Oceanic lavas sampling the high-3He/4He mantle reservoir: Primitive, depleted, or re-enriched?

Abstract

Helium isotopes are used as a tracer for primitive reservoirs that have persisted in the Earth’s mantle. Basalts erupted at several intraplate oceanic islands, including Hawaii, Iceland, Galapagos, and Samoa, have hosted the highest 3He/4He ratios (>30 Ra, where Ra is atmospheric 3He/4He ratio) globally that are far in excess of the 3He/4He typical of the upper mantle sampled at mid-ocean ridges (8 Ra). These lavas have been suggested to be melts of a primitive, or possibly slightly depleted, mantle reservoir, i.e., either fertile or a depleted peridotite. Here we report evidence for geochemical enrichment in the high-3He/4He mantle sampled by lavas with the highest 3He/4He from Hawaii, Samoa, and possibly Galapagos. The titanium concentrations in high-3He/4He lavas from Samoa are too high to be explained by melts of a mantle peridotite, even at infinitesimally small degrees of melting, and the elevated Ti corresponds to elevated Pb-isotopic ratios. The highest 3He/4He lavas from Loihi, Hawaii, also have Ti concentrations that are too high to be melts of primitive mantle peridotite at the degrees of melt extraction proposed for this ocean island. Thus, Ti-rich material must have been added to the high-3He/4He mantle reservoir, and this material is likely to be recycled mafic crust similar to MORB-like eclogite, which is consistent with the elevated Pb-isotopic ratios. We show that fractionation corrected, major element compositions of high-3He/4He alkalic lavas can be satisfactorily modeled by melting and melt-rock interaction scenario in a fertile peridotite-MORB-eclogite hybrid system. Primitive peridotitic and recycled eclogitic reservoirs are suggested to be intimately associated in the deepest mantle and high-3He/4He lavas from several localities may sample a mantle source that hosts a component of recycled oceanic crust.