The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study

Exposed, subduction-related magmatic arcs commonly include sections of ultr amafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite . In this experimental study we examined the effects of variable H2O concen tration on the phase proportions and compositions of igneous pyroxenites an d related ultramafic plutonic rocks. Igneous crystallization experiments si mulated natural, arc magma compositions at 1.2 GPa, corresponding to condit ions of the arc lower crust. Increasing H2O concentration in the liquid cha nges the crystallization sequence. Low H2O concentration in the liquid stab ilizes plagioclase earlier than garnet and amphibole while derivative liqui ds remain quartz normative. Higher H2O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing deriv ative corundum normative andesite liquids. The experiments show that alumin a in the liquid correlates positively with Al in pyroxene, as long as no ma jor aluminous phase crystallizes. Extrapolation of this correlation to natu ral pyroxenites in the Talkeetna and Kohistan arc sections indicates that c linopyroxenes with low Ca-Tschermaks component represent near-liquidus phas es of primitive, Si-rich hydrous magmas.