ORIGIN OF SULFIDE-BEARING METAL IN THE CBA METEORITE GUJBA

The CBa meteorites are metal-rich meteorites, containing up to 65 vol% iron-nickel metal. Material making up the CBa meteorites is hypothesized to have formed as condensates from a vapor plume generated during an impact event between two large asteroids early in the history of the solar system. The CBa meteorite Gujba is characterized by centimeter-sized elliptical iron-nickel metal particles that contain abundant sulfide, predominantly troilite (FeS), occurring in the form of arcuate collections along metal grain boundaries and as rounded blebs 1-20 microns across. Arcuate troilite most likely formed during cooling, when incompatible sulfur-rich liquid was trapped along grain boundaries as metal crystallized. Sulfide blebs formed as precipitates in metal grain interiors, during either cooling or a reheating event. Several models have been proposed to explain how sulfur was incorporated into the metal, the most common being that it condensed from the gas phase into either solid or liquid metal. One problem with evaluating the different models is that sulfur abundances are not well known, largely because S is heterogeneously dispersed within metal grains.

The variation in sulfide textures between individual metal particles can be classified in one of four ways: no sulfides, only blebs, only arcuate, or a mixture of blebs and arcuate troilite. Using image analysis of back-scattered electron images to estimate the amount of sulfur in individual metal particles, we find a range from 0.6-2.6 wt% sulfur. There appears to be no correlation between sulfide textures and amount of sulfur.

Metal particles in CBa meteorites have volatility-controlled siderophile element abundances. However, the S abundances we determined are higher than predicted from the volatility-controlled trend. This is inconsistent with a single stage condensation model of formation. The presence of sulfides in grain interiors is also inconsistent with condensation of S into solid metal, which predicts a small amount of sulfur along grain edges. We propose an alternative model in which S-bearing metal particles represent molten material that was not vaporized during the impact event.