An experiment-based model for the petrogenesis of high-alumina basalts

To understand magmatism at convergent margins, one must know the origin of their characteristic plagioclase-rich high-alumina basalts (HABs). Wet melting experiments on basalts at 3 kilobars yielded high-alumina liquids and a coexisting mineral assemblage with plagioclase crystallization in these melts, while mafic minerals underwent limited crystallization or even resorption. These results suggest that hydrous HAB liquids, presumably formed by fractionation of a hydrous basalt at depth, precipitate voluminous plagioclase as pressure drops during ascent and eruption. Plagioclase accumulation is not necessarily required to explain the petrogenesis of plagioclase-rich HAB.