The solar oxygen-isotopic composition: Predictions and implications for solar nebula processes

The outer layers of the Sun are thought to preserve the average isotopic an d chemical composition of the solar system. The solar O-isotopic compositio n is essentially unmeasured, though models based on variations in meteoriti c materials yield several predictions. These predictions are reviewed and p ossible variations on these predictions are explored. In particular, the tw o-component mixing model of Clayton and Mayeda (1984) (slightly revised her e) predicts solar compositions to lie along an extension of the calcium-alu minum-rich inclusion (CAI) O-16 line between (delta(18)O, delta(17)O)= (16. 4, 11.4)parts per thousand and (12.3, 7.5)parts per thousand. Consideration of data from ordinary chondrites suggests that the range of predicted sola r composition should extend to slightly lower delta(18)O values. The predic ted solar composition is critically sensitive to the solid/gas ratio in the meteorite-forming region, which is often considered to be significantly en riched over solar composition. A factor of two solid/gas enrichment raises the predicted solar (delta(18)O, delta(17)O) values along an extension of t he CAI O-16 line to (33, 28)parts per thousand. The model is also sensitive to the nebular O gas phase. If conversion of most of the gaseous O from CO to H2O occurred at relatively low temperatures and was incomplete at the t ime of CM aqueous alteration, the predicted nebular gas composition (and he nce the solar composition) would be isotopically heavier along a slope 1/2 line. The likelihood of having a single solid nebular O component is discus sed. A distribution of initial solid compositions along the CAI O-16 line ( rather than simply as an end-member) would not significantly change the pre dictions above in at least one scenario. Even considering these variations within the mixing model, the predicted range of solar compositions is disti nct from that expected if the meteoritic variations are due to non-mass-dep endent fractionation. Thus, a measurement of the solar O composition to a p recision of several permil would clearly distinguish between these theories and should clarify a number of other important issues regarding solar syst em formation.