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We found that the particle RA-QD02-0115 returned by the Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa contains the iron carbide haxonite (Fe 21.9-22.7 Co 0.2-0.3 Ni 0.2-0.8 )C 6 and several Fe,Ni alloys, including multi-domain tetrataenite and spinodally decomposed taenite. Ellipsoidal to nearly spherical voids occur throughout the particle and suggest the presence of a fluid phase during textural and chemical equilibration of the host rock within the parent asteroid of 25143 Itokawa. The calculated solubility of carbon in Fe,Ni metal indicates that the carbide formed at temperatures larger than 600 °C during thermal metamorphism of the LL-chondritic mineral assemblage. Haxonite formed metastably with respect to graphite and cohenite, probably due to its high degree of lattice match with neighboring taenite, a low cooling rate at peak metamorphic temperatures, and the hindered nucleation of graphite. Thermodynamic equilibrium calculations indicate that the fluid present was dry (H 2 O-poor) and dominated by methane. The reactive fluid most plausibly had an atomic H/C ratio of 4–5 and was derived from the reduction of macromolecular, insoluble organic matter (IOM) that initially co-accreted with water ice. The initial presence of water is a necessary assumption to provide sufficient hydrogen for the formation of methane from hydrolyzed IOM. Metallic iron was in turn partially oxidized and incorporated into the ferromagnesian silicates during the high-temperature stage of metamorphism. An exemplary bulk reaction from unequilibrated material on the left to an equilibrated assemblage on the right may be written as: 330 CH 0.8 O 0.2(IOM)  + 500 H 2 O (ice/g)  + 681 Fe (in alloy)  + 566 FeSiO 3(in Opx)  → 300 CH 4(g)  + 32 H 2(g)  + 5 Fe 23 C 6(in Hx)  + 566 Fe 2 SiO 4(in Ol) (Opx = orthopyroxene, Hx = haxonite, Ol = olivine, g = fluid species). The best estimate of the fluid/rock ratio in the region of the LL parent body where RA-QD02-0115 formed is about 3 × 10 −3 and corresponds to an initial ice/rock ratio of about 7 × 10 −3 (both by mass). [ABSTRACT FROM AUTHOR]

On April 23rd 2013 at 2:07 a.m., a 1.3 kg meteorite fell in the Braunschweig suburb Melverode (52° 13′ 32.19″ N. 10° 31′ 11.60″ E). Its estimated velocity was 250 km/h and it formed an impact pit in the concrete fall site with a diameter of 7 cm and a depth of 3 cm. Radial dust striae are present around the impact pit. As a result of the impact, the meteorite disintegrated into several hundred fragments with masses up to 214 g. The meteorite is a typical L6 chondrite, moderately shocked (S4) – but with a remarkably high porosity (up to 20 vol%). The meteorite was ejected from its parent body as an object with a radius of about 10–15 cm (15–50 kg). The U,Th-He gas retention age of ∼550 Ma overlaps with the main impact event on the L-chondrite parent body ∼470 Ma ago that is recorded by many shocked L chondrites. The preferred cosmic-ray exposure age derived from production of radionuclides and noble gas isotopes is (6.0 ± 1.3) Ma. [ABSTRACT FROM AUTHOR]

Homogeneity testing of candidate reference materials requires distinguishing the effects of measurement uncertainty of the analytical method from true compositional variations within the material. Many in situ microanalytical techniques do not allow classical ANOVA homogeneity testing due to the infeasibility of truly replicated analyses on the same analysis volume. This also applies to the analysis of beam-sensitive and light element-bearing materials by electron probe microanalysis (EPMA). This reality has led me to reconsider the homogeneity index approach used in the testing of microanalytical reference materials by EPMA. Based on statistical considerations, I show that the homogeneity index is suitable for statistical significance testing using F and chi-squared statistics and allows estimating the contribution of compositional heterogeneity to the total uncertainty budget of the referenced values. However, there are problems of bias and masking of small compositional variations by measurement uncertainty. This contribution shows the strong impact of the total number of measurements on the resolution of a microanalytical homogeneity study and discusses how to quantify the relative contribution of heterogeneity to the total uncertainty budget. I present an example of EPMA to illustrate this approach and show some pitfalls and limitations in its application. [ABSTRACT FROM AUTHOR]

