Helium deficit in high‐3He/4He parent magmas: Predegassing fractionation, not a “helium paradox”

Abstract

We evaluated He‐Ne‐Ar elemental compositions of mid‐ocean ridge basalt (MORB) glasses and basalt glasses from the Hawaiian hot spot (Loihi Seamount, Kilauea) to determine the (3He/22Ne)solar, 4He*/21Ne*, and 4He*/40Ar* MORB‐type and “plume”‐type end‐member compositions within a binary mixing model. Although the 3He/22Ne ratios of “plume” magma and MORB magma marginally agree with current estimates of solar compositions, the calculated 4He*/21Ne* ratios are lower than the theoretical production ratio and indicate that the “plume” magma component experienced a predegassing fractionation event. This fractionation event resulted in a deficit in both radiogenic 4He* and primordial 3He and accounts for the differences in 3He/22Ne, 4He*/21Ne*, and 4He*/40Ar* ratios and 3He concentrations for both mantle domains. This resolves an apparent contradiction previously termed “mantle He paradox,” though the derived 3He concentrations are also at odds with a primitive mantle concept that requires still higher concentrations. We propose that the primary fractionation of He/Ne and He/Ar ratios is related to a more compatible behavior of He relative to Ne, Ar during partial melting. One model is presented: at depths below a shallow MORB source region a rising mantle plume causes formation of low‐degree partial melts, which are deficient in He. During melt ascent, these “plume”‐derived melts mix with melts of the shallow (MORB‐type) mantle source that experienced a higher degree of melting and hence show no He deficit. Depending on the relative proportion of mixing the resulting oceanic basalts will encompass the whole observed range from Loihi‐type and MORB‐type noble gas isotope compositions.