Oxygen Isotopes and the Moon-Forming Giant Impact

We have determined the abundances of ¹⁶O, ¹⁷O, and ¹⁸O in 31 lunar samples from Apollo missions 11, 12, 15, 16, and 17 using a high-precision laser fluorination technique. All oxygen isotope compositions plot within +/-0.016 per mil (2 standard deviations) on a single mass-dependent fractionation line that is identical to the terrestrial fractionation line within uncertainties. This observation is consistent with the Giant Impact model, provided that the proto-Earth and the smaller impactor planet (named Theia) formed from an identical mix of components. The similarity between the proto-Earth and Theia is consistent with formation at about the same heliocentric distance. The three oxygen isotopes (Δ¹⁷O) provide no evidence that isotopic heterogeneity on the Moon was created by lunar impacts.