Intrinsic oxygen fugacity measurements on seven chondrites, a pallasite, and a tektite and the redox state of meteorite parent bodies

Abstract

Intrinsic oxygen-fugacity ( fO 2) measurements were made on five ordinary chondrites, a carbonaceous chondrite, an enstatite chondrite, a pallasite, and a tektite. Results are of the form of linear log f O 2 − 1 T plots. Except for the enstatite chondrite, measured results agree well with calculated estimates by others. The tektite produced fO 2 values well below the range measured for terrestrial and lunar rocks. The lowpressure atmospheric regime that is reported to follow large terrestrial explosions, coupled with a very high temperature, could produce glass with fO 2 in the range measured. The meteorite Salta (pallasite) has low fO 2 and lies close to Hvittis (E6). Unlike the other samples, results for Salta do not parallel the iron-wüstite buffer, but are close to the fayalite-quartz-iron buffer in slope. Minor reduction by graphite appears to have taken place during metamorphism of ordinary chondrites. fO 2 values of unequilibrated chondrites show large scatter during early heating suggesting that the constituent phases were exposed to a range of fO 2 conditions. The samples equilibrated with respect to fO 2 in relatively short time on heating. Equilibration with respect to fO 2 in ordinary chondrites takes place between grades 3 and 4 of metamorphism. Application of P − T − fO 2 relations in the system C-CO-CO 2 indicates that the ordinary chondrites were metamorphosed at pressures of 3–20 bars, as it appears that they lay on the graphite surface. A steep positive thermal gradient in a meteorite parent body lying at the graphite surface will produce thin reduced exterior, an oxidized near-surface layer, and an interior that is increasingly reduced with depth; a shallow thermal gradient will produce the reverse. A body heated by accretion on the outside will have a reduced exterior and oxidized interior. Meteorites from the same parent body clearly are not required to have similar redox states.