Phase equilibria along a basalt-rhyolite mixing line: implications for the origin of calc-alkaline intermediate magmas

One-atmosphere, anhydrous phase equilibria determined for alkali basalt/high-silica rhyolite mixtures provide a model for crystallization of natural calc-alkaline mixed magmas. The compositional trend defined by these mixtures mimics the trends of many continental calc-alkaline volcanic suites. As with many naturally occurring suites, the mixtures studied straddle the low-pressure olivine-plagioclase-augite thermal divide. Magma mixing provides a convenient method for magmas to cross this thermal divide in the absence of magnetite crystallization. For the mixtures, Mg-rich olivine (Fo_82–87) coexists alone with liquid over an exceptionally large range of temperature and silica content (up to 63 wt% SiO₂). This indicates that the Mg-rich olivines found in many andesites and dacites are not necessarily out of equilibrium with the host magma, as is commonly assumed. Such crystals may be either primary phenocrysts, or inherited phenocrysts derived from a mafic magma that mixed with a silicic magma. For the bulk compositions studied, the distribution of Fe and Mg between olivine and liquid (K _D ) is equal to 0.3 and is independent of temperature and composition. This result extends to silicic andesites the applicability of K _D arguments for tests of equilibrium between olivine and groundmass and for modeling of fractional crystallization. In contrast, the distribution of calcium and sodium between plagioclase and liquid varies significantly with temperature and composition. Therefore, plagioclase-liquid K _D s cannot be used for fractional crystallization modeling or as a test of equilibrium. Calcic plagioclase from a basalt will be close to equilibrium with andesitic mixtures, but sodic plagioclase from a rhyolite will be greatly out of equilibrium. This explains the common observation that calcic plagioclase crystals in hybrid andesites are generally close to textural equilibrium with the surrounding groundmass, but sodic plagioclase crystals generally show remelting and armoring with calcic plagioclase.