HYDROGEN INCORPORATION AND RETENTION IN METAMORPHIC OLIVINE FROM THE ECLOGITE FACIES ZERMATT-SAAS SERPENTINITES (WESTERN ALPS)

Incorporation of hydrogen into metamorphic olivine during dehydration reactions in subducted serpentinites provides a mechanism to replenish the deep mantle with water. Metamorphic olivines have been investigated in the eclogite-facies Zermatt-Saas serpentinites that formed at 2.5 GPa and 550 °C through the reaction antigorite + brucite = olivine + fluid. Fourier Transform Infra Red (FTIR) measurements of olivines from massive antigorite serpentinites, shear zones and veins show absorption bands at 3613, 3600, 3580, 3566, 3551, 3535 and 3480 cm^-1, characteristic for Si-vacancy in olivine. Olivine with pure Si-vacancies have been produced in MgO-buffered experiments, but have not been reported in nature so far. The involvement of brucite in the olivine-forming reaction ensures a low Si-activity, and a water activity close to unity favouring hydrogen incorporation in Si-vacancies in olivine.The total integrated absorbance of these bands in clear grains corresponds to water contents between 100 - 140 ppm H₂O. Olivine from all rock types display the same high water contents. Mapping the water distribution by FTIR equipped with a FPA detector shows no signs of gain or loss of H₂O after olivine formation. However, growth zoning is observed in some grains parallel to the crystallographic a-axis and is unrelated to any minor zoning in major and trace elements. This implies that the H in Si-vacancies was retained during exhumation and thus the measured water content represents the amount of water incorporated during peak metamorphic conditions. Using peak conditions of 550 °C and a conservative estimation of 5 My for the exhumation time requires that the diffusivity is ~ 10^-26 m²s^-1 °C. This supports experimental findings that diffusion of H in Si-vacancies is slow. Thus, water incorporated into metamorphic olivine representing 30 wt.% of the total rock mass in the Zermatt-Saas serpentinites at 550 °C, 2.5 GPa could represent an important mechanism to transport water into the deeper mantle by subduction.