Abstract
Olivine, (Mg, Fe)_2SiO_4, is the predominant mineral in the upper mantle and a study of its defect structure is fundamental to an understanding of the rheological laws that describe mantle flow. Existing models are based on creep mechanisms in which point and line defects have a major role. Here we report the first observations, by transmission electron microscopy (TEM), of planar defects in olivine. The displacement vector associated with the defects, R = 1/4 , together with infrared absorption spectra suggest that the structure of the defects resembles that of an OH-bearing monolayer within the olivine, as exists in the humite family of minerals. Because the planar defects were formed in the upper mantle, their discovery has important implications for the deformation of olivine at high pressures and temperatures in the presence of trace quantities of water, as well as revealing another possible reservoir of water in the upper mantle.
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