The Oxygen Isotopic Composition of the Sun

An accurate and precise determination of the oxygen isotopic composition of the Sun is the highest priority scientific goal of the Genesis Mission [1] as such data would provide a baseline from which one could interpret the oxygen isotopic anomalies found at all spatial scales in inner solar system materials. We have measured oxygen isotope compositions of implanted solar wind in 40 spots along a radial traverse of the Genesis SiC target sample 60001 by depth profiling with the UCLA MegaSIMS [2]. Mass-dependent fractionation induced by the solar wind concentrator [3] ion optics was corrected by comparison of the concentrator 22Ne/20Ne with that measured in a bulk solar wind target (diamond-like carbon on Si, [4]). The solar wind captured at L1 has an isotopic composition of (δ18O, δ17O) ≈ (-99, -79)‰, a value which is far removed from the terrestrial mass fractionation line. Profiles from the central portion of the target, where solar concentrations are highest and background corrections minimal, yield a mean Δ17O = -28.3 ± 1.8 ‰ indicating that the Earth and other planetary materials from the inner solar system are highly depleted in 16O relative to the solar wind. A mass-dependent fractionation of ~ -20%/amu in the acceleration of solar wind is required if we hypothesize that the photospheric oxygen isotope value, which represents the bulk starting composition of the solar system, is on the 16O-mixing line characteristic of refractory phase in primitive meteorites [5]. With this assumption, our preferred value for the bulk solar oxygen isotope composition is δ18O ≈ δ17O ≈ -57‰. A mechanism is required to fractionate oxygen isotopes in a non-mass-dependent manner to deplete 16O by ~6 to 7% in the rocky materials of the solar nebula. As oxygen is the third most abundant element in the solar system, and the most abundant in the terrestrial planets, this mechanism must operate on a large scale. Isotope-selective photochemistry, for example as in self-shielding of CO, operating in the solar nebula [6,7] or its precursor cloud [8] is a leading candidate; other models involving molecular symmetry-dependent reactions are also viable [9,10]. References: [1] D. Burnett et al. (2003) Space Sci. Rev. 105, 509. [2] K. McKeegan et al. (2008) LPSC XXIX, #2020. [3] R.Wiens et al. (2003) Space Sci. Rev. 105, 601. [4] V. Heber et al. Space Sci. Rev. 130, 309. [5] R. Clayton and T. Mayeda (1978) EPSL 40, 168. [6] R. Clayton (2002) Nature 415, 860. [7] J. Lyons and E. Young (2005) Nature 434, 317. [8] H. Yurimoto and K. Kuramoto (2004) Science 305, 1763. [9] M. Thiemens (1999) Science 283, 341. [10] R. Marcus (2004) J. Chem. Phys. 121, 8201.