OXYGEN ISOTOPES IN R-CHONDRITE MAGNETITE AND OLIVINE: LINKS BETWEEN R CHONDRITES AND ORDINARY CHONDRITES

Abstract

Ion-microprobe studies yield Δ17O (=δ17O - 0.52 . δ18O) values in magnetite from the Rumurti chondrite (RC) PCA91241 (which is paired with PCA91002) of +3.1 to +3.9%%, slightly higher than that in O from whole-rock R samples. Despite Δ17O values in whole-rock RCs that are much (by ca. 1.6%%) higher than in whole-rock LL chondrites, the Δ17O in R magnetite is much lower (by ca. 2%%) than the values (+4 to +7%%) from LL3 Semarkona and Ngawi (Choi et al., 1998). The δ18O values in PCA magnetite (-15 to -10%%) are the lowest known in meteorites, well below the range in Semarkona (-4 to +9%%). On a δ17O-δ18O 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 to be segments of a single array. This suggests that, in RCs and LL chondrites, magnetite formed from the same raw materials by the same processes, probably by aqueous alteration of metal in an asteroidal setting. We observed Δ17O values in olivines and pyroxene from RCs ranging from -1.2 to +2.9%% and δ18O from +1.4 to +9.1%%. 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 chondrites, 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 chondrites. We postulate that the matrix in R chondrites has Δ17O higher than whole-rock values. We suggest that the original Δ17O value of H2O in the RC body was similar to that incorporated into the ordinary chondrites, previously estimated by Choi et al. (1998) to be ca. +7%% in the LL parent body.