HR: 0830h
AN: P51D-0469    [PDF]
TI: No Evidence for Trapped Noble Gases in CAIs
AU: * Vogel, N
EM: nvogel@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: * Vogel, N
EM: nvogel@bgc.org
AF: Institute for Isotope Geology and Mineral Resources, ETH Center, Zuerich, 8092 Switzerland
AU: Baur, H
EM: baur@erdw.ethz.ch
AF: Institute for Isotope Geology and Mineral Resources, ETH Center, Zuerich, 8092 Switzerland
AU: Leya, I
EM: leya@erdw.ethz.ch
AF: Institute for Isotope Geology and Mineral Resources, ETH Center, Zuerich, 8092 Switzerland
AU: Wieler, R
EM: wieler@erdw.ethz.ch
AF: Institute for Isotope Geology and Mineral Resources, ETH Center, Zuerich, 8092 Switzerland
AB: Refractory inclusions (CAIs) in meteorites probably are the first solids in the solar system. Although formed at high temperatures, CAIs are reported to contain trapped noble gases [1,2,3] which would provide information on CAI formation and solar system evolution. We reassessed this question by measuring Ne and Ar in CAIs of primitive chondrites (Allende, Axtell, Efremovka) by IR-laser extraction suitable for measuring low gas concentrations [4]. We chose meteorites with different preatmospheric radii, exposure ages, and degrees of alteration to take into account those effects on CAI noble gas compositions.\\ $^{20}$Ne/$^{22}$Ne is below 0.9 indicating the absence of common trapped Ne. We suggest that elevated $^{20}$Ne/$^{22}$Ne of [1,2,5] resulted from contamination of their CAI samples with matrix rich in trapped Ne. $^{21}$Ne/$^{22}$Ne is 0.72 to 0.86; more altered CAIs show the lower ratios. The Ne might be a mixture of chondritic cosmogenic Ne and nearly pure $^{22}$Ne, e.g., from presolar SiC [3]. However, calculated cosmogenic Ne for CAI minerals perfectly mimics the observed trend; in particular Na-rich alteration phases shift the $^{21}$Ne/$^{22}$Ne to lower values.\\ $^{36}$Ar/$^{38}$Ar is 0.7 to 4.8, thereby more altered CAIs have higher ratios. The Ar might be a mixture of chondritic cosmogenic Ar (mainly produced from Ca) and trapped Ar [3] or solar wind Ar [2], the latter supporting CAI formation in an X-wind scenario [6]. However, due to high Cl concentrations in CAIs also nearly monoisotopic $^{36}$Ar produced cosmogenically by neutron capture and beta- decay on Cl must be taken into account. Modelling Ar ratios and concentrations using only cosmogenic Ar from Ca and Cl nicely match the measured data. Thereby more Cl-rich altered CAIs show higher $^{36}$Ar/$^{38}$Ar.\\ Although the data do not principally contradict the presence of trapped Ne or Ar in CAIs they can be straightforwardly explained by cosmogenic productions mainly from Na, Ca, and Cl.\\ {[1]} Smith et al. (1977) GCA, 41, 627-647; [2] Shukolyukov et al. (2001) Geochem. Int., 39(1), 110-125; [3] Russel et al. (1998) MAPS, 33, A132; [4] Vogel (2003) PhD-Thesis, ETH Zuerich, Switzerland; [5] Goebel et al. (1982) GCA, 46, 1777-1792; [6] Shu et al. (1997) Science, 277, 1475-1479.
DE: 2104 Cosmic rays
DE: 3662 Meteorites
DE: 6205 Asteroids and meteoroids
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting