O¹⁸/O¹⁶ ratios of Apollo 11 lunar rocks and minerals

Oxygen isotope analyses were obtained on coexisting minerals in 5 lunar microgabbros and basalts. The δO¹⁸ values are: cristobalite (+7·16), plagioclase (+6-06 to +6·33), clinopyroxene (+5·67 to + 5·95), and ilmenite (+3·85 to +4·12). The uniformity of δO¹⁸ suggests a close approach to isotopic equilibrium, with a calculated temperature of formation of about 1200°C, or about the same as terrestrial basalts. These rocks must have crystallized rapidly at low P_(H₂O), as they show no evidence of further oxygen isotope exchange during cooling. The lunar fines and breccias are 0·5 per mil richer in O¹⁸ than the above rocks, indicating addition of an O¹⁸-rich constituent (perhaps carbonaceous chondrites), and/or that O¹⁶ was depleted by vapor fractionation during formation of the abundant glass in these materials. The lunar rocks are very similar in δO¹⁸ to terrestrial mafic and ultramafic igneous rocks and to the chondritic meteorites, suggesting a genetic relationship. However, they are significantly richer in O¹⁸ than several other meteorite groups, such as the basaltic achondrites. The O¹⁸ data also indicate that tektites are unlikely to have originated on the moon.