Mantle neon and atmospheric contamination

Abstract

The apparent distinction between atmospheric and mantle ^(20)Ne/^(22)Ne ratios may provide a technique to quantify air contamination in mantle-derived materials. In the absence of mantle nuclear reactions, which produce either ^(20)Ne or ^(22)Ne in substantial quantities, it is likely that the entire mantle is characterized by a single, uniform ^(20)Ne/^(22)Ne ratio; a value of around 12.5 is suggested by analyses of MORBs, OIBs, diamonds and xenoliths. If this premise is correct, then any measured ^(20)Ne/^(22)Ne ratios in mantle samples that are lower than this must result from addition of an air component, with ^(20)Ne/^(22)Ne= 9.8. This is most likely a syn- or post-eruptive contaminant. The degree of air contamination inferred from ^(20)Ne/^(22)Ne ratios is generally small for diamonds, but is increasingly significant for MORBs and OIBs; many OIB's may carry > 90% air neon. We calculated "air-neon corrected" ^(21)Ne/^(22)Ne and ^(40)Ar/^(36)Ar ratios for the highly degassed MORB mantle and for the less degassed (high ^3He/^4He) "plume" reservoir. The inferred MORB composition is indistinguishable from measurements of some gas-rich glasses. The calculated plume composition is similar to the least air-like measurements from ocean islands, but is less air-like than has been proposed previously. This plume composition is not consistent with a completely undegassed reservoir. From these corrected mantle compositions, we calculated the relative time-integrated rare gas abundances in the mantle, using a simple evolutionary model, which simultaneously considers the isotopic compositions of He, Ne, Ar and Xe. The model shows that both MORB and plume reservoirs have evolved with nearly solar elemental abundances. This provides strong support to suggestions based on Ne isotopes that the Earth accreted with gases nearly solar in composition. Importantly, the inferred mantle Ne/Ar ratios are much higher than atmospheric, which is consistent with simultaneous fractionation of both the atmospheric neon isotope ratio and the Ne/Ar ratio by massive hydrodynamic escape. Mixing between MORB and plume reservoirs (with our calculated elemental and isotopic abundances), plus varying amounts of added air, can account for the rare gases in nearly all mantle-derived rocks.

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