Aragonite, breunnerite, calcite and dolomite in the CM carbonaceous chondrites: High fidelity recorders of progressive parent body aqueous alteration

Abstract

Carbonate minerals in CM carbonaceous chondrite meteorites, along with the silicates and sulphides with which they are intergrown, provide a detailed record of the nature and evolution of parent body porosity and permeability, and the chemical composition, temperature and longevity of aqueous solutions. Fourteen meteorites were studied that range in petrologic subtype from mildly aqueously altered CM2.5 to completely hydrated CM2.0. All of them contain calcite, whereas aragonite occurs only in the CM2.5–CM2.2 meteorites and dolomite in the CM2.2–CM2.0. All of the aragonite crystals, and most of the calcite and dolomite grains, formed during early stages of parent body aqueous alteration by cementation of pores produced by the melting of tens of micrometre size particles of H2O-rich ice. Aragonite was the first carbonate to precipitate in the CM2.5 to CM2.2 meteorites, and grew from magnesium-rich solutions. In the least altered of these meteorites the aragonite crystals formed in clusters owing to physical restriction of aqueous fluids within the low permeability matrix. The strong correlation between the petrologic subtype of a meteorite, the abundance of its aragonite crystals and the proportion of them that have preserved crystal faces, is because aragonite was dissolved in the more altered meteorites on account of their higher permeability, and/or greater longevity of the aqueous solutions. Dolomite and breunnerite formed instead of aragonite in some of the CM2.1 and CM2.2 meteorites owing to higher parent body temperatures. The pore spaces that remained after precipitation of aragonite, dolomite and breunnerite cements were occluded by calcite. Following completion of cementation, the carbonates were partially replaced by phyllosilicates and sulphides. Calcite in the CM2.5–CM2.2 meteorites was replaced by Fe-rich serpentine and tochilinite, followed by Mg-rich serpentine. In the CM2.1 and CM2.0 meteorites dolomite, breunnerite and calcite were replaced by Fe-rich serpentine and Fe–Ni sulphide, again followed by Mg-rich serpentine. The difference between meteorites in the mineralogy of their replacive sulphides may again reflect greater temperatures in the parent body regions from where the more highly altered CMs were derived. This transition from Fe-rich to Mg-rich carbonate replacement products mirrors the chemical evolution of parent body solutions in response to consumption of Fe-rich primary minerals followed by the more resistant Mg-rich anhydrous silicates. Almost all of the CMs examined contain a second generation of calcite that formed after the sulphides and phyllosilicates and by replacement of remaining anhydrous silicates and dolomite (dedolomitization). The Ca and CO2 required for this replacive calcite is likely to have been sourced by dissolution of earlier formed carbonates, and ions may have been transported over metre-plus distances through high permeability conduits that were created by impact fracturing.