carbonaceous chondrites available for study. One grain is a highly crystalline, highly refractory (Fe content < 0.5 at%), structurally undisturbed orthopyroxene (MgSiO3) with an unusually high Al content (1.8 ± 0.5 at%). This is the first TEM documentation of a single crystal pyroxene within the complete stardust silicate data set. We interpret the microstructure and chemistry of this grain as being a direct condensate from a gas of locally non-solar composition (i.e., with a higher-than-solar Al content and most likely also a lower-than-solar Mg/Si ratio) at (near)-equilibrium conditions. From the overabundance of crystalline olivine (six reported grains to date) compared to crystalline pyroxene (only documented as a single crystal in this work) we infer that formation of olivine over pyroxene is favored in circumstellar environments, in agreement with expectations from condensation theory and experiments. The second stardust silicate consists of an amorphous Ca–Si rich material which lacks any crystallinity based on TEM observations in which tiny (<20 nm) hibonite nanocrystallites are embedded. This complex assemblage therefore attests to the fast cooling and rapidly changing chemical environments under which dust grains in circumstellar shells form. [ABSTRACT FROM AUTHOR]

- The oxygen fugacities recorded in the nakhlites Nakhla, Yamato-000593 (Y-000593), Lafayette, and NWA998 were studied by applying the Fe,Ti-oxide oxybarometer. Oxygen fugacities obtained cluster closely around the FMQ (Fayalite-Magnetite-Quartz) buffer (NWA998 = FMQ − 0.8; Y-000593 = FMQ − 0.7; Nakhla = FMQ; Lafayette = FMQ + 0.1). The corresponding equilibration temperatures are 810 °C for Nakhla and Y-000593, 780 °C for Lafayette and 710 °C for NWA998. All nakhlites record oxygen fugacities significantly higher and with a tighter range than those determined for Martian basalts, i.e., shergottites whose oxygen fugacities vary from FMQ − 1 to FMQ − 4. It has been known for some time that nakhlites are different from other Martian meteorites in chemistry, mineralogy, and crystallization age. The present study adds oxygen fugacity to this list of differences. The comparatively large variation in fO2 recorded by shergottites was interpreted by as reflecting variable degrees of contamination with crustal fluids that would also carry a light rare earth element (REE)-enriched component. The high oxygen fugacities and the large light REE enrichment of nakhlites fit qualitatively in this model. In detail, however, it is found that the inferred contaminating phase in nakhlites must have been different from those in shergottites. This is supported by unique 182W/184W and 142Nd/144Nd ratios in nakhlites, which are distinct from other Martian meteorites. It is likely that the differences in fO2 between nakhlites and other Martian meteorites were established very early in the history of Mars. Parental trace element rich and trace element poor regions (reservoirs) of Mars mantle () must have been kept isolated throughout Martian history. Our results further show significant differences in closure temperature among the different nakhlites. The observed range in equilibration temperatures together with similar fO2 values is attributable to crystallization of nakhlites in the same cumulate pile or lava layer at different burial depths from 0.5 to 30 m below the Martian surface in agreement with and is further confirmed by similar crystallization ages of about 1.3 Ga ago (e.g., ). [ABSTRACT FROM AUTHOR]

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa 18.1–28.3 , average Fa 24.9±3.2 , PMD = 12.8; Cr 2 O 3 = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs 3.2–18.7 Wo 0.2–4.5 ; average Fs 14.7±3.7 Wo 1.4±1.3 ; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ17O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa 27) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ17O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ18O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin. [ABSTRACT FROM AUTHOR]