OXYGEN ISOTOPIC COMPOSITIONS OF SOLAR CORUNDUM GRAINS

Abstract

Oxygen is one of the major rock-forming elements in the solar system and the third most abundant element of the Sun. Oxygen isotopic composition of the Sun, however, is not known due to a poor resolution of astronomical spectroscopic measurements. Several ?17O values have been proposed for the composition of the Sun based on (1) the oxygen isotopic measurements of the solar wind implanted into metallic particles in lunar soil (< ?20? by Hashizume & Chaussidon and ~ +26? by Ireland et al.), (2) the solar wind returned by the Genesis spacecraft (?27? ? 6? by McKeegan et al.), and (3) the mineralogically pristine calcium-aluminum-rich inclusions (CAIs) (?23.3? ? 1.9? by Makide et al. and ?35? by Gounelle et al.). CAIs are the oldest solar system solids, and are believed to have formed by evaporation, condensation, and melting processes in hot nebular region(s) when the Sun was infalling (Class 0) or evolved (Class 1) protostar. Corundum (Al2O3) is thermodynamically the first condensate from a cooling gas of solar composition. Corundum-bearing CAIs, however, are exceptionally rare, suggesting either continuous reaction of the corundum condensates with a cooling nebular gas and their replacement by hibonite (CaAl12O19) or their destruction by melting together with less refractory condensates during formation of igneous CAIs. In contrast to the corundum-bearing CAIs, isolated micrometer-sized corundum grains are common in the acid-resistant residues from unmetamorphosed chondrites. These grains could have avoided multistage reprocessing during CAI formation and, therefore, can potentially provide constraints on the initial oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Here we report oxygen isotopic compositions of ~60 micrometer-sized corundum grains in the acid-resistant residues from unequilibrated ordinary chondrites (Semarkona (LL3.0), Bishunpur (LL3.1), Roosevelt County 075 (H3.2)) and unmetamorphosed carbonaceous chondrites (Orgueil (CI1), Murray (CM2), and Alan Hills A77307 (CO3.0)) measured with a Cameca ims-1280 ion microprobe. All corundum grains, except two, are 16O-rich (?17O = ?22.7? ? 8.5?, 2?), and compositionally similar to the mineralogically pristine CAIs from the CR carbonaceous chondrites (?23.3? ? 1.9?, 2?), and solar wind returned by the Genesis spacecraft (?27? ? 6?, 2?). One corundum grain is highly 17O-enriched (?17O ~ +60?, ?18O ~ ?40?) and is probably of the presolar origin; the origin of another 17O-rich grain (?17O ~ ?15?, ?18O ~ ?35?) is unclear. We conclude that the 16O-rich corundum grains in the acid-resistant residues from unequilibrated ordinary and unmetamorphosed carbonaceous chondrites recorded initial oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Our inferred oxygen isotopic composition of the Sun is inconsistent with the more extreme 16O-rich value (?17O ~ ?35?) proposed by Gounelle et al. on the basis of two extremely 16O-rich CAIs from the CH/CB-like chondrite Isheyevo and with the 16O-poor value observed as a component of the solar wind implanted into the metallic particles in lunar soil (Ireland et al.).