Stability of a compressible hydrous melt layer above the transition zone

Seismic and mineral physics structures suggest melt can be generated at the top of the transition zone. Because the melt phase is significantly more compressible than the solid phase, it is expected that a density crossover occurs with increasing pressure. Experimental studies suggest this crossover lies towards the base of the upper mantle. If the density crossover lies above the 410 km discontinuity, melt will collect and form a layer until such time as it reaches the crossover when part of it may become unstable. We investigate the stability of a compressible melt layer which intersects the density crossover. Analytic models of instabilities are used to determine the effects of compressibility and crossover location on instability growth rates. We find that increasing the compressibility increases growth rates due to the fact that rising perturbations expand and become more unstable as they rise. If the density crossover lies above a melt layer at the transition zone, perturbations must be large before becoming unstable, making it likely that the layer would remain. However, when the density crossover lies within the melt layer, all perturbations are unstable and they drain the melt layer completely unless diapiric separation occurs.