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

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

Nov 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000m%26ps...35.1249k&link_type=abstract

Meteoritics & Planetary Science, vol. 35, no. 6, pp. 1249-1258 (2000).

Computer Science

5

Scientific paper

CH carbonaceous chondrites contain a population of ferrous [Fe/(Fe+Mg) ~ 0.1-0.4] silicate spherules (chondrules), ~15-30 μm in apparent diameter, composed of cryptocrystalline olivine-pyroxene normative material, SiO2-rich glass and rounded-to-euhedral Fe,Ni-metal grains. 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 chondrules from CH chondrites. The Fe/(Fe+Mg) ratio in the silicate portions of the spherules is positively correlated with Fe concentration in metal grains, suggesting that this correlation is not due to oxidation/reduction of iron (FeOsil?Femet) during melting of metal-silicate solid precursors. Rather, we suggest that this is a condensation signature of the precursors formed under oxidizing conditions. Each metal grain is compositionally uniform, but there are significant intergrain compositional variations: ~8-18 wt% Ni, <0.09 wt% Cr and sub-solar Co/Ni ratio. The precursor materials of these spherules were thus characterized by extreme elemental fractionations, which have not been observed in chondritic 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 euhedral Fe,Ni-metal grains are lower than those of the coexisting ferrous silicates and we infer that the former crystallized in supercooled silicate melts. The metal grains are compositionally metastable; they are not decomposed into taenite and kamacite suggesting fast postcrystallization cooling at temperatures below 970 K and lack of subsequent prolonged thermal metamorphism at temperatures above 400-500 K.

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