HIGH-PRESSURE AND ULTRA-HIGH-PRESSURE FEATURES OF BOROSILICATES

The most common boron silicates in the earth’s crust, have been identified in a range of high- and ultra-high pressure rocks with clear evidence for their growth and stability at peak metamorphic conditions. This study identifies and quantifies such pressure-temperature indicators, which improve the reliability of such borosilicates, like members of the tourmaline and dumortierite supergroup, as petrogenetic indicators with potential applications in geochemistry and petrology. Although it is known that boron can occur at the silicon position in the tourmaline supergroup, it could also occur at this position in other borosilicates, especially at lower temperatures and higher pressures. Furthermore, it is of particular interest how the occupants of the different crystallographic sites are related to each other and how the occupation of these sites is related to the pressure and temperature conditions. This research project provides an important knowledge and database for future studies of applied geosciences.

A significant MgO content (~1 wt%) in minerals of the dumortierite supergroup is an indication for high-pressure origin, while higher contents (>5 wt% MgO) were only found in ultra-high-pressure (UHP) massifs. Contrary, dumortierites from amphibolite facies rocks only have up to 0.5 wt% MgO, while samples of magmatic to medium-pressure origin (including granites and pegmatites) only exhibit very low MgO contents of ≤0.1 wt% MgO.

Interestingly, it was observed that the distribution of Mg between the two different 6-coordinated crystallographic sites in tourmaline seems to be related to the metamorphic grade. This relation might be used in future for developing a geothermometer. There is an excellent positive correlation between the overall Al content (of the 6-coordinated sites) of the investigated UHP tourmaline samples and the temperature of tourmaline formation. While magmatic tourmalines and samples from low to medium pressure metamorphic units can contain amounts up to 1 apfu (atoms per formula unit) Al at the Si position, especially at higher temperatures, in tourmalines from UHP massifs Al contents >0.1 apfu could not be verified at the Si position, even at temperatures up to almost 900 °C.

This work was funded by the Austrian Science Fund (FWF) project no. P 26903-N19.