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We report the results of petrological, geochemical, and geochronological investigations of the unusual K‐rich L chondrite melt rock Northwest Africa 6486 (NWA 6486). The rock has slightly fractionated siderophile elements and a mostly unfractionated L chondrite pattern of lithophile elements with the exceptions of enrichments in K and Rb and chondritic Sr abundance similar to the K‐rich inclusions found in the ordinary chondrites and indicating a fractionation of alkaline elements through the vapor. We suggest that NWA 6486 and related K‐rich chondritic inclusions were formed in situ on the OC parent bodies and that K and Rb enrichment of these rock most probably is a result of the selective impact evaporation of volatile alkali elements followed by the reaction of a vapor with shock melt. NWA 6486 recorded a breakup event of the L chondrite parent asteroid at 470 Ma during which it was formed. Unusual veins, depleted in K, Na, Ca, and Al relative to the host rock were found in NWA 6486. We suggest that NWA 6486 was affected by aqueous fluids that produced alteration zones depleted in a feldspar component on the walls of opened fractures. The melt veins could be formed during a subsequent impact event by in situ melting of the fracture walls or due to decomposition of an injected supercritical aqueous silicate fluid. The aqueous alteration and the second impact event had no detectable effect on Ar and oxygen isotopic systems. Cosmic ray exposure ages indicate that NWA 6486 was ejected from its parent asteroid ~3–4 Ma ago. [ABSTRACT FROM AUTHOR]

Mineral inventories of enstatite chondrites; ( EH and EL) are strictly dictated by combined parameters mainly very low dual oxygen ( fO2) and sulfur ( fS2) fugacities. They are best preserved in the Almahata Sitta MS-17, MS-177 fragments, and the ALHA 77295 and MAC 88136 Antarctic meteorites. These conditions induce a stark change of the geochemical behavior of nominally lithophile elements to chalcophile or even siderophile and changes in the elemental partitioning thus leading to formation of unusual mineral assemblages with high abundance of exotic sulfide species and enrichment in the metallic alloys, for example, silicides and phosphides. Origin and mode of formation of these exotic chondrites, and their parental source regions could be best scrutinized by multitask research experiments of the most primitive members covering mineralogical, petrological, cosmochemical, and indispensably short-lived isotopic chronology. The magnitude of temperature and pressure prevailed during their formation in their source regions could eventually be reasonably estimated: pre- and postaccretionary could eventually be deduced. The dual low fugacities are regulated by the carbon to oxygen ratios estimated to be >0.83 and <1.03. These parameters not only induce unusual geochemical behavior of the elements inverting many nominally lithophile elements to chalcophile or even siderophile or anthracophile. Structure and mineral inventories in EL3 and EH3 chondrites are fundamentally different. Yet EH3 and EL3 members store crucial information relevant to eventual source regions and importantly possible variation in C/O ratio in the course of their evolution. EL3 and EH3 chondrites contain trichotomous lithologies (1) chondrules and their fragments, (2) polygonal enstatite-dominated objects, and (3) multiphase metal-rich nodules. Mineralogical and cosmochemical inventories of lithologies in the same EL3 indicate not only similarities ( REE inventory and anomalies in oldhamite) but also distinct differences (sinoite-enstatite-graphite relationship). Oldhamite in chondrules and polygonal fragments in EL3 depict negative Eu anomaly attesting a common cosmochemical source. Metal-dominated nodules in both EL3 and EH3 are conglomerates of metal clasts and sulfide fragments in EH3 and concentrically zoned C-bearing metal micropebbles (≥25 μm ≤50 μm) in EL3 thus manifesting a frozen in unique primordial accretionary metal texture and composition. Sinoite-enstatite-diopside-graphite textures reveal a nucleation and growth strongly suggestive of fluctuating C/O ratio during their nucleation and growth in the source regions. Mineral inventories, sulfide phase relations, sinoite-enstatite-graphite intergrowth, carbon and nitrogen isotopic compositions of graphite, spatial nitrogen abundance in graphite in metal nodules, and last but not least 129I/129Xe and 53Mn/53Cr systematics negate any previously suggested melting episode, pre-accretionary or dynamic, in parental asteroids. [ABSTRACT FROM AUTHOR]

