Experimental and Observational Constraints on Halogen Behavior at Depth

International audienceHalogens are volatile elements present in trace amounts in the Earth's crust, mantle, and core. Except for fluorine, they show volatile and incompatible behavior, which makes them key tracers of fluidmediated and/or melt-mediated chemical transport processes. Even small quantities of halogens can profoundly affect many physicochemical processes such as melt viscosity, the temperature stability of mineral phases, the behavior of trace elements in aqueous fluids, or the composition of the atmosphere through magma degassing. Experiments allow us to simulate deep-Earth conditions. A comparison of experimental results with natural rocks helps us to unravel the role and behavior of halogens in the Earth's interior

Vibrational entropy of disordering in omphacite.

Acknowledgements: This work was supported by grants from the Austrian Science Fund (FWF), project number P33904, which is gratefully acknowledged. We thank G. Tippelt for performing the X-ray experiments and E. Forsthofer for maintaining the Materials Studio software at the Salzburg University. We also thank two anonymous reviewers for their detailed and constructive comments.Funder: Paris Lodron University of SalzburgUNLABELLED: The cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00269-023-01260-7

Vibrational entropy of disordering in omphacite

Acknowledgements: This work was supported by grants from the Austrian Science Fund (FWF), project number P33904, which is gratefully acknowledged. We thank G. Tippelt for performing the X-ray experiments and E. Forsthofer for maintaining the Materials Studio software at the Salzburg University. We also thank two anonymous reviewers for their detailed and constructive comments.Funder: Paris Lodron University of SalzburgThe cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order

The speciation of Fluorine in silicate melts

Understanding the behaviour of the halogens (F, Cl, Br, I) is required to explain the dynamic interplay between the silicate interior and volatile-rich hydrospheres and atmospheres of planets, moons, and exoworlds (Clay et al., 2017; 2019). We identify fluorine as a starting point because it is the most abundant and compatible of the halogens in the Earth system, and can directly impact the nature of Earth’s interior. For example, fluorine affects the thermal stability of mineral phases and the viscosity of silicate melts (Webster et al., 2018). Fluorine is a highly electronegative, monoisotopic, negatively charged atom with a low boiling point and ionic radius similar to both oxygen and hydroxyl ions (OH-) (Crepisson et al., 2014; Dalou et al., 2012). Therefore, understanding the pathways followed by fluorine might illuminate the pathways followed by water within silicate systems (McCubbin et al., 2015). Thus, the mechanics of fluorine dissolution in silicate melts (speciation) require attention.We have conducted experiments at high temperatures (1250°C) across a range of pressures (0-3 GPa) in a simple Ca-Mg-Al-Si-O system (CMAS) which reproduces the viscosities of intermediate to mafic silicate melts. The speciation of fluorine is studied using solid-state nuclear magnetic resonance (NMR) spectroscopy. Our results show most of the fluorine is bound with aluminium in the melt structure, but minor abundances of Mg-F and Ca-F are also observed in the 19F NMR spectrum. There is no observable effect of pressure and/or temperature. These observations challenge the assumption that fluorine and hydroxyl ions are kin, because OH- form stable compounds with Mg2+ (i.e. as Mg(OH)2, Mookherjee et al. 2008). These data predict decoupling of F- and OH- irrespective of their similar ionic radius and charge. Therefore, assuming the relative behaviours of F- and OH- follow an equilibrium exchange relationship might be (counterintuitively) flawed