Fine-grained, spinel-rich inclusions from the reduced CV chondrite Efremovka: II. Oxygen isotopic compositions

Abstract

Oxygen isotopes have been measured by ion microprobe in individual minerals (spinel, Al- Ti-diopside, melilite, and anorthite) within four relatively unaltered, fine-grained, spinel-rich Ca-Alrich inclusions (CAIs) from the reduced CV chondrite Efremovka. Spinel is uniformly 16O-rich (Delta-17O is less than or equal to -20 ppm) in all four CAIs; Al-Ti-diopside is similarly 16O-rich in all but one CAI, where it has smaller 16O excesses (-15 ppm is less than or equal to Delta-17O, which is less than or equal to -10 ppm). Anorthite and melilite vary widely in composition from 16O-rich to 16O-poor (-22 ppm is less than or equal to Delta-17O, which is less than or equal to -5 ppm). Two of the CAIs are known to have group II volatilityfractionated rare-earth-element patterns, which is typical of this variety of CAI and which suggests formation by condensation. The association of such trace element patterns with 16O-enrichment in these CAIs suggests that they formed by gas-solid condensation from an 16O-rich gas. They subsequently experienced thermal processing in an 16O-poor reservoir, resulting in partial oxygen isotope exchange. Within each inclusion, oxygen isotope variations from mineral to mineral are consistent with solid-state oxygen self-diffusion at the grain-to-grain scale, but such a model is not consistent with isotopic variations at a larger scale in two of the CAIs. The spatial association of 16O depletions with both elevated Fe contents in spinel and the presence of nepheline suggests that latestage iron-alkali metasomatism played some role in modifying the isotopic patterns in some CAIs. One of the CAIs is a compound object consisting of a coarse-grained, melilite-rich (type A) lithology joined to a fine-grained, spinel-rich one. Melilite and anorthite in the fine-grained portion are mainly 16O-rich, whereas melilite in the type A portion ranges from 16O-rich to 16O-poor, suggesting that oxygen isotope exchange predated the joining together of the two parts and that both 16O-rich and 16O-poor gaseous reservoirs existed simultaneously in the early solar nebula.