Planetary accretion, oxygen isotopes, and the central limit theorem

Abstract

The accumulation of presolar dust into increasingly larger aggregates such as calciumaluminum- rich inclusions (CAIs) and chondrules, asteroids, and planets should result in a drastic reduction in the numerical spread in oxygen isotopic composition between bodies of similar size, in accord with the central limit theorem. Observed variations in oxygen isotopic composition are many orders of magnitude larger than would be predicted by a simple, random accumulation model that begins in a well-mixed nebula, no matter what size objects are used as the beginning or end points of the calculation. This discrepancy implies either that some as yet unspecified but relatively long-lived process acted on the solids in the solar nebula to increase the spread in oxygen isotopic composition during each and every stage of accumulation, or that the nebula was heterogeneous (at least in oxygen) and maintained this heterogeneity throughout most of its nebular history. Depending on its origin, large-scale nebular heterogeneity could have significant consequences for many areas of cosmochemistry, including the application of well-known isotopic systems to the dating of nebular events and the prediction of bulk compositions of planetary bodies on the basis of a uniform cosmic abundance. The evidence supports a scenario wherein the oxygen isotopic composition of nebular solids becomes progressively depleted in 16O with time due to chemical processing within the nebula, rather than a scenario where 16O-rich dust and other materials are injected into the nebula, possibly causing its initial collapse.