The University of Arizona

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

On, J. Aléon, A. N. Krot, K. D. McKeegan, G. J. MacPherson, A. A. Ulyanov

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 (Δ17O ≤ −20‰) in all four CAIs; Al-Ti-diopside is similarly 16O-rich in all but one CAI, where it has smaller 16O excesses (−15‰ ≤ Δ17O ≤ −10‰). Anorthite and melilite vary widely in composition from 16Orich to 16O-poor (−22‰ ≤ Δ17O ≤ −5‰). 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.

Keywords


CV chondrites;Fine-grained inclusions;Iron-alkali metasomatism;Oxygen isotopes;Calcium-aluminum-rich inclusions (CAIs)

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