Molecular hydrogen in minerals as a clue to interpret \u2202D variations in the mantle

B N Moine,A V Korsakov,J Y Cottin,A V Golovin,I B Radu,O B Oleinikov,G Costin,E Deloule,N Bolfan-Casanova,D A Ionov

doi:10.1038/s41467-020-17442-8

Abstract

Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ∂D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2–bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope variations in mantle magmas and minerals.

Highlights

Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle
Water content and H isotopic composition were measured using a Thermal conversion/Elemental Analyser coupled with Isotope-Ratio Mass spectrometer (TC/EA-IRMS), Fourier Transform Infra Red (FTIR) and Secondary Ion Mass Spectrometry (SIMS) analyses
The hydrogen abundances obtained by TC/EA-IRMS and SIMS are consistently higher than those obtained by FTIR, see Table 1

Summary

Introduction

Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. The water concentrations (Table 1) in the omphacite measured by TC/EA-IRMS range from 720 to 5065 wt ppm, consistent with data for orogenic (crustal) eclogites (1200−6000 ppm21–23), and are generally much higher than the maximum of 600 ppm H2O reported previously for mantle pyroxenes[10,24,25].

Results

Conclusion