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Primordial and recycled helium isotope signatures in the mantle transition zone

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Science  16 Aug 2019:
Vol. 365, Issue 6454, pp. 692-694
DOI: 10.1126/science.aax5293

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Diamond window into the deep mantle

Helium isotopes provide a window into the very deepest and oldest parts of Earth's voluminous mantle. However, several processes tend to obscure the helium isotope signal from reservoirs in basaltic lavas that have erupted at the surface. Timmerman et al. identified a set of diamonds that formed deep within Earth and were rapidly erupted, which have avoided near-surface contamination. They find evidence for a deep, primordial rock source along with mixing of sediments from old subducting plates. The signatures extracted from these diamonds have implications for chemical and dynamic models of Earth.

Science, this issue p. 692

Abstract

Isotope compositions of basalts provide information about the chemical reservoirs in Earth’s interior and play a critical role in defining models of Earth’s structure. However, the helium isotope signature of the mantle below depths of a few hundred kilometers has been difficult to measure directly. This information is a vital baseline for understanding helium isotopes in erupted basalts. We measured He-Sr-Pb isotope ratios in superdeep diamond fluid inclusions from the transition zone (depth of 410 to 660 kilometers) unaffected by degassing and shallow crustal contamination. We found extreme He-C-Pb-Sr isotope variability, with high 3He/4He ratios related to higher helium concentrations. This indicates that a less degassed, high-3He/4He deep mantle source infiltrates the transition zone, where it interacts with recycled material, creating the diverse compositions recorded in ocean island basalts.

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