The development of seismic anisotropy in partially molten rocks: Laboratory observations

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• Lars Hansen (University of Oxford)
• Friday 15 April 2016, 14:30-15:30
• Seminar Room 1, Newton Institute.

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MIMW02 - From the grain to the continuum: two phase dynamics of a partially molten, polycrystalline aggregate

Co-authors: Chao Qi (University of Pennsylvania), David Wallis (University of Oxford), Benjamin Holtzman (Lamont-Doherty), David Kohlstedt (University of Minnesota)Seismic anisotropy is a key indicator of the kinematics of flow in the upper mantle. Much insight has been gained into seismic anisotropy that results from the crystallographic alignment of olivine during deformation. This anisotropy is primarily characterized by alignment of the seismically fast axis with the flow direction. This relationship between olivine anisotropy and the macroscopic kinematics allows detailed comparison between simulations of global mantle flow and seismic tomography. However, relatively little is known about the development of seismic anisotropy in partially molten rocks. Some experimental studies on partially molten rocks suggest that the seismically fast direction tends to lie at high angles to the flow direction, leading to a vastly different relationship between anisotropy and kinematics. Thus, the presence of a melt phase appears to fundamentally alter the grain-scale processes leading to crystallographic rotation of the solid phase.
Here we present a new experimental data set detailing the evolution of anisotropy during deformation of partially molten peridotite. Torsion experiments were conducted on samples composed of San Carlos olivine and basaltic melt at a temperature of 1473 K and a confining pressure of 300 MPa. Seismically fast axes of olivine tend to lie at a high angle to the flow direction in a manner similar to previous experiments. The anisotropy in these samples is weak compared to that in dry, melt-free olivine deformed to similar strains. The anisotropy also exhibits relatively little change in strength and orientation with progressive deformation. Detailed microstructural analyses allow us to distinguish between competeing models for the grain-scale deformation processes, favoring one in which intergranular processes control grain rotations. Based on our observations, we extrapolate our results to flow in the oceanic upper mantle, demonstrating good correlation between predicted and obse rved seismic anisotropy.

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