Effects of solute segregation on the grain-growth kinetics of orthopyroxene with implications for the deformation of the upper mantle

The kinetics of static grain growth in a pure synthetic orthopyroxene and orthopyroxene of natural composition were investigated to determine the effects of solute segregation on grain growth. In all experiments the grain size of the synthetic orthopyroxene increases considerably; in contrast, the natural orthopyroxene shows no perceptible grain growth at the same conditions. High-resolution X-ray compositional mapping confirms that the natural orthopyroxene has large concentrations of aluminum and calcium cations segregated on and near its grain boundaries. This segregation is believed to be responsible for the slow grain-growth kinetics in the natural orthopyroxene. Slow grain growth may permit long-term deformation in the diffusion creep regime, as has been observed in some deep mantle xenoliths. As such, it may provide a means of weakening the lithosphere.