Structural and dynamical properties of hydrous magmatic liquids at high pressure: insights from Raman spectroscopy observations during diamond anvil cell experiments

Lauréna Mourand, Clément Ferraina, Salomé Pannefieu, Cyril Aubaud, Charles Le Losq 

Silicate liquids play a central role in the differentiation and internal dynamics of terrestrial planets by controlling the transport of heat and matter. One of their key properties is viscosity, which can be strongly influenced by pressure. Currently, there is not enough data to develop general models of viscosity at high pressure. Besides, data on the short and medium range order structure of high-pressure silicate melts are insufficient to draw general structural laws and use them to express the effect of pressure on melt properties. Here we present a new method involving an internally-heated membrane diamond anvil cell coupled with a Raman spectrometer. This setup allowed us to acquire in situ Raman spectra of water-bearing sodium aluminosilicate glasses and melts with controlled composition, and to observe how the glass transition temperature, a parameter directly related to melt viscosity, changes with pressure. The results reveal a non-linear behavior of the glass transition temperature with pressure, with a minimum around 6 GPa and a strong increase at higher pressures. The interpretation of undercooled melt Raman spectra shows that this behavior results from a competition between structural compaction, melt polymerization and changes in the coordination of Si, Al network former cations. At pressures above 6-7 GPa, once coordination changes dominate, melts with very different chemical compositions exhibit remarkably similar positive Tg(P) slopes.