Exposed, subduction-related magmatic arcs commonly include sections of
ultramafic plutonic rocks that are composed of dunite, wehrlite, and
pyroxenite. In this experimental study we examined the effects of
variable H2O concentration on the phase proportions and compositions of
igneous pyroxenites and related ultramafic plutonic rocks. Igneous
crystallization experiments simulated natural, arc magma compositions
at 1.2 GPa, corresponding to conditions of the arc lower crust.
Increasing H2O concentration in the liquid changes the crystallization
sequence. Low H2O concentration in the liquid stabilizes plagioclase
earlier than garnet and amphibole while derivative liquids remain
quartz normative. Higher H2O contents (>3%) suppress plagioclase and
lead to crystallization of amphibole and garnet thereby producing
derivative corundum normative andesite liquids. The experiments show
that alumina in the liquid correlates positively with Al in pyroxene,
as long as no major aluminous phase crystallizes. Extrapolation of this
correlation to natural pyroxenites in the Talkeetna and Kohistan arc
sections indicates that clinopyroxenes with low Ca-Tschermaks component
represent near-liquidus phases of primitive, Si-rich hydrous magmas.