Deciphering the Complex Alteration Recorded in Primitive Asteroids

Findlay, Ross (2024). Deciphering the Complex Alteration Recorded in Primitive Asteroids. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00096757

Abstract

CM chondrites are unequilibrated breccias sampling primitive asteroids altered by water-rock interactions. While much research has focused on bulk measurements, only tentative attempts have been made to elucidate the complex petrographic and O-isotopic heterogeneity at the clast and component scale utilising this high precision approach.

Clasts of petrologic subtype CM1/2.0 – 2.8 were found in four falls and one very diverse find, LON 94101, the latter of which strengthens the case of a discrete parent body source for CM chondrites. The petrologic subtype of moderately altered clasts ranging from 2.2-2.6 appear to correlate well with variable ‘FeO’/SiO₂ and Mg# compositions within their phyllosilicates, though this is poorly matched from meteorite to meteorite. Fully altered clasts (CM1/CM2.0) host an array of diverse textures ranging from dark, Mg-dominated matrix-rich clasts to those containing abundant chondrule pseudomorphs, comparable in large part to the diversity seen in CM2 material. An array of rarer, petrographically distinct lithologies, including incipiently altered, predominantly anhydrous examples, attest to the diversity of CM material in the meteorite record.

Matrix phyllosilicates from 30 petrographically characterised CM lithologies and 4 cataclastic matrix fractions were micro-sampled to investigate lithology-specific aqueous alteration phenomena through their O-isotopes, obtained via a revised laser fluorination protocol for small, phyllosilicate-rich samples. The exclusion of a significant anhydrous precursor component within these fractions reveals a novel, apparently mass-dependent trend: the CM matrix fractionation line (CMF) at ~ Δ¹⁷O = -2.35 ± 0.46‰ (2σ), indicating the CM lithologies were altered within a narrow range of water to rock (WR) ratios and that historical O-isotope variation primarily just reflects variable amounts of anhydrous precursor. This weakens the case for a closed system style of alteration, where variable WR ratios may be expected; indeed, an open system may provide a better explanation where equilibration of water is achieved over a mineralogically diverse area on the CM asteroid.