On the (mis)behavior of water in the mantle: Controls on nominally anhydrous mineral water content in mantle peridotites

Growth of hydrous minerals suggests higher water activity than in anhydrous peridotites, and therefore hydrous-mineral-bearing xenoliths and anhydrous xenoliths should have different NAM [H2O] and water activities. However, when the two groups are compared no significant differences can be found in either NAM [H2O] or water activity. We propose that the high diffusivity of hydrogen in the mantle allows for equilibration of water activity in the mantle over sub-kilometer length scales over geologic time. Such diffusive equilibration reduces water activity variability and results in the blurring and destruction of correlations between NAM [H2O] and indices of metasomatism or melt extraction. As a result of diffusive equilibration of water, there is a large difference in the variability of concentration between NAM [H2O] (spanning ∼2 orders of magnitude) and similarly incompatible elements such as Ce in the same peridotites (spanning ∼4 orders of magnitude). This difference in behavior explains why H2O/Ce ratios in mantle peridotites are highly variable relative to those of basalts.