Classification of hydrous meteorites (CR, CM and C2 ungrouped) by phyllosilicate fraction: PSD-XRD modal mineralogy and planetesimal environments

Abstract

The relative differences in the degree of hydration should be reflected in any classification scheme for aqueously altered meteorites. Here we report the bulk mineralogies and degree of hydration in 37 different carbonaceous chondrites: Renazzo-like (CR), Mighei-like (CM), and ungrouped (type 2) samples. This is achieved by quantifying the modal abundances of all major (phases present in abundances >1 wt.%) minerals using Position Sensitive Detector X-ray Diffraction (PSD-XRD). From these modal abundances, a classification scheme is constructed that is based on the normalized fraction of phyllosilicate (total phyllosilicate/total anhydrous silicate + total phyllosilicate). Samples are linearly ranked from type 3.0 – corresponding to a phyllosilicate fraction of <0.05, to type 1.0 – corresponding to a total phyllosilicate fraction of >0.95. Powdered meteorite samples from any hydrated carbonaceous chondrite group can be ranked on this single classification scale. The resulting classifications for CRs exhibit a range from type 2.8 to 1.3, while for CMs the range is 1.7–1.2. The primary manifestation of aqueous alteration is the production of phyllosilicate, which ceased when the fluid supply was exhausted, leading to the preservation of anhydrous silicates in all samples. The variability in hydration indicates that either accretion of ices was heterogeneous or fluid was mobilized. From the bulk mineral abundances of the most hydrated samples, we infer that the initial mass fraction of H2O inside of their parent body(ies) asteroids was <20 wt.%. Bulk carbonaceous chondrite mineralogy evolved towards increasingly oxidizing assemblages as the extent of bulk hydration increased. This is consistent with the escape of reducing H2 gas that is predicted to have been produced from water during hydration reactions.