Solar-Gas-Rich Chondrite Pebbles as a Potential Source of Terrestrial Noble Gases, Inferred from Mantle Noble Gas Abundances and Isotopic Compositions

Abstract

Noble gas compositions of oceanic basalts provide valuable constraints on potential primordial sources of mantle noble gases and enable exploration of large-scale events during Earth’s formation, such as magma ocean fractionation processes. Specifically examining solar wind-like ²⁰Ne/²²Ne and ³He/²²Ne in the mantle, a model of nebula gas incorporation by equilibrium dissolution into an early earth magma ocean suggests a scenario explaining noble gases in the Earth’s mantle. However, that model is inconsistent with noble gas elemental ratios of the present-day primordial mantle, which differ greatly from those equilibrated with nebula gas. Some alternative scenario must explain solar wind-like noble gas presence in mantle. We propose alternative potential sources of mantle noble gases based on present-day noble gas compositions of ocean island basalts (OIB) and midoceanic ridge basalts (MORB). As the mantle noble gas source, we assumed chondrite pebbles irradiated by solar wind. Given a mixture with approximately 85–87% contribution from the solar wind in ²²Ne equivalents, the ²⁰Ne/²²Ne, ³⁸Ar/³⁶Ar, and noble gas elemental ratios of the mixture are consistent with those of OIB mantle. Moreover, calculation of noble gas fractionation of disequilibrium outgassing from an early earth magma ocean followed by diffusive ingassing of noble gases, from the deeper mantle into the magma ocean, can explain differences in noble gas elemental and isotopic ratios between the present-day OIB and MORB mantles. These findings can explain mantle noble gas source and formation processes without direct association of nebula gas with the magma ocean, suggesting that nebula gas disappeared before magma ocean emergence on the Earth.