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SUMMARY:Seismic imaging of mantle melting processes beneath volcanic arcs 
 and backarc spreading centers - Douglas Wiens (Washington University in St
 . Louis)
DTSTART:20160606T103000Z
DTEND:20160606T113000Z
UID:TALK66355@talks.cam.ac.uk
CONTACT:INI IT
DESCRIPTION:Co-author:&nbsp\; S. Shawn Wei&nbsp\; (Washington University i
 n St Louis\; now at Scripps Institition of Oceanography)<br><br>Seismologi
 cal studies using land and ocean bottom seismographs can help constrain mo
 dels of mantle melting by imaging velocity anomalies resulting from the pr
 esence of partial melt. &nbsp\;&nbsp\;Arc-backarc systems are particularly
  interesting\, as they involve both flux and decompression melting mechani
 sms\, variable water input into the melting process\, and various levels o
 f interaction between arc and backarc.&nbsp\;&nbsp\; In addition\, an exte
 nsive dataset of petrological and geochemical measurements also provide im
 portant constraints. &nbsp\;&nbsp\;Seismic tomographic results imaging arc
 -backarc systems show extensive upper mantle regions with velocities that 
 are too slow to be explained without invoking partial melt. &nbsp\;&nbsp\;
 In the Mariana arc\, mantle seismic anomalies beneath the arc and backarc 
 spreading center are separated by a high velocity\, low attenuation region
  at shallow depths (< 80 km)\, implying distinct arc and backarc melting r
 egions\, with the anomalies coalescing at greater depths. &nbsp\;&nbsp\;La
 rge slow velocity anomalies delineate the regions with significant melt\, 
 extending from 10-50 km depth beneath the backarc and 40-80 km depth benea
 th the volcanic arc\, consistent with final melt equilibrium depths estima
 ted from basalt thermobarometry.&nbsp\; In the Lau basin\, backarc spreadi
 ng center basalts show a rapid transition from MORB-like chemistry in the 
 north to back-arc basin basalts with strong water and slab-derived geochem
 ical components in the south as the distance between the spreading center 
 and the volcanic arc is reduced. &nbsp\;&nbsp\;Slow seismic velocity anoma
 lies beneath the spreading center extend deeper and farther west in the no
 rth\, suggesting that partial melting occurs along an upwelling limb of ma
 ntle flow originating in the ambient mantle west of the backarc\, but this
  feature is missing in the south\, indicating that the southern ridge samp
 les only mantle in the vicinity of the subducting slab\, consistent with i
 ts high water content. &nbsp\;The amplitude of the observed Lau backarc se
 ismic anomalies have an inverse relationship to inferred mantle water cont
 ent\, suggesting that water reduces the melt porosity.&nbsp\;&nbsp\; Water
  may increase the efficiency of melt transport and reduce porosity by lowe
 ring the melt viscosity\, increasing grain size through faster grain growt
 h\, or by causing a different topology of melt within the mantle rock.&nbs
 p\;&nbsp\;&nbsp\; A lower melt porosity for aqueous melts is also consiste
 nt with the smaller amplitude seismic anomaly seen for the water-rich volc
 anic arc melting regions compared to the backarc melt production zone.&nbs
 p\;&nbsp\;&nbsp\; Seismic attenuation studies show very high shear attenua
 tion beneath the backarc spreading center consistent with high temperature
 s and partial melt.&nbsp\; Perhaps most surprisingly\, we also observe str
 ong bulk attenuation\, suggesting that partially molten mantle absorbs sei
 smic energy with some poorly understood dissipative process.    <br><br>
LOCATION:Seminar Room 1\, Newton Institute
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