Bound water content of the lower oceanic crust estimated from modal analyses and seismic velocities of oceanic diabase and gabbro

Abstract

Hydrous minerals in the oceanic crust are known to carry bound water into subduction zones, but the water content of the lower oceanic crust is poorly constrained. Previous studies suggest that lower crustal diabases and gabbros contain 1–3 and 1–2 wt% H2O. This analysis, based on the modal mineralogy and seismic properties of oceanic diabase and gabbro samples, shows that the H2O content of diabase dike rocks ranges from 1–3%, with an average of 1.5%; in oceanic gabbros, the H2O content decreases with increasing seismic velocity, and gabbros that have velocities typical of the lower oceanic crust (6.7–7.0 km s−1) contain 0.2 to 0.7 wt% H2O, with a mean near 0.5 wt%.