Geophysics

Seismic Spinning

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Science  26 Feb 2010:
Vol. 327, Issue 5969, pp. 1060
DOI: 10.1126/science.327.5969.1060-b
CREDIT: (PHOTO) PHOTOS.COM

Some boundary layers in Earth's interior (including the crust/mantle and core/mantle boundaries) are often identified by sharp seismic discontinuities, whereas finer-scale interlayer structures require constraints from geochemistry and mineral physics to complement less obvious, or sometimes completely absent, seismological signals. For example, because some mantle minerals gradually change in structure or chemical composition with depth (and the associated pressure and temperature shifts), seismic velocities from these regions also vary along a gradient. Based on recent elasticity measurements of the abundant lower mantle mineral ferropericlase, Cammarano et al. computed seismic models that identify one such broad chemical or thermal gradient in the mid—lower mantle. The presence of this transition, which accounts for the preferential stability of high spin states of ferrous iron (Fe2+) in ferropericlase, suggests that the viscosity and thermal conductivity of the mid-mantle may be different than previously thought. Specifically, recycled crustal material driven down into the mantle by subduction zones could flatten or broaden at this depth, which would affect the extent and dynamics of mantle mixing throughout Earth history.

Geophys. Res. Lett. 37, L03308 (2010).

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