Tourmaline is a borosilicate mineral generally known to be confined to superficial systems of Earth's crust as pegmatites, where the most massive specimens crystallize. Because of its extremely variable composition and the very low diffusion rates due to structural constraints, tourmaline is worldwide used to indirectly probe its formation environment and crystallization fluids. Tourmaline is a hydrous mineral, and when incorporated in subducting sediments and the breakdown conditions are reached, water is released from its structure together with B. Such a behavior leads to B-metasomatism of hosting and neighboring rocks, and plays a key role on the overall system rheology. Some of the released B can still be stored in deeply subducted sediments through mineral phases which are stable at higher pressure and temperature conditions, until further breakdown would lead again to water and B release. Mapping out every step of this breakdown sequence would help to trace water and B path, and for this purpose several experimental studies on tourmaline breakdown have been performed.
Maruyamaite, a K-dominant tourmaline, has been observed as stable phase in the ultra-high pressure (UHP) metamorphic complex of Kokchetav (Kazakistan), containing microdiamonds inclusions and thus probably crystallized near the peak pressure conditions of UHP metamorphism in the stability field of diamond. Given the uniqueness of maruyamaite occurrence and its promising insights on tourmaline stability in UHP environments that would lead to a complete revision of the current understandings on the extent of tourmaline stability field, more investigation is needed.
This project aims at constraining the breakdown temperature at room pressure, identifying the breakdown products and defining the breakdown mechanism with a special focus on dehydration process of maruyamaite from Kazakistan and Mn-bearing elbaite from Madagascar through in-situ high-temperature powder X-Rays Diffraction experiments.
This investigation on breakdown conditions, mechanism and the characterization of breakdown products of UHP tourmaline offer new and valuable informations to many Earth Sciences research branches: from the pure crystal-chemical understanding of UHP tourmaline to its structural behavior in temperature with a special focus on dehydration and oxidation processes, to petrological aspects related to B-rich metasomatic fluids and their implications in the geodynamics of subduction systems.