Compositional variations of tourmaline from low to ultrahigh pressure

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The composition of tourmaline has widely been used as a petrogenetic indicator for temperature of formation and for monitoring fluid-driven processes in the Earth s crust and subduction zone environments. There is, however, a need for a better quantification of tourmaline composition in dependence of fluid composition, and also no general agreement if and how the compositional variability of tourmaline could be potentially used as a geobarometer. We want to extend the available data set for fluid-tourmaline from low to very high pressure and will test the hypothesis of enhanced K incorporation on the X-site of tourmaline as well as increased incorporation of tetrahedrally coordinated B into the tourmaline structure by a series of tourmaline-fluid and tourmaline-(K,Na)mica exchange experiments up to ultrahigh pressure. If tourmaline has a significant amount of tetrahedral B this must have a significant effect on the B isotope fractionation between fluid and tourmaline, which will be calibrated. Parallel to this, we will investigate by crystal-chemical and B isotopic analyses of tourmaline from low to high pressure rocks from areas with well-known, previously determined pressure-temperature evolution in order to assess that tourmaline may serve as a geobarometer and moreover represents a monitor of Na-Ca-K concentrations of fluids during metamorphism. Combining the two parts we expect a better understanding of the large-scale, long-term B cycle during subduction processes.