Storage of water in (Mg,Fe)SiO3-perovskite: Synthesis from natural samples

There is significant ambiguity as to the water storage capacity of (Mg,Fe)SiO3 perovskite under lower mantle conditions depending upon the synthesis method and starting materials. Here we present the results of high-pressure synthesis of perovskite from two natural Bamble enstatite samples (Mg/(Fe+Mg) = 0.75; Al2O3 < 0.1 wt%), which were synthesized in the laser-heated diamond anvil cell at 1500-2000 K and 25-65 GPa. One sample is dry with no detectable OH in FTIR, and the second sample contains ~0.08 wt% H2O. The anhydrous sample transforms to perovskite completely with no evidence of other phases present. The hydrous sample, however, forms predominantly perovskite, with some runs resulting in additional minor stishovite, phase D, ferropericlase or a combination of these. The zero-pressure volume and bulk modulus of the resulting perovskite, are indistinguishable between the two sets of experiments (V0 = 163.8(2) Å3 and K0 = 259(4) GPa), indicating little compositional differences. Infrared spectroscopy of the hydrous sample consistently reveals a disordered hydrogen structure consistent with phase D or OH in residual melt (quenched as glass), largely indistinguishable between experiments producing either just perovskite or a combination of perovskite and minor phases. Distinct OH stretching modes become evident upon cooling below 100 K with peaks at 3576, 3378, 3274, and 3078 cm-1, with no evidence of OH in either stishovite or ferropericlase. Additional multiple O-H-O bending modes are evident at between 1255 and 1630 cm-1. We interpret these results as the formation of a minor fraction of hydrous melt at temperatures at or below 1500 K for all pressure to 65 GPa, with a dry residual perovskite phase.