Rayleigh-Taylor Instabilities from Hydrous Melting Propel "Cold Plumes" at Subduction Zones

It is commonly thought hot diapiric flows prevail in the mantle wedge above the subducting slab. However, hydration and partial melting along the slab can create a situation in which a Rayleigh-Taylor instability can develop at the top of a cold subducting slab. We have modelled numerically with a high-resolution 2-D regional model this parodoxically interesting geological phenomenon in which rising diapiric structures, colder than the asthenosphere by 300 to 400 degrees , are driven upward by compositional buoyancy. These "cold plumes" with a compositional, hydrous origin, launched from a depth of greater than 100 km, are lubricated by viscous heating, have an upward velocity in excess of 10 cm/yr, penetrate the relatively hot asthenosphere in the mantle wedge within a couple of million years and thus can cool the surroundings. These cold plumes are fueled by partial melting of the hydrated mantle and subducted oceanic crust due to fluid release from dehydration reactions wihin the slab, including the decomposition of serpentine. There may be a spatial correlation between seismicity and the particular depth of cold plume initiation, which may be related to the initial age of the subducting lithosphere.