We present the first results of a comprehensive investigation aimed at testing the hypothesis of chondrule-matrix complementarity and the four-component model for the compositions of the carbonaceous chondrites and their components. Combining point-counting with electron microprobe analyses, we have determined the bulk compositions of thin sections, as well as the average abundances and compositions of the major chondritic components (chondrules, matrix, refractory inclusions, isolated silicate grains and isolated opaque grains). To minimize the potential for element exchange between components during parent body processing, the two most primitive COs, DOM 08006 and ALH 77307, and the primitive ungrouped CO/CM-like Acfer 094 were selected for this study. To verify our method, we also examined one section of the well-studied CO3.2 Kainsaz, a fall that is free of weathering. We were able to reproduce all major and many minor elemental concentrations reported in the literature for average bulk COs and Kainsaz to better than 10%. The elements most commonly cited as displaying evidence for complementarity are Mg, Si, Al, Ca, Fe and Ti. Iron, however, can be easily affected by chondrule metal-silicate fractionation, redistribution in the parent body and weathering, and our Ti data for matrix are likely compromised by an analytical artifact. Hence, we focused on Mg, Al, Si and Ca - four elements that we can determine very accurately - and show that their relative abundances in chondrules are on average CI-like within the uncertainties of the method. The matrix is not CI-like, but its composition can be explained by the loss of 10-15 wt.% of forsterite from an initially CI-like material prior to or during parent body accretion. These results are inconsistent with chondrule-matrix complementarity. Our average CO chondrule compositions, as well as chondrule and matrix abundances, are in line with the predictions of the four-component model. However, the four-component model assumes a CI-like composition for matrix, and also predicts refractory inclusion abundances that are higher and compositions that are less refractory than we observe. While similar studies of the other carbonaceous chondrite groups are needed, these differences may indicate the limitations of the simplifying assumptions made in the four-component model. (C) 2021 Elsevier Ltd. All rights reserved.

Chondrites are meteorites from undifferentiated parent bodies that provide fundamental information about early Solar System evolution and planet formation. The element Cr is highly suitable for deciphering both the timing of formation and the origin of planetary building blocks because it records both radiogenic contributions from Mn-53-Cr-53 decay and variable nucleosynthetic contributions from the stable Cr-54 nuclide. Here, we report high-precision measurements of the massindependent Cr isotope compositions (epsilon Cr-53 and epsilon Cr-54) of chondrites (including all carbonaceous chondrites groups) and terrestrial samples using for the first time a multi-collection inductively-coupled-plasma mass-spectrometer to better understand the formation histories and genetic relationships between chondrite parent bodies. With our comprehensive dataset, the order of decreasing epsilon Cr-54 (per ten thousand deviation of the Cr-54/Cr-52 ratio relative to a terrestrial standard) values amongst the carbonaceous chondrites is updated to CI = CH >= CB >= CR >= CM approximate to CV >= CO >= CK > EC > OC. Chondrites from CO, CV, CR, CM and CB groups show intra-group epsilon Cr-54 heterogeneities that may result from sample heterogeneity and/or heterogeneous accretion of their parent bodies. Resolvable epsilon Cr-54 (with 2SE uncertainty) differences between CV and CK chondrites rule out an origin from a common parent body or reservoir as has previously been suggested. The CM and CO chondrites share common epsilon Cr-54 characteristics, which suggests their parent bodies may have accreted their components in similar proportions. The CB and CH chondrites have low-Mn/Cr ratios and similar epsilon Cr-53 values to the CI chondrites, invalidating them as anchors for a bulk Mn-53-Cr-53 isochron for carbonaceous chondrites. Bulk Earth has a epsilon Cr-53 value that is lower than the average of chondrites, including enstatite chondrites. This depletion may constrain the timing of volatile loss from the Earth or its precursors to be within the first million years of Solar System formation and is incompatible with Earth's accretion via any of the known chondrite groups as main contributors, including enstatite chondrites. (C) 2021 Elsevier Ltd. All rights reserved.

Like most primitive carbonaceous chondrites, the CM chondrites experienced varying degrees of asteroidal aqueous alteration, which may have overprinted pre-accretionary processing. Several aqueous alteration scales for CM chondrites (and other carbonaceous chondrites) have been proposed based on alteration-dependent changes in various petrological and geochemical characteristics. Given the possibility that the intensity of aqueous alteration could be recorded in the primordial noble gas compositions, we test potential correlations between petrologic, geochemical and noble gas characteristics in a detailed study on 39 CM chondrites, including some of the most pristine CM chondrites identified to date, and 4 CM related carbonaceous chondrites. We mainly compare our noble gas data with the alteration schemes proposed by Alexander et al. (2013) and Howard et al. (2015). In addition to the noble gas analyses, we determined the phyllosilicate fractions of 17 of the CM chondrites using X-ray diffraction (XRD) to complement missing data points in the Howard alteration scheme. The influence of post-hydration thermal modification on noble gases in CM chondrites is investigated by comparison of heated and unheated samples. Cosmic-ray exposure (CRE) ages are determined for all samples in this study as well as for 26 more samples based on CM chondrite literature noble gas data.