Abstract: The crystallographic diversity of pyrrhotite (Fe1− x S), one of the most common iron sulfide minerals, offers insights into how mineral–fluid interactions are controlled by crystal structures. We have conducted oxidative dissolution experiments on monoclinic 4C-pyrrhotite and ‘hexagonal’ NC-pyrrhotite in aqueous H2O2/H2SO4 and FeCl3/HCl media at pH between 1.8 and 2.9 using polished surfaces of single crystals. Quantification and detailed characterization of the reaction interfaces has been accomplished by confocal 3D topometry and transmission electron microscopy (TEM) in conjunction with focused ion beam (FIB) preparation. Crystallographically coherent intergrowths of 4C- and NC-pyrrhotite in a single sample allowed unambiguous identification of strong intrinsic reactivity differences between the two closely related phases. On {110} faces in the H2O2 medium at 35°C and pH below 2.70, NC-pyrrhotite (N ∼4.85) reacts about 50–80% faster than 4C-pyrrhotite. Above pH 2.70, the behavior inverts and 4C-pyrrhotite dissolves faster, while overall reaction rates drop drastically by up to two orders of magnitude. Because the two pyrrhotite phases show only marginally different Fe/S ratios but substantial differences in structural complexity with regards to vacancy ordering, we attribute the reactivity differences to structurally controlled processes at the mineral–water interface. The transition at pH 2.70 is close to the reported isoelectric point of pyrrhotite. We attribute the pH dependent changes in reaction rates and behaviors to protonation/deprotonation of surface sulfhydryl groups and related changes in speciation and bonding mode of reactive oxygen species at the mineral interface. At pH <2.70, we find elemental sulfur as a frequent reaction product in H2O2 and FeCl3 media, indicating incomplete sulfur oxidation. Above pH 2.70, elemental sulfur was not found in H2O2 experiments (no data for FeCl3). Our results show that the effects of crystal anisotropy are strong and directional preference of dissolution changes at the pH 2.70 transition point as well, leading to complex sub-μm-scale textural development at the reaction interfaces throughout the pH range studied. High resolution TEM imaging of cross sections through reacted mineral surfaces show crystalline pyrrhotite up to the reaction interface and the absence of significant non-equilibrium layers or S-enriched (poly)sulfides. [Copyright &y& Elsevier]

Two samples of a unique achondritic lithology of the Almahata Sitta meteorite (MS-MU-019 and MS-MU-036) contain three coexisting pyroxene species: orthoenstatite, clinoenstatite and augite. The silicate assemblage appears to be the restite after extraction of melts of broadly basaltic and metal-sulfide composition from an enstatite chondrite protolith. Transmission electron microscopy (TEM) provides evidence that clinoenstatite within the lithology formed from earlier protoenstatite. The absence of pigeonite despite the successful nucleation of augite and the persistence of orthoenstatite during cooling suggests that the sub-solidus formation of the three coexisting pyroxenes occurred at a pressure of about 0.1 GPa. Rapid cooling at >1 K/h below 1260°C is documented by the cessation of augite equilibration, preservation of the 3-pyroxene assemblage and a low volume fraction of nanoscale orthoenstatite lamellae formed during the transformation of protoenstatite to clinoenstatite. The pressure implies a diameter of roughly 500 km of the differentiated parent body, putting petrological constraints on the size of planetesimals that may have contributed to the accretion of the terrestrial planets including Earth. The high cooling rate indicates a catastrophic disruption of this large planetesimal early in its history. The lithology studied here underlines that planetesimals which existed in the inner Solar System were more diverse than previously thought, and included potentially large differentiated bodies with very FeO-poor, enstatite-dominated mantles. Remains of these bodies are poorly represented in meteorite collections, which points to efficient accretion in the inner Solar System or to removal and little re-distribution of material into the present-day asteroid belt. • A unique enstatite-rich lithology was found among Almahata Sitta meteorite samples. • Petrology indicates a radius of ∼500 km of the differentiated parent planetesimal. • Microstructures indicate rapid cooling and suggest a catastrophic disruption. • This provides empirical insights into lost planetesimals of the inner Solar System. [ABSTRACT FROM AUTHOR]

