H₂O storage capacity of olivine and low-Ca pyroxene from 10 to 13 GPa: Consequences for dehydration melting above the transition zone

The onset of hydrous partial melting in the mantle above the transition zone is dictated by the H₂O storage capacity of peridotite, which is defined as the maximum concentration that the solid assemblage can store at P and T without stabilizing a hydrous fluid or melt. H₂O storage capacities of minerals in simple systems do not adequately constrain the peridotite water storage capacity because simpler systems do not account for enhanced hydrous melt stability and reduced H₂O activity facilitated by the additional components of multiply saturated peridotite. In this study, we determine peridotite-saturated olivine and pyroxene water storage capacities at 10-13 GPa and 1,350-1,450°C by employing layered experiments, in which the bottom ~2/3 of the capsule consists of hydrated KLB-1 oxide analog peridotite and the top ~1/3 of the capsule is a nearly monomineralic layer of hydrated Mg# 89. 6 olivine. This method facilitates the growth of ~200-μm olivine crystals, as well as accessory low-Ca pyroxenes up to ~50 μm in diameter. The presence of small amounts of hydrous melt ensures that crystalline phases have maximal H₂O contents possible, while in equilibrium with the full peridotite assemblage (melt + ol + pyx + gt). At 12 GPa, olivine and pyroxene water storage capacities decrease from ~1,000 to 650 ppm, and ~1,400 to 1,100 ppm, respectively, as temperature increases from 1,350 to 1,450°C. Combining our results with those from a companion study at 5-8 GPa (Ardia et al., in prep.) at 1,450°C, the olivine water storage capacity increases linearly with increasing pressure and is defined by the relation C^olivine _H2O(ppm) = 57.6(±16)× P(GPa) - 169(±18).Adjustment of this trend for small increases in temperature along the mantle geotherm, combined with experimental determinations of D ^pyx/olivine_H2Ofrom this study and estimates ofD^gt/olivine_H2O allows for estimation of peridotite H₂O storage capacity, which is 440 ± 200 ppm at 400 km. This suggests that MORB source upper mantle, which contains 50-200 ppm bulk H₂O, is not wet enough to incite a global melt layer above the 410-km discontinuity. However, OIB source mantle and residues of subducted slabs, which contain 300-1,000 ppm bulk H₂O, can exceed the peridotite H₂O storage capacity and incite localized hydrous partial melting in the deep upper mantle. Experimentally determined values of D ^pyx/olivine_H2O at 10-13 GPa have a narrow range of 1.35 ±0.13, meaning that olivine is probably the most important host of H₂O in the deep upper mantle. The increase in hydration of olivine with depth in the upper mantle may have significant influence on viscosity and other transport properties.