Oxygen isotopes
Oxygen isotopes are the three stable forms of oxygen, whose ratios in a meteorite act like a fingerprint for its parent body.
Oxygen has three stable isotopes: oxygen-16 (about 99.76 per cent of natural oxygen), oxygen-18 (about 0.20 per cent) and oxygen-17 (about 0.04 per cent). Results are reported as δ17O and δ18O, the per mil (‰) deviations of the 17O/16O and 18O/16O ratios from a standard, ocean water. Plotting δ17O against δ18O reveals the key pattern.
On Earth, ordinary physical and chemical processes separate isotopes in proportion to their mass difference, so all terrestrial samples fall on a single line with a slope of about 0.52, the terrestrial fractionation line. Since the 1970s it has been clear that meteorites do not all lie on this line. Many CAIs and carbonaceous chondrites fall along a line of slope close to 1, showing that the early Solar System contained reservoirs with different proportions of oxygen-16 that were not produced by simple mass fractionation. Each parent body therefore inherited its own signature: HED meteorites share one line, martian meteorites another, and ordinary chondrite groups occupy distinct small fields.
Bulk oxygen isotopes are measured by laser fluorination with gas-source mass spectrometry, while ion microprobes measure individual grains. They are now routine for classifying achondrites and testing pairings.
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