The Role of Pressure in Producing Compositional Diversity in Intraplate Basaltic Magmas

Basaltic magmas found in intraplate suites appear to follow more than one differentiation trend. Many ocean island suites follow the ocean island tholeiitic trend, with the basalts differentiating from olivine tholeiite through basaltic andesite, andesite, and dacite to sodic rhyolite. Many continental intraplate magmatic regimes, such as those of the Snake River Plain and the plutonic sequences associated with massif anorthosites, follow the potassic silica-saturated alkalic trend, in which basalt differentiates from olivine tholeiite through ferrobasalt (jotunite or ferrodiorite), Fe-rich intermediate rocks (trachybasalt or monzonite), and trachyte (syenite) to potassic rhyolites and granites. Crystallization experiments on an olivine tholeiite from the Snake River Plain show that the basaltic portions of the ocean island tholeiitic trend and the potassic silica-saturated alkalic trend (which leads to strong alkali, P, Ti, and Fe enrichment and silica depletion) can arise from the same ‘dry’ tholeiitic parental magma. These compositional differences are induced by changes in phase equilibria as a function of pressure, with the ocean island tholeiitic series arising from crystal–liquid differentiation at low pressure and the potassic silica-saturated alkalic series arising via differentiation at elevated pressures.