What happens to borosilicates during subduction and in different geotectonic settings?

The project, planned for three years duration, aims at the structural characterisation and classification of borosilicates of the tourmaline and dumortierite supergroup, as well as minerals from the muscovite-boromuscovite solid solution and dumortierite-related minerals (kornerupine, prismatine). These borosilicates belong to the most common borosilicates of the Earths crust. All terrestrial solar and extrasolar planets (like Venus, Earth, and Mars) have a metallic core, with a surrounding silicate mantle. To get more information about structural features of members of these silicate groups, which crystallised up to ultra-high pressure conditions during subduction into the Earths mantle, it is planned to synthesise such phases at similar conditions. Additionally, similar natural borosilicates will be investigated, which come from different geotectonic settings and also from ultra-high pressure rocks. If necessary, the pressure and temperature conditions of the mineral paragenesis will be estimated using geothermobarometry. Tourmaline and dumortierite minerals 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, but the investigation of their chemical and crystallographic characteristics has so far only been preliminary. The identification and quantification of such pressure-temperature indicators would improve the reliability or tourmaline-supergroup minerals as petrogenetic indicators, provenance tools and geochemical archives, with potential applications in geochemistry, petrology and paleo-tectonic studies. A systematic study of these borosilicates from different geologic environments, which are petrologically well-characterised, can lead to information on important correlations, especially about the ordering of cations between the different atomic positions. Such structural details will be obtained by single-crystal X-ray diffraction. Especially borosilicates from ultra-high pressure massifs could be measured at Earths mantle conditions by using single-crystal X-ray studies in a diamond anvil cell. These borosilicates will also be checked for phase transitions at increasing pressures. Although it is known that boron can occur at the silicon position in tourmalines, it is not clear if it could also occur at the silicon position in dumortierite. 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. Such correlations might help us to learn more about the temperatures during tourmaline crystallisation, including lithium-rich tourmaline, which has not been possible to synthesize to date. This strategic research will provide an important knowledge and data base for future studies of applied petrology and mineralogy, and will be helpful for synthesising relatively large tourmaline crystals for technical applications. In this study, I hypothesize that borosilicates in (ultra)-high-pressure metamorphic rocks that are witnesses of deep subduction of boron carry characteristic crystal-chemical features that can be positively identified and employed to quantify the pressure-temperature history of their formation.

The project's scientific concept was to investigate borosilicates, mainly members of the tourmaline supergroup, as well as associated mica. The focus was on investigations of boron in subducted rocks, but also of borosilicates in different geotectonic settings of the Earth's crust. Hence, borosilicates were characterized and correlations between crystal chemistry, structural features and pressure-temperature conditions were developed, but also petrological and geological investigations were included. Analyses show that plots of fluorine content against the X-site charge will be a useful tool for the future characterization of tourmalines found in various lithologies and in gemstone pegmatites. Another investigation showed for the first time that significant amounts of strontium can also occupy the tourmaline X site. A high-pressure tourmaline, which crystallized under oxidizing conditions in an eclogite, contains the highest amount of Sr found in tourmaline to date. It can be concluded that tourmaline-bearing pegmatites, which seem to be the boron sources in the investigated eclogite, were influenced by a Sr-enriched metacarbonate, because such marbles contain relatively high Sr contents. A graphite deposit of the Moldanubian zone underwent a Variscan medium pressure/high temperature event. Although the graphite schists are assumed to be of Proterozoic origin, the quartz-feldspar dikes, which contain green mica and vanadium-bearing dravite, could be related to decompression-induced anatexis during a younger metamorphic event. The crystal structure of this muscovite, which contains one of the highest amounts of V previously reported in muscovite, has been determined. Extensive studies of pink tourmaline from Lower Austria led to interesting results that received broad attention from the Austrian public (newspapers, TV and radio stations). This tourmaline was named alumino-oxy-rossmanite and represents the most Al-rich end-member composition of all known tourmalines. It is a late-magmatic oxy-tourmaline from Moldanubian pegmatitic bodies embedded in amphibolite and paragneiss. Because of the low mode of associated mica, it was assumed that the silica melt, which formed this pegmatite, crystallized under relatively dry conditions, in agreement with the observation that this tourmaline contains a relatively low amount of OH. The significant content of tetrahedrally-coordinated Al reflects the relatively high-temperature conditions (~700 C) inferred for crystallization of the pegmatite. Investigations on O-H stretching vibration bands of fluor-elbaite indicate that Raman spectroscopy can be applied for the evaluation of the Li and OH content of Al-rich and Li-bearing tourmalines. Based on new findings, it is proposed how Li-rich tourmaline can be successfully synthesized. Because fluorine is positively correlated to the average X-site charge, a smaller amount of F produces higher X-site vacancies in tourmalines that can only incorporate smaller amounts of Li. Hence, the best way to produce Li-rich tourmaline is to synthesize fluor-elbaite at temperatures >550 C. Relatively high amounts of Li and F are required in the starting material.