Oxygen isotopes and the moon-forming giant impact

We have determined the abundances of O-16, O-17, and O-18 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 mit (2 standard deviations) on a single mass-dependent fractionation line that is identical to the terrestrial fractionation tine 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 (Delta O-17) provide no evidence that isotopic heterogeneity on the Moon was created by lunar impacts.