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The isotopic composition of oxygen in the solar system is widely variable. On a three-isotope plot of ¹⁷O/¹⁶O vs ¹⁸O/¹⁶O there is a linear trend indicating that the predominant variable is the abundance of ¹⁶O. Relative to terrestrial compositions, refractory inclusions are enriched in ¹⁶O by up to 7%. Smaller variations are seen between the terrestrial composition and meteoritic compositions, for instance, those of SNC meteorites (Mars), and HED meteorites (Vesta).

The _¹⁶O mixing effect was originally ascribed to the injection of ¹⁶O from a supernova explosion close to the solar nebula, and was also responsible for short-lived ²⁶Al and neutron-rich isotope anomalies in Fe-group elements in refractory inclusions. An alternative theory suggests that chemistry can play a large role in this fractionation with the low abundance isotopes ¹⁷O and ¹⁸O experiencing different chemical reactions than the abundant ¹⁶O. One stumbling point in modelling oxygen isotopes in the solar system is the lack of a definition for the solar composition. For this reason, NASA is flying the Genesis mission to capture a sample of solar wind and bring it back to Earth. In anticipation of this return, we have been analysing metal grains from lunar soil, a sort of natural Genesis experiment.

The metal grains have very low intrinsic oxygen and so the solar wind implant dominates the signal from these grains. The isotope composition we measure is very unusual. It is depleted in ¹⁶O and so represents a reservoir that has not been seen before. Is this the solar composition, we don't know. We'll have to wait for the return of Genesis for confirmation.