Si-rich Fe–Ni grains in highly unequilibrated chondrites

Abstract

Some Fe–Ni grains in the Bishunpur L3 chondrite have Si contents in the range 1.3–4.5 mg per g (mole fractions of 0.003–0.009); the only previous report of Si in ordinary chondrite metal is a lower limit of 0.7 mg per g for metal in the Leighton H4 chondrite¹. The Bishunpur Si contents are 1.1–4 tunes that found in a large metal grain in the Murchison CM2 chondrite². As whole-rock systems both meteorites have high degrees of oxidation and consist of unequilibrated, essentially unmetamorphosed mineral assemblages. Thus the Si was not introduced into the metal during metamorphic reheating, but rather during nebular condensation². Grossman et al.² calculated that the Si mole fraction of 0.0024 in the Murchison grain required a total nebular pressure ≥10⁻⁵atm, which is higher than that generally inferred for the CM formation location. We show here that improvements in the Si activity coefficients and plausible constraints on the nucleation of mafic silicate condensates allow Si mole fractions of 0.003 at nebular pressures of <10⁻⁶atm. Thus such chondrites could have formed over a wide range of distances from the Sun.