Host: The Mineralogical Society of Japan
Segregation of incompatible elements results in a marked difference between the composition of the interiors and the boundaries of grains within the rocks that comprise Earth's mantle. At chemical equilibrium, the molar concentrations of solutes in grain matrices, XGM, and the grain boundaries, XGB, can be related through the grain boundary segregation energy, gamma:
XGB/(XGB0-XGB) = exp (gamma / RT)*XGM/(1-XGM)
where XGB0 is the saturation level of XGB, R is the gas constant, T is the temperature. This thermodynamic model has not been applied to grain boundaries in Earth materials; grain boundaries have simply been treated as possible sinks for impurities in rocks.
To test whether the thermodynamic model describes segregation to grain boundaries in mantle rocks, we investigated partitioning of Ca between the grain interiors and grain boundaries of olivine in natural and synthetic olivine-rich aggregates. We calculated partitioning curves for Ca (ri = 0.1 nm) using Eq. (1) for different temperatures; assuming that the absorption energy, gamma, is equal to misfit strain energy, U, and that the segregated Ca is confined to the M sites at the grain boundary plane (XGB0 = 1). Good overall agreement suggests that the observed Ca partitioning is consistent with the thermodynamic equilibrium model in Eq. (1), with the segregation confined to the grain boundary plane and the partitioning controlled by misfit strain energy. The consistency of the Ca partitioning in both natural and synthetic samples our model predictions suggests that partitioning according to this segregation model accurately describes the partitioning of Ca, and likely other incompatible divalent cations, in Earth's mantle.
