Spontaneous melt localization in a deforming solid with viscosity variations due to water weakening

Extraction of melt is influenced by the rheology of the two phase medium in which it is generated. The viscosity of mantle containing a small melt fra ction is significantly increased as the concentration of water dissolved in olivine decreases. Upon melting, water fractionates into melt, where it is transported-by both advection and diffusion. Advection transports water in to melt rich regions, resulting in higher olivine water contents, lover sol id viscosity, and lower pressure locally. Melt flows into these low pressur e regions, resulting in self-induced localization:of melt. Diffusive transp ort of water opposes advection, creating a preferred wavelength of melt loc alization. An estimated wavelength of melt localization in the range of 100 's of meters to a few kilometers may lead to the formation of high melt fra ction channels which preserve chemical disequilibrium between melt and resi dual mantle.