Oxygen-isotopic evolution of the solar nebula

Abstract

Studies of the three oxygen isotopes in chondrite meteorites demonstrate that diverse O reservoirs (characterized by their Δ 17 O values) were present in the solar nebula. The discovery that some chondritic materials have O-isotopic compositions that cannot be explained by mass-dependent fractionation of an initially well mixed reservoir has important implications for the history of the solar nebula. On a plot of δ 17 O versus δ 18 O (or 17 O/ 16 O versus 18 O/ 16 O), terrestrial samples (with the main exception of stratospheric ozone) fall along a single line (the terrestrial fractionation, or TF, line) having a slope ∼0.52, an indication that the parental reservoir was homogenized. A convenient measure of O-isotopic heterogeneity is the deviation from the TF line, Δ 17 O = δ 17 O - 0.52 x δ 18 O. A popular model to explain the evolution of chondritic oxygen compositions during nebular and asteroidal aqueous alteration processes calls for the nebula to have formed from solids having δ 18 O = -40‰ and δ 17 O = -41‰ (Δ 17 O = -20‰) and a gas having a composition roughly estimated to be δ 18 O = 30‰ and δ 17 O = 24‰ (Δ 17 O = 9‰). This model encounters serious difficulties when examined in detail; in particular, it cannot readily account for the O-isotopic compositions of both chondrules and refractory inclusions from individual chondrite groups, or for the differences between groups, particularly in the compositions of chondrules. Magnetite (Fe 3 O 4) is a key phase for O-isotope studies because during its formation by the oxidation of ferrous metal or FeS, all O comes from the oxidant, probably H 2 O. It appears that most (and, possibly, nearly all) magnetite formed during aqueous alteration processes that occurred in asteroids. In all chondrite groups studied to date, Δ 17 O of the magnetite is greater than or equal to that of the chondrule silicates. If the H 2 O originated in the ambient (local) nebula, then at the time of accretion the Δ 17 O of the nebular gas was generally higher than that of the solids. An alternative view is that the heterogeneity in the O-isotopic composition of chondrites indicates that the nebula formed from diverse batches of presolar materials, the precise O-isotopic composition of the mix varying during the accretion history of the nebula. The large compositional gaps between groups suggest that agglomeration of nebular dust to form chondrites did not proceed at a constant rate but that periods with turbulence levels low enough to allow agglomeration were punctuated by periods of high turbulence during which the composition of the inner nebula sampled by chondrites changed appreciably.