Schorl breakdown at upper mantle conditions. Insights from an experimental study at 3.5 GPa

Hydrogen and B input throughout the Earth’s mantle is continuously fed through a sequence of dehydration and breakdown reactions of hydrous and B-bearing mineral phases stable at different conditions along the subducting slabs. Therefore, the stability of minerals hosting these elements plays a fundamental role. Tourmaline hosts very large amounts of B (up to 14 wt% of B2O3) along with hydroxyl groups (up to 4 wt% of H2O), thus representing a crucial mineral to investigate the fate of B and H in diverse geological settings. The recent finding of tourmaline minerals in ultra-high pressure metamorphic rocks has raised important questions about the actual tourmaline stability field, paying special attention to the high pressure and temperature stability limits of the various tourmaline species. A single-phase system made of natural schorl with the highest Fe2+ concentration known so far (about 18 wt% of FeO) was studied at a fixed pressure (3.5 GPa) and several temperatures (500, 700, 750, 800, 850 and 950 ◦C) to preliminarily constrain its stability conditions, breakdown mechanisms and breakdown products. Experiments at high pressure-high temperature conditions were performed using a multi anvil apparatus under buffered oxygen fugacity through a Re/ReO2 solid mixture. The experimental products were characterized through a multi-analytical approach consisting in Scanning Electron Microscopy imaging and Energy Dispersive System spectra acquisition, Electron MicroProbe analysis, powder X-Ray Diffraction, 57Fe M ̈ossbauer spectroscopy and reflectance Fourier Transform infrared spectroscopy. At 3.5 GPa and T ranging from 500 up to 700 ◦C, the schorl experienced a partial Fe oxidation coupled with dehydrogenation: Fe2+ + (OH) →Fe3+ + O2 + 0.5H2 (g) The observed Fe oxidation was limited to 30% (significantly lower than the full oxidation observed in ex- periments performed in air at room pressure), suggesting that oxidation-dehydrogenation is indeed a thermally activated process, but both environmental pressure and oxygen fugacity are important governing factors. In the pure schorl system at 3.5 GPa, the structural breakdown started at T = 700 ◦C and ended at 850 ◦C, resulting in the formation of almandine garnet as the first breakdown product together with topaz and a B-rich liquid phase: Na Fe2+ 2 Al) Al5 Fe2+)(Si6O18)(BO3 )3(OH)3 (OH, F) schorl → →Fe2+ 3 Al2 (SiO4 )3 almandine + Fe3+, Al)2SiO4(OH, F)2 topaz + 2SiO2 + Al2O3 + 0.5Na2 O + 1.5B2O3 + H2 O melt At 3.5 GPa and T ≥ 850 ◦ C, tourmaline, garnet and topaz were not observed anymore and kyanite, prismatine- and boromullite-like phases and corundum became stable. Both prismatine-like and boromullite-like phases identified by stoichiometry can incorporate B from the B-rich hydrous melt formed after schorl breakdown and may carry it to lower depths. From our work it follows that the schorl-bearing granitoid rocks (or sediments) have the potential to form hydrous B-bearing metasomatic melts at 3.5 GPa and T ≥ 700 ◦C. In cold subduction environments, between the 700–800 ◦C isotherms, the schorl is expected to be stable up to ~100 km depth along the subducting slab, although an excess SiO2 might be responsible for a reduction in tourmaline stability. The role of tourmaline companion minerals on its breakdown conditions and products is left as future issue when a multi-phase system will be considered.