Shock-induced Ti-rich melt pockets in a basaltic eucrite Northwest Africa (NWA) 8003 were studied using scanning and transmission electron microscopy. Unique mineral assemblages consisting of clinopyroxene, ilmenite, vestaite, corundum, and kyanite are observed. Among them, vestaite and corundum in NWA 8003 are first reported to occur in eucrite meteorites. Petrographic and chemical evidences indicate that the Ti-rich melt pockets have formed by in-situ melting of ilmenite, plagioclase, pyroxene, and possibly minor silica and apatite nearby. The temperature rise and melting were caused by the high shock impedance contrast at interfaces between ilmenite and other phases with a distinctly lower density. Crystallization pressure, temperature and cooling time of the Ti-rich melt pockets in NWA 8003 are constrained to be ˜0.9–˜10 GPa, ˜1300–˜1730 °C, and < 1 ms (5–50 μm in size), respectively. [ABSTRACT FROM AUTHOR]

• Space weathering alters lunar iron sulfides and produces iron metal whiskers. • Solar wind and thermal phenomena may cause the alteration of lunar iron sulfides. • Space weathering may contribute to sulfur isotopic fractionation in lunar soils. • The alteration of the iron sulfide might be common among airless bodies. Alteration of iron sulfides on the lunar surface by space weathering is poorly understood. We examined space weathering features of iron sulfides in lunar mature soil grains using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM observations reveal that iron sulfides have vesicular textures and iron whiskers on their surfaces. Iron sulfides observed using TEM are troilite and NC-pyrrhotite. The space-weathered rim on the iron sulfides is characterized by crystallographic misorientations and the disappearance of superstructure reflections of troilite in electron diffraction patterns. These crystallographic modifications are probably produced by solar wind irradiation. The rim contains opened vesicles that are aligned along the c -plane of the sulfides, as well as numerous tiny vesicles. The Fe/S ratio at the surface of the rim is higher than in non-altered regions, indicating selective sulfur loss from the surface. Iron whiskers protrude from the space weathered rim and consist of polycrystalline metallic iron. The sulfide rims and the iron whiskers are both coated with vapor-deposited materials rich in O and Si. The combined processes driven by the solar wind irradiation, heating during impact events, solar UV radiation, and the thermal cycling may cause vesicular textures, selective sulfur escape from the iron sulfides, and the formation of the iron whiskers. The rim textures support the notion that the enrichment of heavy sulfur isotopes in mature lunar soils is caused by space weathering of iron sulfides. The space weathered rims on lunar iron sulfides are similar to those observed in regolith samples from asteroid Itokawa. Therefore, alterations of sulfide surface might be common among airless bodies in the solar system. [ABSTRACT FROM AUTHOR]

