Aqueous Alteration Index for CR Chondrites

The CR chondrite group of meteorites have experienced little to no thermal metamorphism, making them important for the study of early solar-system processes.  However, they have experienced varying degrees of aqueous alteration on the parent asteroid.   A petrographic scale was constructed to gauge aqueous alteration in CM chondrites, but no such scale has been formulated for CR chondrites.  I have used petrographic and compositional parameters to create an alteration index for CR chondrites.  Thin sections of 10 CR chondrites were selected for study.  Reflected light was used to identify alteration rinds on metal grains; transmitted light was used to identify chondrule mesostasis as altered or clear isotropic glass; and wavelength dispersive spectroscopy was used to determine compositions and infer hydration.

Renazzo has been tentatively classified as a 2.4 due to having similar chondrule alteration to the CM2.4 chondrites; it contains incipiently altered olivine phenocrysts, hydrated matrix, and a phyllosilicate mesostasis.  The whole-rock metal abundance does not correlate between CM and CR chondrites because of a higher overall metal abundance in the CR chondrites.  GRO 95577 is assigned to type 2.0; all of the metal in this meteorite is nearly or completely oxidized and the mafic silicate grains have been converted to phyllosilicates.  Most CR chondrites fall in the 2.9-3.0 range.  They can be further distinguished by the presence of glass in the mesostasis and by transmission electron microscopy to determine the extent of matrix hydration.  Application of this scale will enhance our understanding of parent-body aqueous alteration for CR chondrites.