COG3 Seminar – Mark Behn (WHOI)

The role of grain size on the seismic structure of the oceanic upper mantle and melt migration beneath midocean ridges

Grain size is a key microstructural property of the Earth’s mantle, because it influences rheology, deformation mode (e.g., diffusion vs. dislocation creep), seismic attenuation and wave-speeds, electrical conductivity, and the permeability of the mantle to melt migration. In
this talk I will discuss models for grain size evolution in the oceanic upper that combine composite grain-size dependent rheology with the Wattmeter [Austin & Evans, 2007] model for how grain size changes in response to the evolving deformation field. These results will be interpreted in the context of the seismic low velocity zone beneath the ocean basins and the origin of the lithosphere asthenosphere boundary. Finally, I will discuss the implications of grain size variability on the patterns of melt migration beneath mid-ocean ridges and subduction zones and show the results of preliminary two-phase flow models that couple melt migration with the predicted grain-size field.