Hydrogen in the Deep Earth

Abstract

The mechanisms of exchange of hydrogen between the deep interior and surface of Earth, as well as the means of retention and possible abundance of hydrogen deep within the Earth, are examined. The uppermost several hundred kilometers of Earth's suboceanic upper mantle appear to be largely degassed, but significant primordial hydrogen could be retained within the transition zone, lower mantle, or core. Regassing of the planet occurs via subduction: Cold slabs are likely particularly efficient at transporting hydrogen to depth within the planet. Marked changes in hydrogen cycling have taken place throughout Earth's history: Evidence of hydrated ultramafic melts in the Archean and probable hydrogen retention within a Hadean magma ocean indicate that early in its history, the deep Earth was substantially wetter. The largest enigma associated with hydrogen in the deep Earth lies in the core: This region could represent the dominant reservoir of hydrogen on the planet, with up to ∼100 hydrospheres of hydrogen present as a high-pressure iron-alloy.

Nominally hydrous magmatism on the Moon

“…In fact, this conclusion has been incorporated into many petrologic and geophysical models constructed to aid in our understanding of lunar formation and lunar geology (3)(4)(5)(6)(7)(8)(9)(10)(11). The bulk water content of the Moon was recently estimated to be less than 1 ppb (11), which would make the Moon at least six orders of magnitude drier than the interiors of Earth (12,13) and Mars (14). This extremely low water content is in keeping with the pervasive volatile-element depletion signature recorded in all lunar materials, because hydrogen is the most volatile of the elements.…”

Lower mantle hydrogen partitioning between periclase and perovskite: A quantum chemical modelling

“…Such a scheme may lead to the formation of (i) hydroxyl groups (Smyth, 2006;Panero et al, 2015) which are shaped by incorporated H and coordination oxygens, and (ii) X-cation vacancies (V X ) at the expenses of either magnesium (in pe) or silicon (in pvk). In pvk, the substitution of one sixfold coordinated Si with four H atoms, which likely form as many hydroxyl groups with vertex oxygens in the same polyhedron, accounts for an analogous replacement taking place in hydro-garnet, although in this case relating to tetrahedrally coordinated silicon (Williams and Hamley, 2001). The substitution of one Mg versus two hydrogen atoms in pvk has recently been explored (Hernàndez et al, 2013), providing a pe/pvk-partition coefficient for H of %0.01.…”

Thermal, hydrous, and mechanical states of the mantle transition zone beneath southern Africa

“…The south African cratons were formed rapidly in the presence of subduction zones [5,6] and subducting lithosphere may supply water to the transition zone [42,43]. A cold transition zone would be a particularly likely place for accumulation of excess water due to higher water solubility than in ambient mantle [44,45], especially during the Archaean. Water exsolution below 660 km and heavy melts above the 410-km discontinuity could prevent convective mixing of water throughout the mantle over time [46].…”

“…Other constants are b (0.14 km per percent of water saturation [34,35]), c (36U10 35 l/K [49]), and d (30.038 per percent of water saturation [47,48]). Two simpli¢cations in the equation require further experimental constraints: the change in the depth of the phase transition is likely a non-linear function of water content [33] ; and water solubility is a function of temperature [44,45]. Assumptions in tomographic inversions (e.g.…”