Volatile Contents of Izu-Bonin Forearc Volcanic Glasses

Eleven glasses recovered from Holes 786A and 786B were fresh enough for volatile analysis by infrared spectroscopy. The compositions of these glasses range from boninitic to rhyolitic. The glasses from the boninites contain 1.4 to 1.7 wt% H_2O, while the rhyolitic glasses contain 2 to 6 wt% H_2O, and all glasses have less than 30 to 40 ppm CO_2. The highest H_2O contents are probably the result of seawater alteration. The unaltered dacitic to rhyolitic glasses were probably quenched at low pressures corresponding to depths of 0-700 m below the seafloor, also corresponding to the depths of collection of these samples, although the water depths may have been different between the Eocene and the present. The lower boninitic H_2O contents in vesicular glasses suggest shallow quenching on the seafloor.