RESOLVING DIFFERENT NOBLE GAS COMPONENTS IN CRUSTAL FLUIDS

RESOLVING DIFFERENT NOBLE GAS COMPONENTS IN CRUSTAL FLUIDS 

Terrestrial noble gases are dominated by three reservoirs the atmosphere, crust and mantle. The isotopic compositions of noble gases produced by radioactive decay in the crust are distinct from noble gases derived from the mantle, which in turn are distinct from those in the atmosphere (Fig. 1). 

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 

The subscripts crust and tot refer to the crustal and total concentrations while the subscripts air and sample refer to the isotopic composition of the atmosphere (Ozima and Podosek 1983) and sample respectively. 

Once corrected for atmosphere-derived He, the He/ He ratio represents the sum of only two components the crust and the mantle. The contribution of crustal He is then given by 

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