Benioff zone magmatism

In the subduction and transformation of oceanic crust in a Benioff zone the high-pressure eclogitic assemblage includes sanidine; this phase allows the P-T curve of equilibration of andesite magma and quartz eclogite to be calculated by using the concept of K2O activity. Lavas with high K2O concentrations equilibrate at greater depths than those with less K2O. The presence of sanidine buffers the concentration of K2O in the melt, causing it to increase with pressure (depth) and thereby explaining the typical relation between K2O and the depth to the Benioff zone observed in island arcs. Calculations on a specific basaltic andesite lava underlain by a subducted lithospheric plate at 155 km indicate equilibration with its quartz ‘xenocrysts’ (or quartz eclogite) at 1400° ± 50°C. The calculated amount of H2O in this magma at source is less than 0.75 wt %. In contrast to other models of magma genesis in the Benioff zone, we propose a model in which melting takes place only over a very short distance down dip, since the subducted oceanic crust contains only small amounts of potential andesitic magma. Moreover, since active andesite volcanoes do not extend more than 20–30 km across the strike of oceanic volcanic arcs, the source region is likely to be a long, thin, narrow tube of magma that extends laterally along the length of the island arc at a depth equal to the depth to the top of the Benioff zone. Such a liquid ‘layer’ is gravitationally unstable and tends to form equally spaced diapiric conduits along the arc, the approximately equal (∼70 km) spacing of active island arc volcanic centers thus being explained. Fluid dynamic calculations indicate that for an observed spacing of volcanoes of 70 km and a viscosity contrast between the magma and the mantle of 10−9 poise, the magma source is about 20 m thick. However, the viscosity of an andesitic magma under these conditions is highly uncertain.