Mean Oxygen-isotope Composition of the Protosolar Molecular Cloud Silicate Dust

Oxygen-isotope composition of the solar wind returned by GENESIS indicates that the Sun is 16 O-rich (Δ 17 O SMOW = −26±6‰) [1], supporting one of predictions of the CO self-shielding models [2−4]. The self-shielding models [3,4] assume that both the solids and gas were similarly 16 O-rich (Δ 17 O ~ −25‰), and solids subsequently evolved towards the terrestrial value as a result of CO self-shielding, formation of 17,18 O-enriched H 2 O-ice followed by dust/gas fractionation, evaporation of H 2 O-ice inside the snow line, and thermal processing of sili-cates in the H 2 17,18 O-enriched gas resulting in their oxygen-isotope exchange. The existence of 16 O-rich gas in the early solar nebula is supported by oxygen-isotope compositions of the oldest solar system (SS) solids, CAIs, which are either condensates from a gas of solar composition or remelted condensates [5,6]: most CAIs in primitive chondrites are uniformly 16 O-rich (Δ 17 O = −24±2‰) [7,8]. However, (i) the initial mean oxygen-isotope composition of silicates is not known, and (ii) there is no simple correlation between oxygen-isotope compositions of the inner SS solids (CAIs, chondrules, asteroids) and their formation ages [9]. Oxygen-isotope compositions of FUN and F CAIs [10−14, this study] indicate that their precursors had variable Δ 17 O values (−24‰ to ~0‰), implying that the 16 O-rich and 16 O-poor reservoirs coexisted at the time of CAI formation. We suggest that these observations can be explained if the initial solids in the SS were 16 O-depleted relative to the solar nebula gas. This no longer requires the additional stages of dust − 16 O-depleted gas early exchange that the self-shielding models invoke. This interpretation can explain the coexistence of 16 O-rich and 16 O-poor reservoirs at the time of CAI formation as well as 16 O-depleted composition of chondrules, asteroids, and terrestrial planets [15]. Since chondrules formed in regions with high dust/gas ratio (>100×solar) [16], their O-isotope compositions were largely controlled by the 16 O-depleted solids. Although the presence of crystalline silicates in chondritic porous (cometary) IDPs are indicative of radial transport of dust in the protoplanetary disk, this radial mixing was only efficient at the earliest stages of the SS evolution and the amount of dust thermally processed in the inner disk is expected to decrease with time and with increase of radial distance from the Sun [17]. In contrast, all cometary IDPs measured so far …