O-16 enrichments in aluminum-rich chondrules from ordinary chondrites

The oxygen isotopic compositions of seven Al-rich chondrules from four uneq uilibrated ordinary chondrites were measured in situ using an ion microprob e. On an oxygen three isotope plot, the data are continuous with the ordina ry chondrite ferromagnesian chondrule field but extend it to more O-16-enri ched values along a mixing line of slope = 0.83 +/- 0.09, with the lightest value recorded at delta O-18 = -15.7 +/- 1.8 parts per thousand and delta O-17 = -13.5 +/- 2.6 parts per thousand. If Al-rich chondrules were mixture s of ferromagnesian chondrules and CAI material, their bulk chemical compos itions would require them to exhibit larger O-16 enrichments than we observ e. Therefore, Al-rich chondrules are not simple mixtures of these two compo nents. Three chondrules exhibit significant internal isotopic heterogeneity indicative of partial exchange with a gaseous reservoir. Porphyritic Al-ri ch chondrules are consistently O-16-rich relative to nonporphyritic ones, s uggesting that degree of melting is a key factor and pointing to a nebular setting for the isotopic exchange process. Because Al-rich chondrules are c losely related to ferromagnesian chondrules, their radiogenic Mg isotopic a bundances can plausibly be applied to help constrain the timing or location of chondrule formation. (C) 2000 Elsevier Science B.V. All rights reserved .