Re-examining thermal metamorphism of the Renazzo-like (CR) carbonaceous chondrites: Insights from pristine Miller Range 090657 and shock-heated Graves Nunataks 06100

We re-examine the Renazzo-like (CR) chondrite metamorphic trend based on Cr₂O₃ contents of FeO-rich olivine, indicating that it is only appropriate to use such analyses to identify the endmembers of this group (i.e., those that have experienced either no detectable heating or significant heating). As such Miller Range (MIL) 090657 appears to have experienced very minimal (if any) thermal processing and is one of the most pristine CR chondrites analyzed to date, while Graves Nunataks 06100 is the most shock-heated CR chondrite studied. On the basis of bulk H-C-N isotopic compositions, MIL 090657 appears to be of petrological type 2.7. We also report the H-C-N isotopic compositions of extracted insoluble organic matter, in situ chemical compositional data, presolar grain abundances, and a petrologic description of MIL 090657. As a minimally altered CR chondrite of relatively high mass (133.1 g), MIL 090657 provides an invaluable opportunity to perform coordinated, often destructive, analyses on pristine CR chondrite material. By combining a number of petrographic characteristics (Cr₂O₃-content of ferroan olivine, Co/Ni ratios of Fe,Ni metal, ratios of Fe# in chondrule olivine and low-Ca pyroxene, and the presence of excess silica in chondrule plagioclase) with bulk isotopic compositions, we demonstrate their utility as indicators for determining the relative pristinity/heating of low petrographic (type 1–3) chondrites.