A laboratory experiment on the influence of aqueous alteration on noble gas compositions in the Allende meteorite

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We have carried out a hydrothermal experiment on the Allende CV chondrite, in order to understand the observed noble gas loss from CM and CI chondrites, possibly caused by aqueous alteration on their parent bodies (L. B. Browning et al., Geochim. Cosmochim. Acta, 60,2621,1996; T. Nakamura et al., Geochim. Cosmochim. Acta, 63,257,1999). The Allende meteorite was disaggregated and kept with liquid water in a pressurized vessel under 15 atm of water vapor at 200℃ for a week. The abundances of helium, neon, argon, krypton and xenon in the samples have been measured by stepwise heating technique. The abundances of noble gases were decreased by 24% (He), 27% (Ne), 38% (Ar), 23% (Kr) and 34% (Xe) after the hydrothermal treatment, suggesting that the hot water treatment had contributed to reduce the concentrations of noble gases in Allende. This gas loss also affected the isotopic composition of xenon. For the 1200℃ fraction, the amount of ^<132>Xe-HL was decreased from the sample by about 80% during the hot water treatment, while that of ^<132>Xe-Q was decreased to a lesser extent (∿40%). This result seems to indicate that the carrier of Xe-HL is less resistant to the gas loss by the hot water alteration than phase Q for high temperature components. However, further studies are required as this trend becomes less obvious and rather similar if we take into consideration the xenon of 800℃ fraction.
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