On September 12, 2019 at 12:49:48 (UT) a bolide was observed by hundreds of eye-witnesses from the Netherlands, Germany, Belgium, Denmark and the UK. One day later a small meteorite stone was found by accident in Flensburg. The presence of short-lived cosmogenic radionuclides with half-lives as short as 16 days proves the recent exposure of the found object to cosmic rays in space linking it clearly to the bolide event. An exceptionally short exposure time of ∼5000 years was determined. The 24.5 g stone has a fresh black fusion crust, a low density of <2 g/cm3, and a magnetic susceptibility of logχ = 4.35 (χ in 10−9 m3/kg). The rock consists of relict chondrules and clusters of sulfide and magnetite grains set in a fine-grained matrix. The most abundant phases are phyllosilicates. Carbonates (∼3.9 vol.%) occur as calcites, dolomites, and a Na-rich phase. The relict chondrules (often surrounded by sulfide laths) are free of anhydrous silicates and contain abundant serpentine. Lithic clasts are also surrounded by similar sulfide laths partly intergrown with carbonates. 53Mn-53Cr ages of carbonates in Flensburg indicate that brecciation and contemporaneous formation of the pyrrhotite-carbonate intergrowths by hydrothermal activities occurred no later than 4564.6 ± 1.0 Ma (using the angrite D'Orbigny as the Mn-Cr age anchor). This corresponds to 2.6 ± 1.0 or 3.4 ± 1.0 Ma after formation of CAIs, depending on the exact absolute age of CAIs. This is the oldest dated evidence for brecciation and carbonate formation, which likely occurred during parent body growth and incipient heating due to decay of 26Al. In the three oxygen isotope diagram, Flensburg plots at the 16O-rich end of the CM chondrite field and in the transition field to CV-CK-CR chondrites. The mass-dependent Te isotopic composition of Flensburg is slightly different from mean CM chondrites and is most similar to those of the ungrouped C2 chondrite Tagish Lake. On the other hand, 50Ti and 54Cr isotope anomalies indicate that Flensburg is similar to CM chondrites, as do the ∼10 wt.% H 2 O of the bulk material. Yet, the bulk Zn, Cu, and Pb concentrations are about 30% lower than those of mean CM chondrites. The He, Ne, and Ar isotopes of Flensburg show no solar wind contribution; its trapped noble gas signature is similar to that of CMs with a slightly lower concentration of 20Ne tr. Based on the bulk H, C, and N elemental abundances and isotopic compositions, Flensburg is unique among chondrites, because it has the lightest bulk H and N isotopic compositions of any type 1 or 2 chondrite investigated so far. Moreover, the number of soluble organic compounds in Flensburg is even lower than that of the brecciated CI chondrite Orgueil. The extraordinary significance of Flensburg is evident from the observation that it represents the oldest chondrite sample in which the contemporaneous episodes of aqueous alteration and brecciation have been preserved. The characterization of a large variety of carbonaceous chondrites with different alteration histories is important for interpreting returned samples from the OSIRIS-REx and Hayabusa 2 missions. [ABSTRACT FROM AUTHOR]

The distributions of major and minor elements in Earth’s core and mantle were primarily established by high pressure, high temperature metal–silicate partitioning during core segregation. The partitioning behaviors of moderately siderophile elements can be used to constrain the pressure–temperature conditions of core formation and the core’s composition. We performed experiments to study the partitioning of Ni, Co, V, Cr, Si, and O between silicate melt and Fe-rich metallic melt in a multianvil press and diamond anvil cell, up to 100 GPa and 5700 K. Combining our new results with data from 18 previous studies, we parameterized the effects of pressure, temperature, and metallic melt composition on partitioning. Ni and Co partitioning are insensitive to composition. At low pressures, these elements become less siderophile with increasing temperature, with this trend reversing above ∼45 GPa. V and Cr partitioning are much more sensitive to metallic melt composition and less sensitive to pressure. Partitioning of Si and O are insensitive to pressure, but with strong and moderate temperature dependences, respectively. Our new parameterizations of Ni and Co partitioning suggest that the Earth’s distributions of these elements can be matched by single-stage core–mantle equilibration at 54 ± 5 GPa and 3300–3400 K. These conditions would result in 8.5 ± 1.4 wt% Si and 1.6 ± 0.3 wt% O in the core, compatible with the core’s measured density. However, this single-stage model matches the Earth’s V and Cr distributions less well. We also incorporated our parameterizations into models of multi-stage core formation over evolving pressure–temperature-oxygen fugacity conditions, reproducing the Earth’s Ni and Co distributions while simultaneously producing a core whose light element composition is consistent with its density. [ABSTRACT FROM AUTHOR]