One of the major persisting controversies about the Earth's interior concerns its hydrogen (H) storage inventory, especially below the 660 km discontinuity (Novella et al., 2024). Even though locked in trace amounts as defects within mineral lattices, H-bearing species (e.g., H2O, OH, H2) can potentially add up to several oceanic masses of hidden reservoirs in the bulk mantle. These species (colloquially referred to as “water or H2O”) have played an important role in the planet's geological, geochemical, and geodynamical evolution. Water is, together with temperature, the major driving force for the plate-tectonic regime by lowering both the viscosity and density of the solid-mineral phases (Nestola and Smyth, 2016), driving slab subduction and the deep Earth water cycle. It is one of the primary sources of current-day deep mantle melting, thereby promoting major chemical differentiation (Kawamoto, 2004). The fact that its internal cycle and global budget remain poorly constrained today is a major caveat in our understanding of the planet's evolution and sustenance.

We conduct high pressure-temperature multi-anvil experiments in sub-solidus conditions such that the synthesized run products are representative of a water-saturated fertile lower mantle i.e., Bridgmanite-Ferropericlase-Davemaoite coexisting with hydrous melt. In conjunction with nano ion-probe technique (NanoSIMS), we characterize the hydrogen abundance of the lower mantle nominally anhydrous minerals (NAMs). As such forms of analysis are calibration dependent (Bureau et al., 2009), we synthesize several hydrated standards of matrix composition similar to the target minerals to build calibration curves. Absolute H2O abundance in the standards is quantified using a combination of FTIR-ERDA analysis. Such a composite approach allows us to determine empirical relations between the major elements chemistry (quantified using EPMA) and H2O content of the NAMs. Finally, we determine the bulk water storage capacity of a chemically homogenous lower mantle and suggest plausible protonation mechanisms in play.