Intrinsic Oxygen Fugacities Of Some Inclusions From The Allende Carbonaceous Chondrite, A Primitive Meteorite

Abstract

The redox states of phases in primitive meteorites provide constraints for the composition of the early nebu­lar gas in which the planets and asteroids formed. The bulk composition nebula from solar spectra abundances and equilibrium condensation theory predicts that oxygen was bound up in CO at high temperatures so that the H2/H20 ratios of the early solar nebula were high (100-2000) and thus the nebular gas was very reducing. Equilibrium calcu­lations and experimental extrapolations of fO2-composition equilibria for primitive, Ca, Al-rich inclusion (CAI) and chondrule phases in the Allende CV3 carbonaceous chondrite show a range of oxidation states, with fassaite, a Ti-rich clinopyroxene, possibly equilibrating with the inferred reduced nebular gas while other phases and components equilibrated with more oxidized gases, 1 to 4 orders of magnitude more oxidized than the inferred, reduced nebular gas. Intrinsic oxygen fugacity (IOF) measurements using the double-cell, solid electrolyte technique developed by Sato (1971) and modified by Ulmer et al., (1976) were taken for whole rock samples of one olivine porphyry and two CAI in the Allende meteorite at temperatures of 800°to 1150° C. Results show their oxidation states to be 5-8 orders of magnitude more oxidized than the inferred nebu­lar gas at these temperatures. Melilite separates from the two CAI gave IOF measurements 8-10 orders of magnitude more oxidized than the calculated, reduced nebular gas and act as a strong oxidation buffer to the redox state of Ti3+-rich fassaite in the whole rock samples. The high oxidation state of melilite is due to its equilibration at high temperatures (>700°C) with a relatively oxidized gas and is also due to its high oxygen exchange and diffusion rates (Hayashi and Muelenbachs, 1986). IOF measurements of fine-grained alteration mineral clumps in one CAI yield an oxidation state more oxidized than the whole rock CAI IOF but more reduced than melilite from the same CAI. IOF measurements show that melilite and alteration minerals are in disequilibrium with respect to fO2 yet these phases are close to equilibrium with respect to oxygen isotope composition. CAI alteration minerals have cores of a Ti­rich phase which is reduced and inferred to be fassaite. IOF experiment results indicate either heterogeneity in the composition of the nebula across time and/or space or disequilibrium between primitive material and the nebu­lar gas.