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Discontinuous precipitation of rutilated quartz: grain-boundary migration induced by changes to the equilibrium solubility of Ti in quartz

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A Correction to this article was published on 18 April 2020

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Abstract

We experimentally investigated changes to the titanium concentrations of quartz-rich rocks caused by grain-boundary migration. Synthesis experiments were performed to make high-Ti quartz starting materials for usage in recrystallization experiments. We isothermally recrystallized quartz starting material in separate experiments at 10–25 kbar to reduce the solubility of Ti in quartz. Microstructural and geochemical results indicate that the high-Ti quartz starting material recrystallized to form low-Ti quartz with acicular rutile inclusions. During recrystallization, high-Ti quartz was consumed to form low-Ti quartz and acicular rutile crystals precipitated behind moving grain boundaries via the discontinuous precipitation unmixing process. Experimentally imposed changes to the equilibrium solubility concentration of Ti in quartz by amounts as low as ~ 0.02 weight % (200 µg/g or ppm by weight) created a free energy driving force of 8.7 kJ/mol that was sufficient to drive large-scale recrystallization of the quartzites. Microstructures of relict grains of the high-Ti quartz starting materials are characterized by cuspate and irregular grain boundaries, whereas new quartz grains exhibit subhedral to euhedral grain morphologies and abundant three-grain junctions. Discontinuous precipitation can significantly change mineral compositions, and in the absence of a free fluid phase can operate much faster than other types of mass transfer processes commonly associated with solid-state reactions.

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  • 18 April 2020

    The original version of this article contained a mistake.

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Acknowledgements

This research was made possible through grants provided by the National Science Foundation to JBT. Most results were supported by EAR1551343; pilot experiments were performed with funds from EAR1543627. An MRI grant through the NSF Earth Sciences Division (EAR1625835) and support from Syracuse University provided electron microprobe facilities used for much of this research. The authors thank F. Spear for providing migmatite samples from New Hampshire. An anonymous reviewer greatly improved an earlier version of the manuscript.

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Correspondence to Jay B. Thomas.

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Communicated by Mark S Ghiorso.

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The original version of this article was revised: The reaction was incorrect and it has been corrected.

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Thomas, J.B., Nachlas, W.O. Discontinuous precipitation of rutilated quartz: grain-boundary migration induced by changes to the equilibrium solubility of Ti in quartz. Contrib Mineral Petrol 175, 38 (2020). https://doi.org/10.1007/s00410-020-01676-2

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  • DOI: https://doi.org/10.1007/s00410-020-01676-2

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