Isotopes radiogenic excess component

Nonradiogenic Xe isotopes in the atmosphere. The isotopic compositions of Xe components in the solar system have been more difficult to unravel. There is no suitable widespread solar system Xe component to use as a reference composition (see Ott 2002, this volume). The light isotopes of atmospheric Xe are related to both bulk chondritic and solar Xe by fractionation of -4.2% per amu (first noted by Krummenacher et al. 1962), with a clear radiogenic excess of -7% in Xe from decay of However, these components cannot be precisely related to the composition of the heavy Xe isotopes in... 

The meteorite data (Fig. 10) are not related by simple mass fractionation. They exhibit a clear negative trend in A Mg -6 Mg space that can be explained by a combination of the presence of excess Mg due to decay of short-lived and extinct A1 and a mass-fractionation component (cf, Fig. 4). This is most easily seen on a plot of A Mg vs. 5 Mg where decay of Al in the absence of fractionation would result in a vertical trend (Fig. 11). The trend is dominated by the Allende chondrules with negative A Mg in the data set but there are hints (e.g., the Bjurbole chondrules) that the trend may extend to positive A Mg as well. Apositive A Mg may signify that bulk Earth has more radiogenic Mg than these primitive samples. Alternatively, it may mean that there are small anomalies in the Mg isotopic system (e.g., excesses in Mg) at the 0.03%o level. 

Two-component mixing. The atmosphere however, does contain a significant amount of noble gases, such as Ne, " °Ar and Xe that are also derived from crustal/radiogenic sources. To distinguish which of these species are derived from the immediate crustal system from those that are derived from atmosphere sources, the isotopic ratios can be compared with the atmospheric ratio to identify the crustal excess. In a two-component crust/air mixture... 

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