Geology

How Heat Stirs the Mantle

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Science  05 Jun 2009:
Vol. 324, Issue 5932, pp. 1242
DOI: 10.1126/science.324_1242a

In the aftermath of earthquakes, seismic tomography offers a window into mantle dynamics through large-scale inversion of recorded seismic velocities along different paths. Observed variations in velocity generally reflect mantle density, but the patterns could also indicate shifts in temperature, composition, grain size, or water content. To address this conundrum, Dalton et al. conducted a global inversion of seismic attenuation, which responds differently to variations in these parameters than does the shear-wave velocity. The data (constrained further by recent laboratory studies of the mineral olivine, abundant in the upper mantle) imply that temperature variations of about 150° to 200°C can explain most of the patterns beneath ocean basins in the upper mantle but that a different composition must be invoked beneath continents at depths less than 250 km. In a complementary approach, Schuberth et al. focused on the lower mantle, comparing seismic data with a numerical mantle circulation model. They also found that much of the observed seismic data, including an apparent large upwelling beneath Africa, can be well accounted for by temperature variations alone (of up to 1000°C) produced by high heat flow from the core.

Earth Planet. Sci. Lett. 10.1016/j.epsl.2009.04.009 (2009); Geochem. Geophys. Geosyst. 10, 10.1029/2009GC002401 (2009).

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