MELT PERCOLATION IN A PARTIALLY MOLTEN MANTLE MUSH - EFFECT OF A VARIABLE VISCOSITY

Data collected in ophiolite massifs indicate that some horizons with v ery high melt fractions have been frozen during the accretion of the h arzburgites just below the crust. We show that the existence, distribu tion and amplitude of these impregnations are better understood if the ological bifurcations due to the increase in melt fraction, in additio n to a power-law dependence of permeability on porosity, are taken int o account in the compaction models. As shown by deformation experiment s on partially molten rock, at melt fractions of around 5-10% a first theological bifurcation occurs when the melt wets the grain boundaries of mantle minerals, enhancing diffusion processes. A second one occur s when the melt fraction exceeds the second percolation threshold (aro und 25-30%). Our 1D numerical experiments based on the compaction equa tions integrating the effect of these bifurcations show striking featu res reminicent of observations in ophiolites.