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SUMMARY:How much water is subducted into the lower mantle?  Evidence from 
 transition zone anisotropy - Dr Andy Nowacki\, University of Leeds
DTSTART:20160309T160000Z
DTEND:20160309T170000Z
UID:TALK63721@talks.cam.ac.uk
CONTACT:David Al-Attar
DESCRIPTION:The rate and style of chemical interchange between the Earth's
  upper and lower mantle is still poorly understood\, yet knowing this is e
 ssential to understanding how the planet's composition has evolved since i
 ts formation.  One fundamental question concerns whether tectonic plates c
 an transport significant volumes of volatile compounds\, water in particul
 ar\, to great depths.  If so\, this provides a way to recycle this materia
 l back into the Earth.\n\nTo address this question\, I examine the seismic
  signals from earthquakes at depths of over 400 km\, whose origin is itsel
 f highly uncertain.  Using the the method of 'source-side shear wave split
 ting'\, I examine the anisotropy in the region of these events\, looking a
 t subduction zones around the Pacific.  I find that in these regions at le
 ast\, the transition zone (410 to 660 km depth) shows seismic anisotropy t
 hat is as strong as the upper or lowermost mantle.  By testing a range of 
 possible causative mechanisms\, I show that the most likely is the alignme
 nt of dense hydrous magnesium silicate crystals\, known sometimes as the '
 alphabet phases' because they are called 'A'\, 'B'\, 'D'\, and so on.  Thi
 s implies water is carried at least to 660 km depth by subducting slabs.  
 My results also suggest that the earthquakes may not be caused by the tran
 sition of metastable olivine in the slab interior.\n\n\n
LOCATION:Marine/Wolfson Building lecture hall\, Bullard Labs.
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