Why is the Degree of Aqueous Alteration Variable?

K. T. Howard and B. Zanda

82nd Annual Meeting of The Meteoritical Society 2019

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“Minerals containing water form inside of asteroids during aqueous alteration. We have quantified the range in degrees of aqueous alteration for carbonaceous chondrites from all groups [1]. Here we focus on the CM chondrite Paris and selected CM falls. We aim to answer the question: why did aqueous alteration inside of the CM parent body asteroid(s) vary?
Our approach: The fraction of hydrated silicates to anhydrous silicates (PSF) measures the degree of aqueous alteration [1]. The key parameter controlling hydration reactions is water/rock ratio. Water/rock ratios have only been inferred to date, e.g., from O-isotope compositions [2] and O-stoichiometry [1]. Paris preserves large volumes of primary matrix in the form of amorphous Fe-(Mg)-silicate [3], quantifying its abundance allows us to place an upper limit on the fraction of ice accreted in CM matrix. From bulk modal mineralogy and component (chondrule vs. matrix) abundances, we reconstruct the initial H2O contents of CMs to reveal the cause of variable hydration.”