Water solubility of olivine under redox-controlled deep upper mantle conditions: effects of pressure, temperature and coexisting fluids and implications

Water as structural hydroxyl in olivine plays an important role in determining the water budget of the upper mantle and its numerous physicochemical properties. However, the solubility of water in olivine in the deep upper mantle (i.e., 300–410 km depth), which defines the maximum water content under given conditions, still needs to be known with high precision. We examined the water solubility by annealing experiments under conditions controlled by Fe-FeO buffer and peridotite assemblages at 10–13 GPa and 1100–1450 ºC, using a starting olivine of representative chemistry and different fluid materials. The experimental conditions were broadly consistent with those prevailing in the deep upper mantle. The attainment of equilibrium water incorporation in the H-annealed olivine samples was ensured by H diffusion kinetics, water profile analyses and time-series studies. The annealed samples demonstrate infrared hydroxyl bands at 3650–3000 cm⁻¹, but the relative band patterns are different from those observed in the available H-annealing experiments at 1–7 GPa under otherwise comparable conditions (including starting materials). The obtained solubility of water increases with increasing both temperature and pressure over the run conditions, and differs apparently between the runs equilibrated by different fluids that are relevant to the deep upper mantle and water solubility studies. In general, the water solubility of olivine increases nonlinearly with increasing depth in the upper mantle, and can be described as: C_w = (290 ± 78) × exp ((0.0043 ± 0.0006) × depth (km))– (268 ± 89) (H₂O as coexisting fluid) and C_w = (149 ± 72) × exp ((0.0046 ± 0.0011) × depth (km))–(132 ± 85) (CH₄-H₂O as coexisting fluid), where C_w is water solubility (ppm wt. H₂O). The water solubility of olivine in the realistic upper mantle should be defined from the runs coexisting with CH₄-H₂O, and the highest value is only ~ 800 ± 80 ppm wt. H₂O, implying that the actual water contents of olivine in the upper mantle must be mostly (if not exclusively) lower. The inferred storage capacity of water in peridotite in the upper mantle reaches its maximum of 600 ± 100 ppm wt. H₂O (95% confidence level) at the bottom boundary of ~ 410 km depth, and a minimum of 350 ± 50 ppm wt. H₂O (95% confidence level) is expected at mid-depths of 190–230 km. During the upwelling of relatively water-rich materials from the source regions of enriched mid-ocean ridge basalts or ocean island basalts, hydrous melting would be much easier to trigger at the mid-depths of the upper mantle. The data further suggest that, to produce a pervasive hydrous melting at the ~ 410 km depth, the prevailing water content of the mantle transition zone should be greater than ~ 600 ppm wt. H₂O.