Large spinel grains in a CM chondrite (Acfer 331): Implications for reconstructions of ancient meteorite fluxes

By dissolving 30400kg of marine limestone in HCl and HF acid, our group has previously recovered common relict chromite grains (approximately 63250m) from ordinary chondritic micrometeorites that fell on ancient sea floors, up to 500Myr old. Here, we evaluate if CM group carbonaceous chondritic material, which makes up an important fraction of the micrometeorite flux today, contains analogous grains that can be searched for in acid residues. We dissolved 8g of CM2 meteorite Acfer 331 in HF, which yielded a characteristic assemblage of both transparent Mg-Al- and opaque Cr-spinels >28m. We find on average 4.6 and 130 Mg-Al-spinel grains per gram in the 63250 and 2863m size fractions, respectively. These grains are mostly pink or colorless, and often characterized by heterogeneous Cr-content. Black, opaque Cr-spinel grains are absent from the >63m fraction, but in the 2863m fraction we find approximately 65 such grains per gram meteorite. The individual grains have a characteristic composition, with heterogeneous major element compositions (e.g., 44.461.7wt% Cr2O3), but narrow ranges for maximum TiO2 (0.61.6wt%) and V2O3 (0.51.0wt%) concentrations. The content of spinel grains in the 2863m fraction of CM meteorites appears comparable at the order of magnitude level with the content of >63m sized chromite grains in fossil L-chondrites from Ordovician limestone. Our approach of recovering meteoritic spinel from sediment may thus be extended to include CM meteorites, but the smaller size fraction of the acid residues should be searched.