Compositional variations of tourmaline from low to ultrahigh pressure

电气石的成分已被广泛用作地层温度的岩石成因指示器，并被用于监测地球S地壳和俯冲带环境中的流体驱动过程。然而，需要根据流体成分更好地量化电气石的成分，也没有普遍的共识，电气石的成分可变性是否以及如何潜在地用作地压计。我们希望通过一系列的电气石-流体和电气石-(K，Na)云母交换实验，将现有的流体-电气石数据集从低压扩展到超高压，并通过一系列的电气石-流体和电气石-(K，Na)云母交换实验，验证电气石X位K掺入增强的假说，以及电气石-流体和电气石-(K，Na)云母交换实验增加四配位B进入电气石结构的假设。如果电气石含有大量的四面体B，这肯定会对流体和电气石之间的B同位素分馏产生重大影响，这将被校准。与此同时，我们将通过晶体化学和B同位素分析研究电气石从低压岩石到高压岩石的变化，以评估电气石作为地压计的可能性，并代表变质过程中流体的Na-Ca-K浓度。将这两部分结合起来，我们希望能更好地理解俯冲过程中的大范围、长期的B旋回。

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.