Constraints on the origin of the HIMU reservoir from He-Ne-Ar isotope systematics

We present a new set of He-Ne-Ar isotopic compositions for HIMU and EM1 lavas from the Cook-Austral Islands in the south Pacific. He-3/He-4 of the HIMU lavas are lower than MORB values, as previously demonstrated, but a coherent variation in He-3/He-4 with Pb isotopes indicates two-component mixing to form the lavas. One component is the HIMU reservoir with He-3/He-4 of 6 Ra or lower, and the other is the local lithosphere, which is commonly involved in EM1 lavas. Relative abundances of radiogenic and nucleogenic He-4, Ne-21 and Ar-40 show systematic variations. In He-4/Ar-40*-He-4/Ne-21* space, the HIMU lavas define a trend that is parallel to, but offset from the trend previously observed for other OIBs. Using He-4/Ne-21* as a monitor of elemental fractionation of noble gasses, fractionation-corrected He-4/Ar-40* is higher than the He-4/Ar-40* production ratio in the mantle, reflecting a HIMU reservoir with a lower K/U (approximately 3000) than canonical mantle value (13000). Radiogenic He-3/He-4 and the low K/U are best explained by a model where the HIMU reservoir was formed by direct accumulation of, or deep mantle metasomatism with, ancient subducted oceanic crust modified by hydrothermal alteration and dehydration during its subduction. If the subducted oceanic crust with fractionated K/U forms a large isolated reservoir in the mantle, previous estimates of K/U and K concentrations for the bulk silicate Earth, that did not take this reservoir into consideration, will be too high. Moreover, the mass balance calculation indicates that the subducted oceanic crust may make a significant contribution to the U mantle budget, requiring there to be less in the primitive mantle. However, a contribution from the primitive mantle is not ruled out entirely, unless the subducted oceanic crust had a relatively high U concentration and low K/U over geologic time and was totally preserved in a convecting mantle. (C) 2011 Elsevier B.V. All rights reserved.