Oxygen isotopes in R-chondrite magnetite and olivine: Links between R chondrites and ordinary chondrites

Ion-microprobe studies yield Delta O-17 (= delta O-17 - 0.52.delta O-18) va lues in magnetite from the Rumurti chondrite (RC) PCA91241 (which is paired with PCA91002) of +3.1 to +3.9 parts per thousand, slightly higher than th at in O from whole-rock R samples. Despite Delta (1)7O values in whole-rock RCs that are much (by ca. 1.6 parts per thousand) higher than in whole-roc k LL chondrites, the Delta O-17 in R magnetite is much lower (by ca. 2 part s per thousand) than the values (+4 to +7 parts per thousand) from LL3 Sema rkona and Ngawi (Choi et al., 1998). The delta O-18 values in PCA magnetite (-15 to -10 parts per thousand) are the lowest known in meteorites, well b elow the range in Semarkona (-4 to +9 parts per thousand). On a delta O-17- delta O-18 diagram both magnetite data sets form linear arrays with slopes of ca. 0.7, indicating mixing of O from different isotopic reservoirs; the slopes and intercepts of the two arrays are similar enough to permit them t o be segments of a single array. This suggests that, in RCs and LL chondrit es, magnetite formed from the same raw materials by the same processes, pro bably by aqueous alteration of metal in an asteroidal setting. We observed Delta O-17 values in olivines and pyroxene from RCs ranging from -1.2 to +2 .9 parts per thousand and delta O-18 from +1.4 to +9.1 parts per thousand. These compositions scatter in the same general range observed in chondrules from ordinary chondrites. The similarity in the O-isotopic composition of minerals that preserve a record of formation in the solar nebula supports a model in which RCs formed from nebular components similar to those in H ch ondrites, but with a matrix/chondrule ratio several times higher in the RCs , and with more extensive aqueous alteration in the RCs than in known H cho ndrites. We postulate that the matrix in R chondrites has Delta O-17 higher than whole-rock values. We suggest that the original Delta O-17 value of H 2O in the RC body was similar to that incorporated into the ordinary chondr ites, previously estimated by Choi et al. (1998) to be ca. +7 parts per tho usand in the LL parent body. Copyright (C) 2000 Elsevier Science Ltd.