MANTLE NEON AND ATMOSPHERIC CONTAMINATION

Abstract

The apparent distinction between atmospheric and mantle²⁰Ne²²Ne ratios may provide a technique to quantify air contamination in mantle-derived materials. In the absence of mantle nuclear reactions, which produce either²⁰Ne or²²Ne in substantial quantities, it is likely that the entire mantle is characterized by a single, uniform²⁰Ne²²Ne ratio; a value of around 12.5 is suggested by analyses of MORBs, OIBs, diamonds and xenoliths. If this premise is correct, then any measured²⁰Ne²²Ne ratios in mantle samples that are lower than this must result from addition of an air component, with²⁰Ne²²Ne= 9.8. This is most likely a syn- or post-eruptive contaminant. The degree of air contamination inferred from²⁰Ne²²Ne 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”²¹Ne²²Ne and⁴⁰Ar³⁶Ar ratios for the highly degassed MORB mantle and for the less degassed (high³He⁴He) “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.