Water in boninite glass and coexisting orthopyroxene: concentration and partitioning

Abstract

Spectroscopic measurements of water in glass inclusions in pyroxene from boninite samples from the Bonin Islands conclusively document the high (2.8–3.2 wt%) primary water contents of boninite magmas. Associated quenched glass from pillow lava rims have slightly lower (2.2–2.4 wt%) water contents, suggesting that minor amounts of degassing occurred between the time of melt entrapment in the orthopyroxenes and subsequent eruption on the sea floor. Some zonation of molecular water contents in pillow rim glasses was observed. OH contents of the host orthopyroxene phenocrysts were also measured, allowing for the calculation of partition coefficients for water between boninite melt and orthopyroxene. These values (0.003–0.004) for water partitioning between orthopyroxene and mafic melts may help constrain petrogenetic models of mantle-derived magmas.