It is possible that the majority of Earth’s H2O budget is present as hydroxyl (OH) structurally incorporated into the major nominally anhydrous minerals (NAMs) of the mantle (e.g., Martin and Donnay 1972). Ringwood (1966) thought as much as five times the surface H2O-mass could be present in the mantle, amounting to ~0.2 wt% H2O if distributed throughout the entire mantle (Harris and Middlemost 1969). We know now the (Mg,Fe)2SiO4 polymorphs of the upper mantle and transition zone can incorporate up to several weight percent of water in their...

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