[S-IT17] Property and role of liquids inside terrestrial planets

[SIT17-02] Atomic structure of CO₂-bearing melts along the carbonatite-basalt join at high pressure and temperature

*Veronica Stopponi¹, Annalisa D'Arco^2,3, Rostislav Hrubiak⁴, Yoshio Kono⁵, Stefano Lupi^2,6, Craig E Manning⁷, Manuela Nazzari⁸, Brent T Poe⁹, Claudia Romano¹⁰, Vincenzo Stagno^1,8 (1.Department of Earth Sciences, Sapienza University of Rome, Rome, Italy, 2.INFN National Institute of Nuclear Physics, Rome, Italy, 3.SBAI Department of Basic and Applied Sciences for Engineering, Physics, Sapienza University of Rome, Rome, Italy, 4.High Pressure Collaborative Access Team (HPCAT), X-ray Science Division, Argonne National Laboratory, Argonne, IL, USA, 5.Geodynamic Research Center, Ehime University, Matsuyama, Japan, 6.Department of Physics, Sapienza University of Rome, Rome, Italy, 7.Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA, 8.Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy, 9.Department of Engineering and Geology, University of Chieti-Pescara, Chieti Scalo, Italy, 10.Department of Sciences, University of Studies Roma Tre, Rome, Italy)

Keywords:High pressure, Melt structure, Carbonate-bearing melts, Paris-Edinburgh press, Vibrational spectroscopy, Mantle melting

Melting of carbonated peridotite rocks in the Earth’s interior produces a variety of magmas the composition of which is controlled by pressure, temperature and oxygen fugacity (Gudfinnsson and Presnall 2015; Stagno 2019). Within a depth interval of 25 and 250 km, the continuum from carbonatitic, kimberlitic, melilititic, picritic and basaltic melts is characterized by CO₂ content that decreases gradually with increasing temperature and, therefore, melt fraction. As a consequence, the rheology of these magmas is expected to change dramatically with implications for their migration rate and the geodynamic transport of carbon.
The aim of this study was to investigate the atomic structure and vibrational properties of a variety of synthetic glasses representative of CO₂-bearing melts (i.e. carbonatite-kimberlite-melilitite-nephelinite-picrite-basalt) as a function of the increasing SiO₂ content, pressure and temperature.
The atomic structure of molten glasses was investigated at pressures of about 1-7 GPa and temperatures varying from about 1150 to 2000°C using the Paris-Edinburgh press installed at beamline 16BM-B of the Advanced Photon Source (Argonne, IL, USA) combined with synchrotron radiation to perform in situ multi-angle energy dispersive X-ray diffraction measurements. The vibrational properties were studied on the recovered quenched glasses using both micro-Raman and micro-reflectance-FTIR.
Preliminary results shed light on different compressibility mechanisms upon increasing pressure both in the intermediate-range ordering and in the local structure of melts. The position of the first sharp diffraction peak in the structure factor S(q) shows that kimberlitic, melilititic and nephelinitic melts display a much less closely packed structure with respect to volatile-free depolymerized silicate melts. Bond lengths between tetrahedrally coordinated cations, such as Si⁴⁺, Al³⁺, and oxygen (T-O) increase for all the compositions investigated in the experimental pressure range, with the exception of T-O lengths of the basalt melt where a turnover at about 4.5 GPa is observed as in Sakamaki et al. (2013).
The vibrational spectroscopic investigation shows significant changes in the speciation of C-O molecular bonds that likely reflect a more complex C-dominated atomic environment than what is known so far.






Gudfinnsson and Presnall 2005, J Petrol 46, 8, 1645-1659.
Sakamaki et al. 2013, Nat Geosci 6, 1041-1044.
Stagno 2019, J Geol Soc London 176, 375-387.