SULFUR DEPLETION ON 433 EROS: ANALYZING METEORITIC ANALOGS TO COMPARE POSSIBLE CAUSES

One of the most surprising findings of the NEAR mission was a marked depletion of S on the surface of asteroid 433 Eros (Nittler et al., 2001). Although micrometeorite bombardment producing S volatilization was the favored mechanism to explain this depletion, partial melting and removal of an Fe,Ni-FeS cotectic partial melt could not be excluded as the cause. We have studied possible meteoritic analogs to 433 Eros to further constrain these two models.

Derivation of Mn/Si and Cr/Si ratios (Lim et al., 2003) from NEAR X-ray spectra might provide additional constraints. We have studied the acapulcoites and lodranites Acapulco, EET 84302 and Lodran to document the effect of partial melting on these ratios. Modeling suggests that efficient removal of chromite should reduce Cr/Si dramatically and Mn/Si slightly. Evidence for chromite removal is observed in reduced chromite abundances in Lodran compared to Acapulco. Chromite appears to be removed with the Fe,Ni-FeS melts, with which it is associated in EET 84302. Non-chondritic Mn/Si and, particularly, Cr/Si ratios for 433 Eros would strongly favor partial melting.

If impact volatilization is the primary cause of the sulfur depletion, we would expect to see similar effects in ordinary chondrite regolith breccias. The dark portions of these meteorites are rich in solar wind gases and sample ancient regolith. The Dwaleni H6 regolith breccia has a bulk S/Si ratio typical of ordinary chondrites (Jarosewich, 1990) and we found no difference in troilite abundance between the dark, regolith-exposed portions and the light, unexposed clasts. It is possible that ancient regolith breccias do not sample, in bulk, the same material present in the upper hundred microns of modern regolith. We have, however, observed a single taenite-troilite particle which includes numerous orthopyroxene blebs less than 5 microns in diameter. This particle appears to have formed by melting and may be the equivalent of a sulfur-cemented, asteroidal agglutinate. To examine the possible role of volatilization during formation of this particle, ToF-SIMS analyses of chalcophile elements with a range of volatilities are planned.