Oxygen Isotopic Compositions of Solar, Micrometer-sized Corundum, Hibonite and Spinel Grains in Acid-resistant Residues from Ordinary and Carbonaceous

Introduction: Isotopic studies of the very refractory solar condensates can potentially provide important constraints on the initial O-isotope composition of the solar nebula and its evolution [1,2]. Based on the narrow range of O-isotope composition (Δ 17 O ~ −24±2‰, 2σ) of primary minerals in FUN and non-FUN CAIs, which avoided postcrystallization isotope exchange, from CR and CV chondrites, we concluded that this value may represent the initial composition of the solar nebula and, possibly, of the Sun [3−5]. A more extreme value, Δ 17 O ~ −35‰, was inferred from compositions of two gros-site-rich CAIs from the CB/CH-like chondrite Isheyevo [6,7]. Both values are generally consistent with the O-isotope measurements of the solar wind returned by the Genesis mission, which, however, have relatively large uncertainty [8]. The currently favored self-shielding models assume that the initial O-isotope composition of the Sun and the solar nebula was 16 O-rich (δ 17,18 O ~ −50‰) and evolved with time as a result of CO photo-dissociation, dust-gas fractionation, and radial mixing of dust and gas in the protoplanetary disk [9−12]. Corundum is the first mineral thermodynamically predicted to condense from a gas of solar composition (T cond = 1770 K at P tot = 10 −3 bar [13]). At lower temperature , corundum reacts with nebular gas to form hibonite (1728 K), grossite (1698 K), perovskite (1680 K), meli-lite ss (1580 K), and spinel ss (1488 K). Corundum-bearing CAIs are very rare, possibly indicating efficient reaction with the cooling solar nebula gas [14−18]. The isolated µm-sized corundum grains in primitive chondrites are more common [19−21] and may represent primordial gas-solid condensates, which could have avoided multi-stage reprocessing during formation of CAIs (some co-rundum grains may represent evaporation residues [15]), and can potentially constrain the initial O-isotope composition of the solar nebula. Most studies of the isolated corundum grains, however, have been largely focused on presolar corundum grains [e.g., 22,23]; corundum grains of solar origin have not received much attention [19−21,24,25]. We report here the preliminary results of O-isotope compositions of µm-sized corundum, hibonite, and spinel grains from acid-resistant residues of Semark-ona (LL3.0), Bishunpur (LL3.1), and Allende (CV3.6). Samples and Analytical Techniques: Acid-resistant residues of Semarkona, Bishunpur, and Allende are from [26]. The obtained grains, 1−10 µm in size, were mounted onto clean gold foils. Corundum, hibonite, and spinel grains were identified using a JEOL JSM-5900LV