Introduction: Olivine is one of the major minerals in chondritic meteorites and occurs in all chondritic components, including amoeboid olivine aggregates (AOAs), forsterite-bearing Type B (FoB) Ca,Al-rich inclusions (CAIs), chondrules, and matrices (e.g., [1-4]). As a result, olivines could preserve chemical and isotopic characteristics of their formation environments. There are significant variations in oxygen isotopic compositions of chondritic olivine. Forsteritic olivine in AOAs and FoB-CAIs has solar-like 16O-rich compositions (D17O ~ -23±2‰); the former condensed from an 16O-rich gas of ~solar composition, whereas the latter crystallized from remelted condensates in an isotopic similar gaseous reservoir. Olivine phenocrysts in chondrules are significantly 16O-depleted compared to CAIs and AOAs: D17O ranges from ~ -7 to +3‰. Only rare relict olivines in chondrules have CAI/AOA-like 16O-rich compositions; these grains most likely represent fragments of CAIs and AOAs incompletely melted during chondrule formation (e.g., [5]). Olivine in matrices of weakly metamorphosed meteorites is extremely fine-grained (<100 nm) and has predominantly forsteritic composition. In metamorphosed chondrites, matrix olivines are coarse-grained (up to 10 μm) and enriched in FeO. Due to small grain sizes, oxygen isotopic compositions of matrix olivines are poorly known. Mapping of matrices of Vigarano [6] and Kakangari [7] using isotope microscopes (secondary ion mass-spectrometer (SIMS) plus SCAPS detector) revealed the presence of 16O-rich olivine grains. While most matrix olivines in Vigarano have 16O-poor compositions, nearly half of olivines measured in Kakangari matrix are 16O-rich. Here, we present for the first time new lithologies largely composed of 16O- and Fe-rich olivine. We attempt to address the question of whether these 16O-rich olivine grains were formed by condensation in the solar nebula or by a secondary process during the evolution of the parent body. Mineralogical characteristics of these unusual olivines, as well as in-situ measured oxygen-isotope compositions of different phases are reported in detail. Samples and analytical methods: The new lithologies occur as large clasts in two different ordinary chondrites: Northwest Africa 5206 (LL3) and Northwest Africa 6925 (L3) Scanning electron microscopy (SEM) and electron microprobe analysis (EMPA) were performed at the University of Münster. Two electron-transparent lamellae from one clast were prepared using a Zeiss CrossBeam 340 focused ion beam system (FIB). Subsequent transmission electron microscope (TEM) analyses were performed using an aberration-corrected Thermo Scientific "Themis" (300 kV). In-situ oxygen isotope analyses in olivine were carried out using SIMS at both the University of Heidelberg (Cameca ims-1280 HR) and the University of Hawai'i (Cameca ims-1280). In-situ trace and rare earth element (REE) concentrations were measured by inductively coupled mass spectrometry by laser ablation (LA-ICP-MS). Results and discussion: The lithologies consist mainly of FeO-rich olivine (Fa~34) ranging from a few up to ~20µm, hydrous amphiboles (intergrowth of tremolite and cummingtonite), phyllosilicates, and some Fe-Ni metal and Fe-Ni sulfides. FeO-rich olivine represents the major phase (~60 vol.%) forming the matrix, and commonly occurs as rounded objects (enclosing amphiboles), similar to donut-shaped AOAs with metal or sulfide inside. In addition, fragments of 2 POP chondrules (16O-poor) were found in both clasts. The clasts show flat CI-normalized REE patterns (~1.4 x CI). Oxygen-isotopic compositions in FeO-rich olivine grains plot along ~slope-1 line in 3O-isotope diagram, ranging from ~-25 to ~3? in D17O; by far most olivines have ?17O ~-24?, consistent with typical CAIs and AOAs. Some olivines have positive D17O values similar to chondrules in ordinary and R chondrites. The others have intermediate D17O values. The 16O-rich and 16O-depleted olivines have similar Fa contents (~34 mol%). The 16O-rich olivines are texturally and isotopically similar to olivines in