Effect of grain size on the distribution and transport of deep-seated fluids and melts

Because permeability increases with the square of the grain size for a give n fluid fraction, it is commonly assumed that porous flow of fluids and mel ts is more effective in coarse-grained domains of deep-seated rocks. This r elative behavior is accurate, however, only as a description of two systems operating in isolation. We demonstrate that if coarse- and fine-grained do mains are in chemical communication, then equilization of pore-wall curvatu re across the system results in concentration of fluid or melt in domains o f finer grain size. Because of this localization of fluid, the permeability of the finer-grained domains will match or exceed that of domains with coa rser grains. Fluid focussing due to differences in grain size has potential ly important implications for grain-scale transport of metamorphic fluids - including those released from subducting slabs into the sheared mantle wed ge - and also for partial melts in the mantle and deep crust.