EXPERIMENTAL CONSTRAINTS ON HYDROGEN IN EARTH'S INTERIOR

Hydrogen is the least well constrained compositional variable in the Earth. Earth’s oceans have long been considered to comprise the bulk of the planet’s hydrogen, but constitute only 0.023 percent of the planet’s mass. However, H is soluble in all of the solid and liquid phases of the interior in sufficient quantities to constitute a reservoir many times the size of the oceans. Because nearly all of the minerals of the crust and mantle are oxygen minerals, relatively small amounts of hydrogen incorporated into even the nominally anhydrous phases may greatly exceed the amount of water in the oceans. On subduction, there exists a sequence of hydrous phases occurring on breakdown of serpentine and talc that can convey significant quantities of water into the transition zone.

The oceanic and continental crust together is likely to incorporate between three and thirty percent of an ocean mass, but if fully hydrated (serpentinized) could incorporate more than a full ocean of water. The lithospheric mantle is likely to contain between one and fifteen percent of an ocean volume, whereas the asthenospheric mantle (100 to 410 km depth) between four and ninety percent of an ocean. With increasing pressure H becomes increasingly compatible so that in the transition zone, nominally anhydrous wadsleyite, ringwoodite, and garnet can incorporate between five percent and three full ocean masses of water. If saturated at temperatures about 200 ºC below likely mantle geotherms, these phases can incorporate nearly ten full ocean masses of water. Seismic velocities are consistent with up to three ocean masses of water in the transition zone and upper mantle.

Experiments indicate that hydrogen solubility in silicate perovskite of the lower mantle is low at less than about 500ppm H₂O by weight, but that water in solid phases of this region may depend on the stability of phase D. Phase D, however, is likely instable at mantle geotherm temperatures and could break down to perovskite plus melt which could return to the transition zone trapping water in this region. Hydrogen contents of the deep lower mantle and core are much less well constrained, however He and Ne isotopic studies in plume magmas support abundant hydrogen in the early differentiation of the planet, so that H is a viable candidate for the light element in the core.