Explosive volcanism and the graphite-oxygen fugacity buffer on the parent asteroid(s) of the ureilite meteorites

Abstract

The ∼39 ureilite meteorites are in many respects primitive objects. They are highly carbonaceous, averaging ∼3.0 wt% C, mostly as graphite. Yet their consistently coarse-grained textures and plagioclase-depleted compositions imply that they are igneous. Despite diverse mafic-silicate Mg/(Mg + Fe) and pyroxene/olivine ratios, plagioclase is totally absent among ∼35 monomict/unbrecciated ureilites; even among 4 polymict/brecciated ureilites the average Al concentration is only 0.28 ± 0.04 wt%. We suggest that the ureilites are residues from partial melting of graphite-rich source materials, which formed sufficiently deep in a relatively large, low- f(O 2) asteroid (or more likely asteroids) for graphite to be stable even at magmatic T. The Al-rich basaltic magmas that ascended from the residues were nearly quantitatively lost from the asteroid during pyroclastic eruptions. Graphite entrained in the ascending magmas was oxidized by an extremely pressure-sensitive reaction into CO x , the expansion of which into the vacuous environment of the asteroid engendered pyroclastic exit velocities greatly in excess of the asteroid's escape velocity. Carbon would oxidize long before the attainment of magmatic temperatures in small asteroids, but we infer that basalt-dissipative explosive volcanism was common on all C-rich asteroids that were roughly 100–200 km in radius and heated to widespread, deep melting; and most unmetamorphosed chondritic meteorites are C-rich. Based on the ureilites' consistently modest depletions of light rare-earth elements, siderophile elements, and FeNi metal, we infer that the melt fraction was removed mostly by equilibrium, “batch” partial melting. Late-stage ureilite evolution involved an abrupt increase in the cooling rate, implying a drop in environmental temperature, and a slight amount of graphite oxidation, implying a drop in pressure. The simplest model to account for this non sequitur (and for the conversion of some of the graphite into diamonds) is catastrophic impact disruption of the primary asteroid. The remarkably efficient removal of the final traces of melt from the ureilites is difficult to explain by any model, but possibly was caused by thermal migration along steep thermal gradients engendered after the catastrophic disruption.