Patrick F. Dobson, Henrik Skogby,
George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125
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 orthoproxene phenocrysts were also measured,
allowing for the calculation of partition coefficients for water
between boninite melt and orthoproxene. These values (0.003-0.004) for
water partitioning between orthopyroxene and mafic melts may help
contrain petrogenetic models of mantle-derived magmas.