Ambient Xenon : A Key to the History of Meteorites

Abstract

Xenon exhibits a special anomaly in many meteorites in which Xe/sup 129/ is greatly enhanced with respect to terrestrial Xe. Smaller general anomalies also affect the remainder of the Xe isotopes. It is postulated that the xenon special anomalies are a result of the meteorite parent bodies having formed quickly enough after the last synthesis of I/sup 129/ to permit an excess of the radiogenic daughter Xe/sup 129/ to accumulate in the iodine-bearing minerals. Since it is not certain which processes contributed to the nucleosynthesis of iodine, relative rather than absolute decay intervals can be derived. It is suggested that radiogenic Xe/sup 129/, from I/sup 129/, and Xe/sup 131/ to Xe/sup 136/, from fission of heavy elements, was produced in those regions of the parent body that were at relatively high temperatures and mixed freely with the ambient xenon, so that the isotopic abundance in ambient xenon of Xe/sup 129/ and Xe/sup 131/ to Xe/sup 136/ increased with time. Applications of xenon chronometry in studies on the interiors of meteorite parent bodies are discussed. Data on the xenon content of the Sardis iron troilite and of Abee are compared. An ideal model is presented and theoretical measurements of xenon isotopicmore » abundance are compared with measurements on Abee. Equations are presented for the correlation of Xe/sup 129//Xe/sup 132/ and Xe/sup 128//Xe/sup 132/, in w hich consideration is given to the effects of temperature on the release of xenon from iodine-bearing minerals. It is suggested that the method may be applied in studies on environmental parameters of meteorites of different types and should give new insight into the significance of the high- and low-iron groups of chrondrites, the galliumgermanium group of meteorites, and the clusters of cosmic ray ages. (C.H.)« less