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Seismic evidence for melt in the upper mantle

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MIMW03 - From the continuum to the tectonic: the magma/mantle dynamics of planet earth

Michael Kendall, School of Earth Sciences, University of Bristol, UK.   
(Co-author) James Hammond, Department of Earth and Planetary Sciences, Birkbeck, University of London, UK.  
A range of seismic measurements can be used to map melt distribution in the deep Earth. These include seismic P- and S-wave velocities derived from tomography, Vp/Vs ratios obtained from receiver functions, and estimates of seismic anisotropy and attenuation. The most obvious melt parameter that seismic data might be sensitive to is volume fraction. However, in many cases such data are more sensitive to the aspect ratio of melt inclusions, which is controlled by the wetting angle. In many active regions these observations are readily explained by silicate melt in the upper 100 km of the mantle. While low wavespeeds may be attributed to thermal effects in tectonically young or actively volcanic regions, in older, tectonically stable regions low velocity anomalies apparently persist even past the decay time of any thermal perturbation, rendering such a mechanism implausible. Low volume melts can also reduce wavespeeds, but their buoyancy should drain them upward away from source regions, preventing significant accumulation if they are able to segregate. Sulfide, ubiquitous as inclusions in lithospheric mantle xenoliths, forms dense, non‐segregating melts at temperatures and volatile fugacities characteristic of even old lithospheric mantle. Modest amounts of sulfide melt can lead to long-term reductions in seismic wavespeeds in areas of the lithosphere and the asthenosphere disturbed by prior melting events that carry and concentrate sulfide.      

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