Hydrogen isotopic composition of diamond-hosted silicates from the lithospheric mantle of the Guiana Shield

Abstract The hydrogen isotope composition of the mantle provides insight into the advection, melting and metasomatism of the mantle, and the Earth’s water cycle. Because of sampling challenges, most estimates for the hydrogen composition of the mantle have been derived from basaltic glasses rather than from direct samples of the mantle. We present the first direct measurement of D/H ratios in nominally anhydrous orthopyroxene and olivine inclusions hosted within diamonds. The diamond hosts were recovered from placer deposits in Guyana, South America. The diamond-hosted inclusions preserve a population with δD of –31 ± 59‰ (normalised to Vienna Standard Mean Ocean Water). This value is D-enriched relative to that expected for a homogeneous upper mantle and is similar to values measured for arc basalts. We introduce three explanations for the observed D-enrichment in the anhydrous silicate inclusions. The hydrogen isotopic composition of the mantle might simply be heterogeneous in space and/or time. The measured D-enrichment could instead preserve local heterogeneity produced by fluids genetically related to the crystallisation of the diamond hosts. Local and mantle-wide processes might also have operated in concert, and overprint each other. Diamond-hosted silicate inclusions preserve an underexplored record of Earth’s water cycle.