Noble Gases as Tracers of Mantle Processes and Magmatic Degassing

Abstract

Noble gas geochemistry provides powerful tools for constraining mantle degassing through geological time. However, noble gas elemental and isotopic ratios are often disturbed by melting, magma degassing and atmospheric contamination. It is necessary to understand and quantify these shallow influences in order to obtain the noble gas elemental and isotopic ratios in the mantle. In this chapter, we present an overview of the key parameters that are necessary to derive mantle compositions. We discuss solubilities in silicate melts, crystal/melt partition coefficients during melting, and different models for vesiculation and degassing, along with the preferred method to correct for atmospheric contamination, using neon isotopic compositions. Using selected samples from mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), we give the probable mantle contents and elemental and isotopic compositions of He, Ne, Ar and Xe in the depleted mantle and in the high 3He source. These estimates cannot be reconciled with ancient depletion models for unradiogenic noble gas isotopic compositions, found in some oceanic island basalts, which are best explained by relatively undegassed sources deep in the mantle.