Aluminium drives hydrogen incorporation in pyrolitic lower mantle: Constraints from a combined FTIR-ERDA-nanoSIMS analysis

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Hydrogen is present as hydroxyl (OH) defects in the crystal lattices of the nominally anhydrous minerals (NAMs) composing the Earth’s mantle. The NAMs comprising the lower mantle: bridgmanite, ferropericlase, and davemaoite are the three most abundant minerals on Earth, and thus their combined hydrogen solubility exerts a first-order control on the planetary H 2 O capacity and overall H 2 O cycle. In this study, we equilibrated the three principal NAMs with hydrous melt at the adiabatic geotherm and pressure corresponding to the shallow lower mantle. The H 2 O content of the coexisting NAMs is quantified by nanoSIMS using matrix-matched standards characterized by Fourier Transform Infrared (FTIR) spectroscopy combined with Elastic Recoil Detection Analysis (ERDA). Our results show that bridgmanite, ferropericlase, and davemaoite incorporate ∼0.03, ∼0.16, and ∼0.18 wt% H 2 O, respectively. Our data shows that hydrogen is primarily incorporated via Al 3+ -H + insertion at the Si 4+ (octahedral) site in perovskite-type structures and the metal (octahedral) site in ferropericlase. Finally, we refine a maximum H 2 O content of 0.058 (0.009) wt% H 2 O in a pyrolitic-shallow lower mantle, which is equivalent to ∼1.26 (0.2) ocean masses when extrapolated to a chemically homogeneous lower mantle.

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