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Quantifying the effects of non-hydrostatic stress on multi-component minerals
  • Benjamin L Hess,
  • Jay J. Ague,
  • Peter Voorhees
Benjamin L Hess
Yale University

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Jay J. Ague
Yale University
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Peter Voorhees
Northwestern University
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Mineral compositions are used to infer pressures, temperatures, and timescales of geological processes. The thermodynamic techniques underlying these inferences assume a uniform, constant pressure. Nonetheless, convergent margins generate significant non-hydrostatic (unequal) stresses, violating the uniform pressure assumption and creating uncertainty. Materials scientists F. Larché and J. Cahn derived an equation suitable for non-hydrostatically stressed geologic environments that links stress and equilibrium composition in elastic, multi-component crystals. However, previous works have shown that for binary solid solutions with ideal mixing behavior, hundreds of MPa to GPa-level stresses are required to shift mineral compositions by a few hundredths of a mole fraction, limiting the equation’s applicability. Here, we apply Larché and Cahn’s equation to garnet, clinopyroxene, and plagioclase solid solutions, incorporating for the first time non-ideal mixing behavior and more than two endmembers. We show that non-ideal mixing increases predicted stress-induced composition changes by up to an order of magnitude. Further, incorporating additional solid solution endmembers changes the predicted stress-induced composition shifts of the other endmembers being considered. Finally, we demonstrate that Larché and Cahn’s approach yields positive entropy production, a requirement for any real process to occur. Our findings reveal that stresses between tens and a few hundred MPa can shift mineral compositions by several hundredths of a mole fraction. Consequently, mineral compositions could plausibly be used to infer stress states. We suggest that stress-composition effects could develop via intracrystalline diffusion in any high-grade metamorphic setting, but are most likely in hot, dry, and strong rocks such as lower crustal granulites.