S-IN-ENSTATITE OXYBAROMETER RECORDS EVOLVING OXYGEN FUGACITY IN THE EARLY SOLAR NEBULA

The early solar system experienced a wide range of oxygen fugacity (fO₂) as recorded in different meteorites. Enstatite chondrites (EC) and aubrites represent the most reduced endmembers [e.g. Wadwha 2008] based on Si in kamacite, high sulfur, and low water. Mercury [McCubbin et al., 2012], and perhaps the early Earth [Javoy et al., 2010], may have formed at such low fO₂, suggesting reduced conditions were widespread in the inner nebula. However, enstatite and other components in these meteorites are not uniformly reduced, and exhibit evidence of variable reduction of silicates [e.g. Lusby et al., 1987]. Quantifying spatial and temporal variations in fO₂ requires multiple oxybarometers. In this contribution we apply a new sulfur-in-enstatite oxybarometer. Oxygen fugacity estimates from this oxybarometer do not correlate with Si-in-kamacite but do correlate with metamorphic degree. We suggest the two oxybarometers record evolving fO₂ conditions in the solar nebula.

To calibrate the S-in-enstatite oxybarometer, we analyzed 4 high pressure (1 GPa), highly reduced experiments that contained melt and enstatite. The results were compared to 13 enstatite chondrite and achondrite meteorites (including 3 EL’s, 4 EH’s, 1 E-impact melt, and 5 aubrites). Silicate phases in the experiments and meteorites were analyzed for volatile elements (S, C, F, and Cl) using the IMS 1280 SIMS instrument at WHOI. S-in-enstatite fO₂’s for each meteorite group were similar to but skewed toward more oxidized conditions than fO₂ from kamacite and also varied up to 2.5 log units in a given meteorite.

The S-in-enstatite fO₂ estimates do not correlate with the estimate from kamacite Si content but do correlate well with degree of metamorphism. This suggests low fO₂ conditions after meteorite parent body accretion that were reset to more oxidized fO₂ by later metamorphism as well as incomplete reduction of silicates in contrast with metal during sulfidization. A correlation between S and Cl in the enstatites and metal/sulfide microinclusions in EC’s indicate high Cl fugacity during the sulfidization event. F is very abundant with concentrations up to ~180 ppm. High scatter of F suggest it was undersaturated and mobile. With increasing metamorphism feldspar grains coarsen from <2um to up to 200 um [Brearley & Jones 1998], which may diffuse F into enstatite.