Vol 279

This study experimentally investigates the patterns of hydrogenation of olivine crystals by an aqueous fluid in equilibrium with olivine and orthopyroxene at a pressure of 5.5 GPa and temperatures of 850, 940, 1030, and 1100 °C. It is shown that at P-T parameters characteristic of slabs and the portion of the mantle wedge mechanically coupled to slabs under the thermal regimes of warm and hot subduction, the interaction of the aqueous fluid with olivine is largely controlled by the concentration of pre-existing silicon vacancies in olivine crystal structure. As a result, at a temperature of 850 °C, the concentration of OH defects related to Si vacancies increases noticeably relative to the initial value; however, calculated as H₂O, it does not exceed 110 ppm. At 1030 °C, no new Si vacancies are formed, and the water content in olivine remains at the same level as after experiments at lower temperatures. The formation of Si vacancies and their protonation during olivine recrystallization in an aqueous fluid are recorded only at 1100 °C, near the peridotite solidus. In newly formed olivine, the water content reaches 350 ppm. It has been established that an increase in oxygen fugacity from the values of the Ni-NiO (NNO) buffer to those of the Fe₂O₃-Fe₃O₄ (HM) buffer, as well as an increase in NaCl concentration in the fluid from 0 to 9 wt.%, have little or no effect on the solubility of water in olivine. It is concluded that due to the low solubility of water in olivine immediately following the complete dehydration of serpentinized peridotites at depths of ~150-200 km, the efficiency of water transport into the mantle by slabs under the thermal regimes of warm and hot subduction must decrease sharply. Under such conditions, a significant volume of aqueous fluid is released within the slabs, which can participate in the generation of deep magmas and mantle metasomatism.