Modal mineralogy of CM chondrites by X-ray diffraction (PSD-XRD): Part 2. Degree, nature and settings of aqueous alteration

Abstract

Within 5 million years after formation of calcium aluminium rich inclusions (CAI), high temperature anhydrous phases were transformed to hydrous phyllosilicates, mostly serpentines, which dominate the matrices of the most primitive carbonaceous chondrites. CMs are the largest group of meteorites to provide samples of this material. To understand the nature of the availability, and role of H2O in the early solar system – as well as the settings of aqueous alteration – defining CM petrogenesis is critical. By Position Sensitive Detector X-ray Diffraction (PSD-XRD), we determine the modal abundance of crystalline phases present in volumes >1% for a suite of CMs – extending Part 1 of this work that dealt only with CM2 falls (Howard et al., 2009) to now include CM2 and CM1 finds. CM2 samples contain 13–31% Fe,Mg silicates (olivine + pyroxene) and from 67% to 82% total phyllosilicate (mean 75% ± 1.3 2σ). CM1 samples contain 6–10% olivine + pyroxene and 86–88% total phyllosilicate. Magnetite (0.6–5.2%), sulphide (0.6–3.9%), calcite (0–1.9%) and gypsum (0–0.8%) are minor phases across all samples. Since phyllosilicate forms from hydration of anhydrous Fe,Mg silicates (olivine + pyroxene), the ratio of total phyllosilicate to total anhydrous Fe,Mg silicate defines the degree of hydration and the following sequence results (in order of increasing hydration): QUE 97990 < Y 791198 < Murchison < Murray < Mighei < ALHA 81002 < Nogoya ≤ Cold Bokkeveld ≤ Essebi < QUE 93005 < ALH 83100 < MET 01070 < SCO 06043. High activities of Al (mostly from reactive mesostasis) and Si help to explain the composition and structure of CM serpentines that are distinct from terrestrial standards. Our data allows inference as to CM mineralogy at the point of accretion and challenges the conceptual validity of progressive alteration sequences. Modal mineralogy also provides new insights into CM petrogenesis and hints at a component of aqueous alteration occurring in the nebula, in addition to on the CM parent body(ies).