Basalt Convection and Plagioclase Retention: A Model for the Generation of High-Alumina Arc Basalt

Recent investigations of crystal settling in convecting magma have suggested that crystals may be spatially sorted according to both size and mineralogy. For a basalt magma simultaneously crystallizing plagioclase, clinopyroxene, and olivine, a possible result is the selective removal of mafic minerals and the preferential retention of plagioclase crystals along the intrusion margin. This represents a possible mechanism for generating plagioclase-phyric magmas of high-alumina basalt composition. In this study, a combined basalt crystallization and convection model has been developed to assess the extent to which crystallization and convection leads to mineralogic sorting and preferential plagioclase retention in a primitive mantle derived calc-alkaline basalt intrusion. The model assumes a simple rectangular chamber geometry, steady-state non-turbulent single cell convection, and spatially uniform crystallization. The results indicate that over a narrow range of convective velocities: (1) crystal sorting by size should occur, with progressively smaller crystals concentrated near the intrusion margin; (2) mineralogic sorting should also occur, with a pronounced region of plagioclase enrichment and mafic mineral depletion near the intrusion margin; (3) regions of plagioclase enrichment contain magmas of high-alumina basalt composition; and (4) the depth range over which high-alumina basalt formation occurs is a function of the convective velocity. Though specific to calc-alkaline basalt, these results have general application to late stage tholeiitic basalt, and plagioclase-bearing alkaline basalt systems. For this model to have application to natural high-alumina basalt magmas, actual convection must be non-turbulent, convective velocities must be appropriate for preferential plagioclase retention, and crystallization must extend at least 0.5 km inward from the intrusion margin. At two high-alumina basalt occurrences in the Aleutian Islands, observed variations in bulk chemical composition, mineralogy, and texture agree fully with those predicted by the model, thus suggesting that the required conditions exist. An important consequence of the model is that, if correct, high-alumina basalts are restricted to magmatic arc environments not by virtue of a special source component, but rather by unique thermal characteristics conducive to the appropriate conditions of convection and crystallization.