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SUMMARY:The development of seismic anisotropy in partially molten rocks: L
 aboratory observations - Lars  Hansen (University of Oxford)
DTSTART:20160415T133000Z
DTEND:20160415T143000Z
UID:TALK65512@talks.cam.ac.uk
CONTACT:INI IT
DESCRIPTION:Co-authors: Chao Qi (University of Pennsylvania)\, David Walli
 s (University of Oxford)\, Benjamin Holtzman (Lamont-Doherty)\, David Kohl
 stedt (University of Minnesota)Seismic anisotropy is a key indicator of th
 e kinematics of flow in the upper mantle. Much insight has been gained int
 o seismic anisotropy that results from the crystallographic alignment of o
 livine during deformation. This anisotropy is primarily characterized by a
 lignment of the seismically fast axis with the flow direction. This relati
 onship between olivine anisotropy and the macroscopic kinematics allows de
 tailed comparison between simulations of global mantle flow and seismic to
 mography. However\, relatively little is known about the development of se
 ismic anisotropy in partially molten rocks. Some experimental studies on p
 artially 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 m
 elt phase appears to fundamentally alter the grain-scale processes leading
  to crystallographic rotation of the solid phase.<br>Here we present a new
  experimental data set detailing the evolution of anisotropy during deform
 ation of partially molten peridotite. Torsion experiments were conducted o
 n samples composed of San Carlos olivine and basaltic melt at a temperatur
 e 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 simi
 lar to previous experiments. The anisotropy in these samples is weak compa
 red to that in dry\, melt-free olivine deformed to similar strains. The an
 isotropy also exhibits relatively little change in strength and orientatio
 n 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 ro
 tations. Based on our observations\, we extrapolate our results to flow in
  the oceanic upper mantle\, demonstrating good correlation between predict
 ed and obse rved seismic anisotropy.
LOCATION:Seminar Room 1\, Newton Institute
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