Geochemistry of glasses from ODP Site 125-786

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% H2O, while the rhyolitic glasses contain 2 to 6 wt% H2O, and all glasses have less than 30 to 40 ppm CO2. The highest H2O 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 H2O contents in vesicular glasses suggest shallow quenching on the seafloor.从786A和786B钻孔中回收的11件玻璃质样品保存完好，足以通过红外光谱法（infrared spectroscopy）开展挥发性组分分析。这些玻璃质样品的成分覆盖玻安岩（boninite）质至流纹岩质（rhyolitic）范围：玻安岩质玻璃的H₂O含量为1.4~1.7重量百分比，流纹岩质玻璃的H₂O含量为2~6重量百分比，且所有样品的CO₂含量均低于30~40ppm。最高的H₂O含量可能是海水蚀变作用的结果。未蚀变的英安岩（dacite）质至流纹岩质玻璃质样品应形成于对应海底以下0~700米深度的低压快速淬火环境，该深度也与本次样品的采集深度相符——虽然始新世（Eocene）与现今的水深可能存在差异。多孔玻璃质样品中较低的玻安岩质H₂O含量则表明其是在海底浅部环境中快速淬火形成的。