  Nominally anhydrous minerals in rocky planet mantles can sequester oceans of
water as a whole, giving a constraint on bulk water inventories. Here we
predict mantle water capacities from the thermodynamically-limited solubility
of water in their constituent minerals. We report the variability of mantle
water capacity due to (i) host star refractory element abundances that set
mineralogy, (ii) realistic mantle temperature scenarios, and (iii) planet mass.
We find that planets large enough to stabilise perovskite almost unfailingly
have a dry lower mantle, topped by a high-water-capacity transition zone which
may act as a bottleneck for water transport within the planet's interior.
Because the pressure of the ringwoodite-perovskite phase boundary defining the
lower mantle is roughly insensitive to planet mass, the relative contribution
of the upper mantle reservoir will diminish with increasing planet mass. Large
rocky planets therefore have disproportionately small mantle water capacities.
In practice, our results would represent initial water concentration profiles
in planetary mantles where their primordial magma oceans are water-saturated.
We suggest that a considerable proportion of massive rocky planets' accreted
water budgets would form surface oceans or atmospheric water vapour immediately
after magma ocean solidification, possibly diminishing the likelihood of these
planets hosting land. This work is a step towards understanding planetary deep
water cycling, thermal evolution as mediated by rheology and melting, and the
frequency of waterworlds.
