Oxygen isotope tracing of the Solar System

Oxygen isotope measurements have played a key role in establishing the way the solar system has formed. On Earth mass-dependent fractionation constrains conditions under which oxygen distributes itself among different phases. In the solar system, oxygen can experience kinetic fractionation as well, but the predominant cosmochemical signature is variability in ¹⁶O. This was first recognised in refractory inclusions, the oldest known materials in the solar system, where 4% excesses in ¹⁶O were apparent. Subsequently the range has increased to nearly 50% with the extreme values being an 8% excess of ¹⁶O (relative to terrestrial oxygen) in a chondrule, and to an apparent deficit in ¹⁶O of 40% in chondrite matrix. Originally interpreted as a nucleosynthetic signature, these compositions appear to be due to photochemical speciation either in the solar system or the precursor molecular cloud.