Colloquium: Wenlu Zhu

Wenlu Zhu

Associate Professor, Department of Geology, University of Maryland

Abstract:  High-resolution three-dimensional (3-D) data on melt distribution are required to determine permeability of partially molten mantle peridotite. In a monomineralic system with isotropic interfacial energy, mineral-melt interfaces are characterized by a constant mean curvature at textural equilibrium, and melt distributes evenly among all grains. For such an idealized system, a power-law permeability-melt fraction relationship with an exponent of 2 can be derived. However, if more than one mineral is present in a system, melt likely partitions unevenly between the different minerals to minimize the total energy of the system. The grain-scale redistribution of fluid among various solid phases is called lithologic partitioning. At ocean ridges, lithologic partitioning could produce permeability heterogeneity, with olivine-rich regions acting as high permeability conduits, and pyroxene-rich regions as low permeability barriers. To date, there is no clear experimental evidence of lithologic partitioning in partially molten mantle rocks. However, interfacial energy driven fluid redistribution has been documented in analog systems containing quartz, calcite and fluorite [Watson, 1999]. Using an approach that integrates high temperature and pressure laboratory experiments, x-ray synchrotron micro-tomography, and image-based numerical simulations, we evaluated the effect of melt partitioning on permeability and 3-D melt distribution in partially molten harzburgite. Our results show strong evidence for lithologic partitioning, in a ~2 to 1 ratio of local melt fraction between olivine and orthopyroxene. Such data place important new constraints on rates of melt migration and melt extraction within the partially molten regions beneath mid-ocean ridges.