Rayleigh–Taylor instabilities from hydration and melting propel ‘cold plumes’ at subduction zones

Abstract

It is commonly thought that 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 numerically modeled this parodoxically interesting geological phenomenon, in which rising diapiric structures, colder than the asthenosphere by 300–400°C, are driven upward by compositional buoyancy, with a high-resolution two-dimensional regional model. 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.