Oxygen isotopes in eucrites, shergottites, nakhlites, and chassignites

Oxygen isotopic analyses ( ¹⁸O/ ¹⁶O and ¹⁷O/ ¹⁶O) have been carried out on three shergottites, two nakhlites, two chassignites, nine eucrites, and seventeen terrestrial samples. The eucrites define a fractionation line displaced from the terrestrial line, with an ¹⁶O excess of 0.4‰. The shergottites, nakhlites, and Chassigny define another fractionation line with an ¹⁶O deficiency of 0.6‰ relative to the earth. Within each of these two groups, the meteorites have been derived from a common oxygen reservoir, and perhaps a common parent body. The difference between the two groups requires separate reservoirs. Surprisingly, Brachina, previously classified as a chassignite, has an oxygen isotopic composition that lies within the eucrite group.