Ferrous silicate spherules with euhedral iron-nickel metal grains from CH carbonaceous chondrites: Evidence for supercooling and condensation under oxidizing conditions

The CH carbonaceous chondrites contain a population of ferrous (Fe/(Fe + Mg ) approximate to 0.1-0.4) silicate spherules (chondrules), about 15-30 mum in apparent diameter, composed of cryptocrystalline olivine-pyroxene normat ive material, +/-SiO2-rich glass, and rounded-to-euhedral Fe,Ni metal grain s. The silicate portions of the spherules are highly depleted in refractory lithophile elements (CaO, Al2O3, and TiO2 <0.04 wt%) and enriched in FeO, MnO, Cr2O3, and Na2O relative to the dominant, volatile-poor, magnesian cho ndrules from CH chondrites. The Fe/(Fe + Mg) ratio in the silicate portions of the spherules is positively correlated with Fe concentration in metal g rains, which suggests that this correlation is not due to oxidation, reduct ion, or both of iron (FeOsil <reversible arrow> Fe-met) during melting of m etal-silicate solid precursors. Rather, we suggest that this is a condensat ion signature of the precursors formed under oxidizing conditions. Each met al grain is compositionally uniform, but there are significant intergrain c ompositional variations: about 8-18 wt% Ni, <0.09 wt% Cr, and a sub-solar C o/Ni ratio. The precursor materials of these spherules were thus characteri zed by extreme elemental fractionations, which have not been observed in ch ondritic materials before. Particularly striking is the fractionation of Ni and Co in the rounded-to-euhedral metal grains, which has resulted in a Co /Ni ratio significantly below solar. The liquidus temperatures of the euhed ral Fe,Ni metal grains are lower than those of the coexisting ferrous silic ates, and we infer that the former crystallized in supercooled silicate mel ts. The metal grains are compositionally metastable; they are not decompose d into taenite and kamacite, which suggests fast postcrystallization coolin g at temperatures below 970 K and lack of subsequent prolonged thermal meta morphism at temperatures above 400-500 K.