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SUMMARY:Shear Induced Melt Bands: The Mechanics of their Formation and the
 ir Possible Role as Melt Conduits Beneath Mid-Ocean Ridges - Samuel Butler
  (University of Saskatchewan)
DTSTART:20160609T103000Z
DTEND:20160609T113000Z
UID:TALK66407@talks.cam.ac.uk
CONTACT:INI IT
DESCRIPTION:The compaction equations predict that an instability will occu
 r if the matrix viscosity decreases with porosity and the system is subjec
 ted to shear. The instability is manifest as a series of bands of high and
  low porosity. Porosity bands have been seen in experimental investigation
 s of sheared partial melt systems and the bands are always oriented roughl
 y 25&deg\; from the direction of maximum compression and occur on length s
 cales similar to the compaction length and significantly larger than the g
 rain size. Linear instability analysis of the compaction equations predict
 s that bands should grow fastest at the smallest possible length scale and
 \, for purely porosity-dependent matrix viscosity\, parallel to the direct
 ion of maximum compression. Various additions to the matrix rheology law h
 ave successfully been used to produce bands with orientations similar to t
 hose seen in experiments\, including strain-rate dependent viscosity\, ani
 sotropic viscosity and grain-size and roughness dependent or damage rheolo
 gy.   Furthermore\, melt bands have been proposed as high permeability con
 duits that could channel melt towards the mid-ocean ridge. &nbsp\;In order
  to be effective channels\, the bands must be oriented towards the ridge a
 nd their amplitude must be sufficient to result in a significant permeabil
 ity variation after evolving through a mid-ocean ridge corner flow.   In t
 his presentation\, I will first present linearized theory and numerical mo
 deling results for melt band formation in 2D in simple and pure shear geom
 etries with the rheology laws listed above in order elucidate the process 
 of melt band formation. I will then present an explanation for the growth 
 of the width of melt bands to sizes greater than that of the initial heter
 ogeneity. I will then present linear theory and numerical modeling results
  for bands formed when the background velocity field is that of a mid-ocea
 n ridge corner flow in order to assess the efficacy of melt bands as a cha
 nneling mechanism for melt to mid-ocean ridges. I will show that the rotat
 ion of bands by the mid-ocean ridge flow field causes bands to be poorly o
 riented to channel melt to the mid-ocean ridge and that the amplitude of t
 he bands is only likely to be sufficient to cause the significant permeabi
 lity heterogeneity that is necessary to cause channelization if the matrix
  bulk viscosity is small.
LOCATION:Seminar Room 1\, Newton Institute
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