AOAs and most likely condensed directly in the early solar nebula at high temperatures. Subsequently they accreted together with less 16O-rich olivines and water ices (as indicated by abundant hydrated phases), and experienced Fe-Mg interexchange during fluid-assisted thermal metamorphism in an asteroidal setting but largely escaped from O-isotope equilibration. The extremely high proportion of 16O-rich condensates in the clasts may suggest their early accretion either in the inner or outer disk. Bulk Cr and Ti isotopic compositions of the clasts may help to understand their genetic relationship to carbonaceous or noncarbonaceous meteorites. This work is in progress. [ABSTRACT FROM AUTHOR]

Aims. Radiometric ages for chondritic meteorites and their components provide information on the accretion timescale of chondrite parent bodies, and on cooling paths within certain areas of these bodies. However, to use this age information for constraining the internal structure, and the accretion and cooling history of the chondrite parent bodies, the empirical cooling paths obtained by dating chondrites must be combined with theoretical models of the thermal evolution of planetesimals. Important parameters in such thermal models include the initial abundances of heat-producing short-lived radionuclides (26Al and 60Fe), which are determined by the accretion timescale and the terminal size, chemical composition and physical properties of the chondritic planetesimals. The major aim of this study is to assess the effects of sintering of initially porous material on the thermal evolution of planetesimals, and to constrain the values of basic parameters that determined the structure and evolution of the H chondrite parent body. Methods. We present a new code for modelling the thermal evolution of ordinary chondrite parent bodies that initially are highly porous and undergo sintering by hot pressing as they are heated by decay of radioactive nuclei. The pressure and temperature stratification in the interior of the bodies was calculated by solving the equations of hydrostatic equilibrium and energy transport. The decrease of porosity of the granular material by hot pressing due to self-gravity was followed by solving a set of equations for the sintering of powder materials. For the heat-conductivity of granular material we combined recently measured data for highly porous powder materials, relevant for the surface layers of planetesimals, with data for heat-conductivity of chondrite material, relevant for the strongly sintered material in deeper layers. Results. Our new model demonstrates that in initially porous planetesimals heating to central temperatures sufficient for melting can occur for bodies a few km in size, that is, a factor of ≈10 smaller than for compact bodies. Furthermore, for high initial 60Fe abundances small bodies may differentiate even when they had formed as late as 3-4 Ma after CAI formation. To demonstrate the capability of our new model, the thermal evolution of the H chondrite parent body was reconstructed. The model starts with a porous body that is later compacted first by "cold pressing" at low temperatures and then by "hot pressing" for temperatures above ≈700K, i.e., the threshold temperature for sintering of silicates. The thermal model was fitted to the well-constrained cooling histories of the two H chondrites Kernouvé (H6) and Richardton (H5). The best fit was obtained for a parent body with a radius of 100 km that accreted at t = 2.3Ma after CAI formation, and had an initial 60Fe/56Fe = 4.1 × 10-7. Burial depths of 8.3 km and 36 km for Richardton and Kernouvé were able to reproduce their empirically determined cooling history. These burial depths are shallower than those derived in previous models. This reflects the strong insulating effect of the residual powder surface layer, which is characterised by a low thermal conductivity. [ABSTRACT FROM AUTHOR]

Highlights: [•] Sintering by cold compaction and hot isostatic pressing of asteroid material. [•] Optimised fit of parent body model to thermal history data of nine H chondrites. [•] Comparing instantaneous formation approximation with protracted growth. [•] Retrieving accretion time of H chondrite parent body from meteoritic record. [•] Influence of pre-heating of asteroid building blocks negligible. [Copyright &y& Elsevier]