Effect of Water on Olivine Stability in Tholeiites and the Production of Silica-Saturated Magmas in the Island-Arc Environment

Experimental crystallization of olivine tholeiite and SiO₂-saturated tholeiite compositions has demonstrated that addition of water up to the limit of solubility markedly increases the maximum pressure of olivine stability. At depths within the upper mantle from which picritic magmas originate under dry conditions, water-saturated partial melting of peridotite can produce SiO₂-saturated tholeiite liquids. Unless they segregate from residual mantle at very shallow depths, magmas of this type with water contents >5% must fractionate at least olivine during ascent to the earth's surface, thus yielding quartz tholeiites or basaltic andesites. The geochemistry of basalts and basaltic andesites of the island-arc tholeiitic series is compatible with origin involving olivine fractionation from primitive olivine-poor tholeiite magmas. The sites of initiation of partial melting and segregation of these magmas from peridotite mantle will be controlled by availability of water from dehydration of downgoing oceanic lithosphere.