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.

The bulk S elemental abundances and delta S-34 values for 83 carbonaceous chondrites (mostly CMs and CRs) and Semarkona (LL3.0) are reported. In addition, the S elemental abundances and delta S-34 values of insoluble organic material (IOM) isolated from 25 carbonaceous chondrites (CMs, CRs, and three ungrouped) are presented. The IOM only contributes 2-7% of the S to the bulk meteorites analyzed and exhibits no systematic variations. The average group bulk S abundances are similar to previous measurements. In-group variations likely reflect variations in matrix abundances, as well as parent body processes and weathering. The S and C abundances are roughly correlated and scatter about a mixing line between CI-like matrix and C-free and S-depleted chondrules. Systematic deviations from this mixing line may indicate different degrees of heating of matrix material in the nebula. There are no systematic variations in average group delta S-34 values, in contrast to what is seen for the volatile chalcophiles Zn, Te, Se, and Ag, as well as the less volatile siderophile Cu. Renormalization of the elemental and isotopic compositions indicates that the elemental and isotopic fractionations of Zn, Te, and Ag were controlled by the same process, whereas Se is intermediate in its behavior between these three elements and S. The isotopic fractionations could be associated with diffusion of volatile chalcophiles into sulfide at the end of chondrule formation. Copper appears to be distinct in its behavior from the chalcophiles, perhaps because it is more refractory and more siderophile.

Little is known about the origin of the spectral diversity of asteroids and what it says about conditions in the protoplanetary disk. Here, we show that samples returned from Cb-type asteroid Ryugu have Fe isotopic anomalies indistinguishable from Ivuna-type (CI) chondrites, which are distinct from all other carbonaceous chondrites. Iron isotopes, therefore, demonstrate that Ryugu and CI chondrites formed in a reservoir that was different from the source regions of other carbonaceous asteroids. Growth and migration of the giant planets destabilized nearby planetesimals and ejected some inward to be implanted into the Main Belt. In this framework, most carbonaceous chondrites may have originated from regions around the birthplaces of Jupiter and Saturn, while the distinct isotopic composition of CI chondrites and Ryugu may reflect their formation further away in the disk, owing their presence in the inner Solar System to excitation by Uranus and Neptune.