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Grain boundaries as reservoirs of incompatible elements in the Earth's mantle

Abstract

The concentrations and locations of elements that strongly partition into the fluid phase in rocks provide essential constraints on geochemical and geodynamical processes in Earth's interior. A fundamental question remains, however, as to where these incompatible elements reside before formation of the fluid phase. Here we show that partitioning of calcium between the grain interiors and grain boundaries of olivine in natural and synthetic olivine-rich aggregates follows a thermodynamic model for equilibrium grain-boundary segregation. The model predicts that grain boundaries can be the primary storage sites for elements with large ionic radius—that is, incompatible elements in the Earth's mantle. This observation provides a mechanism for the selective extraction of these elements and gives a framework for interpreting geochemical signatures in mantle rocks.

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Acknowledgements

We thank M. Hirschmann and T. Morishita for discussions and M. Zimmerman for experimental assistance. T.H. acknowledges receipt of a JSPS research fellowship. This collaboration was performed under the ORNL SHaRE User Program and with support from the NSF.

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Correspondence to Takehiko Hiraga.

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Further reading

Figure 1: Chemistry and structure of olivine-olivine grain boundaries in a sample of olivine + anorthite annealed at 1,473 K.
Figure 2: Calcium concentrations in olivine grain boundaries versus concentration in olivine grain matrices.
Figure 3: Ratio of solute concentration in a system composed of grain matrices + grain boundaries, CGM+GB, to that of a system composed of grain matrices alone, CGM, versus ionic radius at a temperature of 1,473 K.

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