Neon Isotopic Composition of the Mantle Constrained by Single Vesicles Analyses

Abstract:

The origin of volatiles on Earth is still matter of debates. Noble gases turn out to be an efficient tool due to their inertness in chemical reactions and can then allow us to constrain Earth’s atmosphere formation processes. In that way several studies have focused on lower mantle neon isotopes because the ²⁰Ne/²²Ne ratio is thought to reflect that of Earth’s primordial components. Two models for light noble gases origin on Earth have hence been proposed: either solar wind implantation of the Earth's precursors or dissolution into the mantle of a primordial atmosphere captured from the solar nebula gas. In order to support one of the two models, we analyzed the noble gas compositions (helium, neon and argon) of oceanic island basalts from Fernandina (Galápagos hotspot). The samples are studied both by step-crushing and by laser ablation analyses of single vesicles. Results of step-crushing are consistent with those of laser ablation analyses. But the latter results permit to get rid of atmospheric contamination and to identify which crushing steps are subject to such contamination. Helium isotopic ratios R/Ra (where R is the ³He/⁴He ratio and Ra the atmospheric ratio) are about 23 for the two Galápagos samples. We obtain ²⁰Ne/²²Ne and ⁴⁰Ar/³⁶Ar isotopic ratios as high as 12.85-12.87 and 7000-9400 respectively for the source of the Galápagos hotspot. Hence, we show that step-crushing and laser ablation analyses are two complementary methods that should be used together to derive the noble gas ratios in uncontaminated samples. The results of neon compositions are consistent with other studies on other hotspots and support the model of solar wind implantation associated with sputtering to explain helium and neon origins on Earth.