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SUMMARY:Experimental constraints on melt distribution and its effect on th
 e rheology and seismic properties of polycrystalline olivine - Ulrich Faul
  (Massachusetts Institute of Technology)
DTSTART:20160412T123000Z
DTEND:20160412T133000Z
UID:TALK65414@talks.cam.ac.uk
CONTACT:INI IT
DESCRIPTION:Coauthors: Gordana Garapi&#x107\; (SUNY New Paltz)\, Ian Jacks
 on (Australian National University)<br><br> Bulk properties of partially m
 olten rocks are significantly affected by the melt distribution and geomet
 ry. Surface energy minimisation determines the melt geometry\, both locall
 y at the junction of two grains and melt (dihedral angle)\, and for the ag
 gregate as whole\, in the form of grain growth. Grain growth is a continuo
 us process and partially molten rocks therefore constitute a dynamic syste
 m. This contrasts with the assumptions of the model melt distribution in a
  static system with isotropic grains of a single size that can be fully ch
 aracterised by measuring dihedral angles. In the dynamic system the local 
 melt geometry is not always in its minimum energy configuration\, as grain
  growth continuously creates new neighbours that need to adjust their grai
 n boundary plane orientations. In this dynamic system\, the melt distribut
 ion can not be characterised by only measuring dihedral angles.&nbsp\; <br
 > As a somewhat more comprehensive assessment of the melt distribution\, w
 e measure the proportion of grain boundaries wetted by melt (grain boundar
 y wetness/contiguity). While deformation experiments in the diffusion cree
 p regime by necessity need to be carried out on fine-grained samples\, the
  melt distribution can be determined on significantly more coarse-grained 
 samples\, hot-pressed at high temperatures up to two weeks in a piston cyl
 inder apparatus. The wetness data from these samples\, obtained at suitabl
 y high resolution\, allows augmentation of the experimentally measured dif
 fusion creep rheology. <br> For the direct experimental determination of t
 he effect of small amounts of melt on both large-strain rheology and seism
 ic properties it is important to characterise genuinely melt-free material
 s for reference. For this reason we use synthetic Fo90 olivine aggregates 
 that contain no melt or trace elements\, unless deliberately added. Experi
 ments with melt-bearing samples show that both the large strain rheology a
 nd seismic properties are substantially affected by small amounts of melt\
 , consistent with the observations of the melt distribution described abov
 e. For seismic properties the presence of melt affects both the shear modu
 lus and attenuation in the seismic frequency band. Important for the effec
 t of melt on seismic properties are wetted grain boundaries with sufficien
 tly low aspect ratio for local fluid flow to take place (i.e. on the scale
  of a single grain\, &rsquo\;melt squirt&rsquo\;). <br> Similar materials 
 are also used to determine the effect of water (hydroxyl) on the rheology 
 and seismic properties of olivine. This allows comparison of the relative 
 effects of water and melt in the upper mantle.<br>
LOCATION:Seminar Room 1\, Newton Institute
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