Oxygen-isotopic evolution of the solar nebula

Studies of the three oxygen isotopes in chondrite meteorites demonstrate th at diverse O reservoirs (characterized by their Delta O-17 values) were pre sent in the solar nebula. The discovery that some chondritic materials have O-isotopic compositions that cannot be explained by mass-dependent fractio nation of an initially well mixed reservoir has important implications for the history of the solar nebula. On a plot of delta O-17 versus delta O-18 (or O-17/O-16 versus O-18/O-16), terrestrial samples (with the main excepti on of stratospheric ozone) fall along a single line (the terrestrial fracti onation, or TF, line) having a slope similar to0.52, an indication that the parental reservoir was homogenized. A convenient measure of O-isotopic het erogeneity is the deviation from the TF line, Delta O-17 = delta O-17 -0.52 X delta O-18. A popular model to explain the evolution of chondritic oxyge n compositions during nebular and asteroidal aqueous alteration processes c alls for the nebula to have formed from solids having delta O-18 = -40 part s per thousand and delta O-17 = -41 parts per thousand (Delta O-17 = -20 pa rts per thousand) and gas having a composition roughly estimated to be delt a O-18 = 30 parts per thousand and delta O-17 = 24 parts per thousand (Delt a O-17 = 9 parts per thousand). This model encounters serious difficulties when examined in detail; in particular, it cannot readily acr count for the O-isotopic compositions of both chondrules and refractory inclusions from individual chondrite groups, or for the differences between groups, particu larly in the compositions of chondrules. Magnetite (Fe,O,) is a key phase f or O-isotope studies because during its formation by the oxidation of ferro us metal or FeS, all O comes from the oxidant, probably H2O. It appears tha t most (and, possibly, nearly all) magnetite formed during aqueous alterati on processes that occurred in asteroids. In all chondrite groups studied to date, Delta O-17 of the magnetite is greater than or equal to that of the chondrule silicates. If the H2O originated in the ambient (local) nebula, t hen at the time of accretion the Delta O-17 of the nebular gas was generall y higher than that of the solids. An alternative view is that the heterogen eity in the O-isotopic composition of chondrites indicates that the nebula formed from diverse batches of presolar materials, the precise O-isotopic c omposition of the mix varying during the accretion history of the nebula. T he large compositional gaps between groups suggest that agglomeration of ne bular dust to form chondrites did nor proceed at a constant rate but that p eriods with turbulence levels low enough to allow agglomeration were punctu ated by periods of high turbulence during which the composition of the inne r nebula sampled by chondrites changed appreciably.