Evidence for oxidizing conditions in the solar nebula from Mo and W depletions in refractory inclusions in carbonaceous chondrites

Abstract

Depletions of Mo and W relative to other refractory metals of similar volatility (Re, Os, Ir, Ru, Pt) are common in a suite of 16 Ca,Al-rich inclusions (CAIs) from 5 carbonaceous chondrites. Twelve of the 16 CAIs from Allende, Grosnaja, Leoville, and Ornans show Mo depletions; six of these 12 inclusions also show W depletions. The one CAI analyzed from the Essebi chondrite shows no depletions. The Mo and W depletions have a very characteristic pattern with Mo always more depleted than W. The same Mo and W depletion pattern occurs in calculated refractory metal alloy compositions formed at oxygen fugacities 10 3 to 10 4 times greater than the canonical solar nebula oxygen fugacity. We conclude that Mo and W depletions are common in CAIs from carbonaceous chondrites and that the depletions result from high temperature oxidation. The oxidation may have occurred during the evaporation of primitive dust into CAIs but is also consistent with condensation at high oxygen fugacities. No potential alternative processes appear to be capable of producing the observed Mo and W depletion patterns.