IR investigation of peridotite from ODP-Leg125, Mariana-Izu-Bonin forearc

Orthopyroxene was analyzed as a proxy for water in melt-depleted, supra-subduction-zone peridotite by polarized infrared radiation. Samples from Conical (site 779) and Torishima seamount (site 784), located c. 1300 km apart in the Mariana-Izu-Bonin forearc (ODP-Leg 125), have very similar average H2O contents of 215 ppm (Conical) and 191 ppm (Torishima). Conical peridotite equilibrated at lower temperature (760 ºC) and lower oxygen fugacity (site average: ΔlogFMQ -0.65) compared to samples from Torishima (830 ºC; ΔlogFMQ +0.33). The degree of partial melting is almost identical for the two sites (18 and 20 %). The recorded H2O concentrations are considerably higher than those in samples from the Bismarck forearc (31-92 ppm H2O; Tollan and Hermann, 2019). Instead, the average values resemble those of peridotitic orthopyroxene from mid-ocean-ridge (MOR) settings.