Oxygen Isotopes in Meteorites

Abstract

Oxygen isotope abundances display a remarkable variability on all scales studied, from micrometers to planetary dimensions. Two types of processes have been identified and associated with some of this variability: (i) a nebular interaction between condensates and the ambient gas, probably at a temperature sufficiently high that mass-dependent fractionation effects were relatively small, and (ii) low-temperature aqueous alteration, probably occurring within the parent bodies. The first process begins with 16O-rich condensates, as are seen in CAIs, and enriches them in 17O and 18O along the CCAM line. The second process modifies these materials, especially in the carbonaceous chondrites, with further enrichment of the two heavier isotopes, but on lines of shallower slope as a consequence of large low-temperature fractionation effects. Although these two processes may be adequate to account for the observed variability in the chondritic meteorites, they do not seem to provide an explanation for the range of compositions of the achondrites. Some ad hoc explanations have been proposed, such as the existence of additional oxygen reservoirs or time-variable oxygen sources (Wasson, 2000), but these are basically untestable. Many questions remain unanswered. What was the anhydrous precursor for the CR–CH–CB group? What were the precursors of the metamorphosed CM and CI meteorites? What was the relationship between ureilites and carbonaceous chondrites? Why are so few differentiated parent bodies represented by achondrites? Why is the isotopic composition of the Earth identical to that of the Moon, but different from that of Mars? What is the relationship (if any).