The oxygen isotope evolution of parent body aqueous solutions as recorded by multiple carbonate generations in the Lonewolf Nunataks 94101 CM2 carbonaceous chondrite

摘要

The CM2 carbonaceous chondrite LON 94101 contains aragonite and two generations of calcite that provide snapshots of the chemical and isotopic evolution of aqueous solutions during parent body alteration. Aragonite was the first carbonate to crystallize. It is rare, heterogeneously distributed within the meteorite matrix, and its mean oxygen isotope values are δ~(18)O 39.9±0.6‰, δ~(17)O -0.3±1.0‰ (1σ). Calcite precipitated soon afterwards, and following a fall in solution Mg/Ca ratios, to produce small equant grains with a mean oxygen isotope value of δ~(18)O 37.5±0.7‰, δ~(17)O 1.4±1.1‰ (1σ). These grains were partially or completely replaced by serpentine and tochilinite prior to precipitation of the second generation of calcite, which occluded an open fracture to form a millimetre-sized vein, and replaced anhydrous silicates within chondrules and the matrix. The vein calcite has a mean composition of δ~(18)O 18.4±0.3‰, δ~(17)O -0.5±0.5‰ (1σ). Petrographic and isotopic results therefore reveal two discrete episodes of mineralisation that produced calcite generations with contrasting δ~(18)O, and mean δ~(17)O values. The aragonite and equant calcite crystallized over a relatively brief period early in the aqueous alteration history of the parent body, and from static fluids that were evolving chemically in response to mineral dissolution and precipitation. The second calcite generation crystallized from solutions of a lower δ~(17)O, and a lower δ~(18)O and/or higher temperature. As two generations of calcite whose petrographic characteristics and oxygen isotopic compositions are similar to those in LON 94101 occur in at least one other CM2, multiphase carbonate mineralisation could be the typical outcome of the sequence of chemical reactions during parent body aqueous alteration. It is equally possible however that the second generation of calcite formed in response to an event such as impact fracturing and concomitant fluid mobilisation that affected a large region of the common parent body of several CM2 meteorites. These findings show that integrated petrographic, chemical and isotopic studies can provide new insights into the mechanisms of parent body alteration including the spatial and temporal dynamics of the aqueous system.