CSEDI: Collaborative Research: New Experimental Approaches to He/U-Th Partitioning of the Mantle

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

Grove EAR-0215547 It is paradoxical that the mantle sources with the highest 3He/4He and 22Ne/21Ne, and argued to be the least-degassed and therefore the most primitive or primordial, have uniformly depleted (not primitive) Sr, Nd, and Hf isotope signatures. This is an unexplained and long-standing flaw in the "Standard Model" for rare gas evolution in the earth. One of the assumptions underlying the Standard Model is that He is more incompatible than Th and U during upper mantle melting processes, and that it will be preferentially removed during melting and subsequent eruptive degassing. Existing data for mineral-melt partitioning of He ranges over several orders of magnitude, and does not place any real constraints on this most important assumption of the Standard Model. This project will address this issue by methodical experimental measurement of He, Th and U partitioning between olivine, clinopyroxene and silicate melt. The study will provide new solubility data for true structural-bound He in both natural and synthetic olivine and clinopyroxene, and in silicate melt, over a wide range of temperature, pressure, and oxygen fugacity. Helium in single-phase experimental charges will be analyzed first by crushing in vacuo (to release helium from cracks, voids and defects) followed by fusion in an utrahigh vacuum furnace. He in multi-phase charges will be analyzed by in-situ laser-ablation. U and Th partitioning data will also be determined over the same range of conditions for olivine/melt and clinopyroxene/melt equilibrium pairs in simple Fe-free haplobasaltic systems. The experiments will be designed to provide a direct comparison between Th-U partitioning and He partitioning. Ultimately, the resulting data will allow an assessment of a key question in terrestrial helium isotope evolution: is high 3He/4He synonymous with an "undegassed" or "relatively primordial" mantle source? The answer to this question has broad significance to the CSEDI community, as helium is one of the only lines of evidence for relatively undegassed material in the deep earth. --