Creepy Mantle

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Science  14 Feb 2003:
Vol. 299, Issue 5609, pp. 981
DOI: 10.1126/science.299.5609.981a

Periclase (MgO) is a minor component of Earth's lower mantle, yet it may be essential for understanding the rheological and chemical characteristics of solid-state creep (mantle convection) because of its elastic anisotropy and ionic conductivity. Van Orman et al. measured lattice and grain boundary diffusion of 25Mg and 18O in isotopically marked, synthetic periclase at 2273 kelvin and 25 gigapascals in a multianvil device, and then extrapolated to conditions (∼4500 kelvin and 140 gigapascals) at the core-mantle boundary (CMB). A transition from dislocation creep to diffusion creep is predicted to occur near the CMB for grains smaller than 1 mm and shear stresses of 1 to 10 megapascals. Substantial chemical exchange near the CMB is allowed by the relatively fast rate of diffusion in periclase. In particular, grain boundary diffusion is very efficient, and over the 4.5 billion years of Earth's history, a mixed layer of 100 kilometers in thickness could have formed. Thus, the rheological and chemical properties of the mantle are consistent with the seismically observed D” layer at the CMB. — LR

Geophys. Res. Lett.30, 1056, 10.1029/2002GL016343 (2003).

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