Critical Phenomena between Magmas and Aqueous Fluids

A series of direct observation of complete miscibility between aqueous fluids and island arc andesite/rhyolite magmas has been conducted using modified Bassett's type externally heated diamond anvil cell. The critical phenomena are observed between aqueous fluids and Fuji 1707 magmas with chemical compositions from andesite (62 wt.% SiO2) to rhyolite (69% SiO2). Such critical phenomena between aqueous fluids and magmas equilibrated with mantle peridotite system remain uncertain. This study is the first observation of critical phenomena of natural andesite and rhyolite with aqueous fluids, suggesting a second critical point between magmas and aqueous fluids in the depths of the Earth. Such a supercritical fluid can be separated into a melt and a fluid eventually along its migration to the surface. Under such circumstances elemental fractionation between the aqueous fluid and the melt should take place.