Cosmochemical Study of Halogens Based on Halogen-derived Noble Gas Isotopes

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We have observed very high concentrations of Cl, Br and I in Yamato-74191 and in CAIs of Allende(CV3) chondrite. Because halogens are generally thought to have behaved as volatile elements when solid condensates were produced in solar nebula, mechanism to produce the observed high concentrations of halogens in CAIs is difficult to understand. This is due to a lack of knowledge about the cosmochemical behavior of halogens. Laser microprobe noble gas analysis was applied to Allende meteorite traversing the CAI and matrix with a space resolution of about 150μm. The high concentrations of ^<129>Xe show good correlation with the chlorine concentrations determined with EPMA analysis. ^<80>Kr/^<84>Kr and ^<128>Xe/^<132>Xe ratios are also high in the measured portions with high ^<129>Xe/^<132>Xe ratios. The ^<80>Kr and ^<128>Xe can be produced by neutron capture on ^<79>Br and ^<127>I, respectively, in space.The present results show that halogens have penetrated into the CAIs from outside through the disordered boundary of CAI, when aqueous alteration produced sodalite inside the CAIs. Since we could not find halogen-derived noble gases in surrounding matrix materials, the halogen-rich materials in CAIs must have been produced in the solar nebular, and not in parent body after embedded in matrices. The high concentrations of ^<129>Xe in sodalite-rich materials indicate that the aqueous alteration must occur at the very early stage of solar system when abundant ^<129>I was still alive.