NOBLE GASES IN METEORITES - TRAPPED COMPONENTS

Abstract

Fundamental insights in the field of cosmochemistry have come from the study of the abundances and isotopic compositions of trapped noble gases. A case in point is the discovery of presolar grains in primitive meteorites: isotope abundance anomalies in noble gases were known long before the discovery of presolar grains such as nanodiamonds, silicon carbide and graphite, and it was the search for the carrier phases of these anomalies that ultimately led to the identification and isolation of these types of grains (e.g., Anders and Zinner 1993; Ott 1993). Starting with the work of John Reynolds and his colleagues at Berkeley, much of the work on trapped noble gases has centered on xenon, so much that an own term “xenology” was coined to describe this field of noble gas cosmochemistry (Reynolds 1963). The first of the so-called “gas-rich meteorites” were discovered by Russian workers in 1956 (Gerling and Levskii 1956), the same year that Reynolds published details of his innovative static noble gas mass spectrometer (Reynolds 1956). Soon thereafter it was discovered that the elemental abundance patterns observed in meteorites basically fell into one of two patterns: (a) the gas-rich meteorites with a “solar” pattern—high abundances of the lightest gases He and Ne compared to the heavy ones, the origin being implanted solar wind; and (b) the “planetary” pattern with strong elemental fractionation, i.e., strong enrichment of the heavy noble gases relative to the light ones, when compared to the solar abundance pattern (Signer and Suess 1963; Fig. 1⇓). “Planetary” gases are most abundant in the most primitive carbonaceous chondrites (Marti 1967; Mazor et al. 1970). In this chapter I will deal exclusively with the type of noble gases in meteorites included in the “planetary component” as originally defined. Solar wind noble gases as well …