Deep mantle matters

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Science  23 May 2014:
Vol. 344, Issue 6186, pp. 800-801
DOI: 10.1126/science.1254399

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The lower mantle, lying between ~670- and ~2890-km depth, comprises most of the rocky portion of Earth. Convection processes within this region transfer heat from the iron core upward, advecting the heat flow that drives the near-surface tectonic engine. As many of the largest volcanic events appear to be correlated with seismic features in the deep mantle (1), the deep lower mantle may also represent an occasional scourge of our surface environment. In this issue, two sets of challenging experiments yield new pictures for how different deep seismic anomalies might be generated. On page 892, Andrault et al. (2) examine the melting temperature of oceanic crust (basalt) to core-mantle boundary (CMB) pressures and temperature, and use that to explain the genesis of areas with ultralow seismic velocities near the CMB. On page 877, Zhang et al. (3) report the startling discovery of a new, iron-rich silicate phase that may be a major component of the lowermost ~700 km of Earth's mantle.