Calculation of the energetics of water incorporation in majorite garnet

Abstract

Interpretation of lateral variations in upper mantle seismic wave speeds requires constraints on the relationship between elasticity and water concentration at high pressure for all major mantle minerals, including the garnet component. We have calculated the structure and energetics of charge-balanced hydrogen substitution into tetragonal MgSiO 3 majorite up to P = 25 GPa using both classical atomistic simulations and complementary first-principles calculations. At the pressure conditions of Earth's transition zone, hydroxyl groups are predicted to be bound to Si vacancies (o) as the hydrogarnet defect, [o Si +4OH O ] X , at the Si2 tetrahedral site or as the [o Mg +2OH O ] X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [o Mg +2OH O ] X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt% Al 2 O 3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [o Mg +2OH O ] X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnv s /dlnv p , in MgSiO 3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H 2 O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnv s /dlnv p for ringwoodite. The incorporation of 3.2 wt% Al 2 O 3 also decreases R(H 2 O) by ~0.2-0.4. Therefore, to accurately estimate transition zone compositional and thermal anomalies, hydrous majorite needs to be considered when interpreting seismic body wave anomalies in the transition zone.

Diffusion in garnet: a review

“…It is worth noting that Wright et al (1994) first used computer simulation methods to model the structure and energetics of the hydrogarnet defect in grossular. Recently, similar studies have been reported by Pigott et al (2015) for MgSiO 3 majorite up to 25 GPa employing both classical atomistic simulations and complementary first-principles calculations. However, owing to the complexity of the structure and composition of garnet, only Wright et al (1995) and Carlson et al (2014) have carried out computer simulation methods to calculate O and trace element diffusivity in garnet so far.…”

Stability and Solid Solutions of Hydrous Alumino-Silicates in the Earth’s Mantle

“…Many synthesis experiments exploring phase D report greater hydrogen concentrations than expected from stoichiometry [15] with less than ideal Mg/Si or Al/Si ratio. They imply an additional, unexplored defect mechanism that should include either a hydrogarnet substitution of Si 4+ = 4 H + or a vacancy on the Mg-site as Mg 2+ = 2 H + , two of the most common defects stable under transition zone pressures [26,27]. Such additional mechanisms may also help explain the mismatch with multiple synthesis experiments in the a-axis as a function of aluminum content ( Figure 2) as well as explain the scatter in structural results.…”

Dehydration Melting Below the Undersaturated Transition Zone

“…We calculate the energetics of the V ′′ M +2(H · ), V ′′′′ Si +4(H · ), Al ′ Si +H · , and Fe ′ Si +H · defects for majorite, akimotoite, calcium perovskite, periclase, and wadsleyite, complimented by previously reported results on ringwoodite, bridgmanite, and majorite garnet (Panero, 2010;Panero et al, 2015;Pigott et al, 2015).…”

“…Geochemistry, Geophysics, Geosystems 2015; Pigott et al, 2015). Crystal defects are denoted using Kröger-Vink notation, in the form of A C S , in which A denotes the species occupying lattice site S with net ionic charge C. Here, we use V to denote crystallographic vacancies, M for metal species (Mg, Fe, or Ca), "·" and " ′ " to denote a net positive and negative charge, respectively.…”

“…In wadsleyite, this defect is most stable on the Mg3 site, with OH bonds along opposite edges of vacant octahedral. The V ′′ Mg +2(H · ) defects in ringwoodite and majorite are as in Panero (2010) and Pigott et al (2015). In ringwoodite, the H defects lie within the vacant Mg octahedron bonded to opposite oxygen atoms.…”