WATER IN BONINITE GLASS AND COEXISTING ORTHO-PYROXENE - CONCENTRATIONAND PARTITIONING

Spectroscopic measurements of water in glass inclusions in pyroxene fr om boninite samples from the Bonin Islands conclusively document the h igh (2.8-3.2 wt%) primary water contents of boninite magmas. Associate d quenched glass from pillow lava rims have slightly lower (2.2-2.4%) water contents, suggesting that minor amounts of degassing occurred be tween 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 orthopyrox ene phenocrysts were also measured, allowing for the calculation of pa rtition coefficients for water between boninite melt and orthopyroxene . These values (0.003-0.004) for water partitioning between orthopyrox ene and mafic melts may help constrain petrogenetic models of mantle-d erived magmas.