We investigated the stable isotope fractionation of chromium (Cr) for a panorama of chondrites, including EH and EL enstatite chondrites and their chondrules and different phases (by acid leaching). We observed that chondrites have heterogeneous delta Cr-53 values (per mil deviation of the Cr-53/Cr-52 from the NIST SRM 979 standard), which we suggest reflect different physical conditions in the different chondrite accretion regions. Chondrules from a primitive EH3 chondrite (SAH 97096) possess isotopically heavier Cr relative to their host bulk chondrite, which may be caused by Cr evaporation in a reduced chondrule-forming region of the protoplanetary disk. Enstatite chondrites show a range of bulk delta Cr-53 values that likely result from variable mixing of isotopically different sulfide-silicate-metal phases. The bulk silicate Earth (delta Cr-53 = -0.12 +/- 0.02 parts per thousand, 2SE) has a lighter Cr stable isotope composition compared to the average delta Cr-53 value of enstatite chondrites (-0.05 +/- 0.02 parts per thousand, 2SE, when two samples out of 19 are excluded). If the bulk Earth originally had a Cr isotopic composition that was similar to the average enstatite chondrites, this Cr isotope difference may be caused by evaporation under equilibrium conditions from magma oceans on Earth or its planetesimal building blocks, as previously suggested to explain the magnesium and silicon isotope differences between Earth and enstatite chondrites. Alternatively, chemical differences between Earth and enstatite chondrite can result from thermal processes in the solar nebula and the enstatite chondrite-Earth, which would also have changed the Cr isotopic composition of Earth and enstatite chondrite parent body precursors.

Knowing how the major chondritic components evolved and what their initial compositions were is pivotal for our understanding of the processes that shaped the early Solar System. Here, we have extended to the CR chondrites our testing of chondrule-matrix complementarity and the four-component model, i.e., two very different explanations for the bulk compositions of the carbonaceous chondrites and their components. Combining point-counting with electron microprobe analyses, we have analyzed four relatively primitive Antarctic CRs and the fall Renazzo. Our results for the abundances of chondrules and matrix are in good agreement with literature data, and confirm that these abundances vary considerably amongst the CRs (80.4 +/- 2.3 wt.% and 18.5 +/- 2.8 wt.%, respectively, in the four Antarctic CRs vs. 62.3 +/- 3.4 wt.% and 33.2 +/- 2.2 wt.% in Renazzo). The significant differences make the determination of the average properties and bulk compositions of the CRs problematic. This is particularly true for the volatile elements that were predominantly accreted in matrix. Nevertheless, all major and many minor element concentrations reported in the literature for average bulk CRs are reproduced here to better than 10%. By comparing our results to conventionally determined bulk compositions, we were able to verify the accuracy of our approach and identify elements likely affected by alteration or analytical artifacts (e.g., Ti, K, Co). Two particular compositional details of the CR chondrites investigated are (a) the relatively high contents of Mn in the chondrules compared to CO chondrules, and (b) the depletion of S in the matrix, relative to CI. In terms of the major elements Mg, Al, Si and Ca, our data suggest that unaltered chondrules and matrix exhibited CI-like relative abundances, supporting previous conclusions for the CO chondrites. Where observed, deviations of element abundances in the matrix from CI (Na, Mg, S, Ca, Fe, Ni) can be explained in terms of alteration (parent body and terrestrial) and pre-accretionary loss of forsterite and, possibly, sulfides. Overall, our results are more consistent with the predictions of the four-component model than they are with chondrulematrix complementarity. (C) 2022 Elsevier Ltd. All rights reserved.

The D/H ratio is a clue to the origin and evolution of hydrogen-bearing chemical species in Solar system materials. D/H has been observed in the coma of many comets, but most such measurements have been for gaseous water. We present the first in situ measurements of the D/H ratios in the organic refractory component of cometary dust particles collected at very low impact speeds in the coma of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by the COSIMA instrument onboard Rosetta. The values measured by COSIMA are spatial averages over an approximately 35 x 50 mu m(2) area. The average D/H ratio for the 25 measured particles is (1.57 +/- 0.54) x 10(-3), about an order of magnitude higher than the Vienna Standard Mean Ocean Water (VSMOW), but more than an order of magnitude lower than the values measured in gas-phase organics in solar-like protostellar regions and hot cores. This relatively high averaged value suggests that refractory carbonaceous matter in comet 67P is less processed than the most primitive insoluble organic matter (IOM) in meteorites, which has a D/H ratio in the range of about 1 to 7 x 10(-4). The cometary particles measured in situ also have a higher H/C ratio than the IOM. We deduce that the measured D/H in cometary refractory organics is an inheritance from the presolar molecular cloud from which the Solar system formed. The high D/H ratios observed in the cometary particles challenges models in which high D/H ratios result solely from processes that operated in the protosolar disc.