Oxygen isotopes in chondrules and coarse-grained chondrule rims from the Allende meteorite

Abstract

The oxygen isotopic compositions of porphyritic chondrules and coarse-grained chondrule rims from the Allende CV chondrite form a 16O-mixing trend situated near the carbonaceous chondrite anhydrous minerals (CCAM) mixing line (lying ∼ 0.1–0.7‰ higher in δ 17O ). Coarse-grained rims are depleted in 16O relative to their enclosed chondrules. Chondrules and rims exhibit significant correlations between their oxygen isotopic and chemical compositions: objects rich in 16O tend to have higher contents of refractory lithophiles and alkalies; objects poor in 16O tend to have higher contents of FeO and volatile siderophiles. CV chondrite matrix material is chemically and isotopically similar to some coarse-grained chondrule rims. Previous work showed that O-isotope data for CV barred olivine (BO) chondrules plot just below the terrestrial fractionation (TF) line, near δ 18O ⋍ 4‰ , well off the CCAM line. The Allende data suggest that three solid precursor components are required to account for the O-isotopic compositions of CV chondrules and rims: (1) a high-temperature, refractory-, alkali- and 16O-rich component, perhaps near δ 18O = −40‰ , δ 17O = −42‰ (relative to SMOW), (2) a low-temperature, FeO-rich, refractory- and 16O-poor component lying near the CCAM line, perhaps near its intersection with the TF line at δ 18O ≈ 10‰ , and (3) an additional component required to explain the composition of BO chondrules.