Abstract
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 little or no plagioclase. An isothermal pressure drop to 1 kilobar caused 20 to 30 percent plagioclase crystallization in these melts, while mafic minerals underwent limited crystallization or even resorption. These results suggest that hydrous (>/=4 percent H(2)O) HAB liquids, presumably formed by fractionation of a hydrous basalt at depth, will precipitate voluminous plagioclase as pressure drops during ascent and eruption. Plagioclase accumulation is not necessarily required to explain the petrogenesis of plagioclase-rich HAB.
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