Optical probing of glass transition at high pressure allows whole-mantle silicate melt viscosity models

Upon heating, the glass transition is a temperature interval over which an unrelaxed glass transforms to a supercooled liquid relaxed on the experimental time scale. The term standard glass transition temperature (Tg) refers to the temperature at which the viscosity is 1012 Pa s [1]. One of the many reasons why Tg is important in geosciences is that it successfully parametrizes models of silicate melt viscosity over the temperature range of magmatic systems [2]. The pressure-dependence of Tg of a given silicate glass/melt is thus a crucial parameter that can provide insights into the viscosity of the corresponding natural silicate liquid in the Earth’s mantle. Although the detection of the glass transition at 1 atm is relatively straightforward, high-pressure measurements present challenges mainly because of the miniscule sample volume available for probing. As a consequence, previous high-pressure measurements of Tg in silicate glasses/melts were carried out in large-volume devices at P < 2-3 GPa [3,4], drastically losing the sensitivity to detect glass transition at higher pressures. The emerging picture is that in silicates systems the pressure-dependence of Tg is governed by the glass/melt structure, specifically the number of non-bridging oxygens (NBO) per tetrahedron (TET). Apparently, the Tg of polymerized glasses (NBO/TET < 1) decreases with pressure, while the Tg of depolymerized glasses (NBO/TET > 2) increases with pressure [3,4]. One unanswered question is how these trends are affected by pressure-induced structural changes in the glass, such as the rearrangement of the TET network or the increase in Si and Al coordination by O (i.e., progressive decrease of TET associated with the increase of higher coordination polyhedra). Extending the pressure range of Tg measurements to P > 2-3 GPa is needed to address this question. Yet, to the best of our knowledge, no successful attempt has been made to constrain glass transition temperatures in silicate systems in a diamond anvil cell device. At EGU 2024, we will report on ongoing time-controlled optical experiments in laser-heated diamond anvil cells which reveal spectroscopic discontinuities in Fe-bearing basaltic glasses over the glass transition. We will attempt to link these discontinuities to Tg in order to model the viscosity of basaltic melts at mantle P-T. References [1]        B. Mysen and P. Richet, Silicate Glasses and Melts (Elsevier, 2019). [2]        M. Cassetta, D. Di Genova, M. Zanatta, T. B. Ballaran, A. Kurnosov, M. Giarola, and G. Mariotto, Sci. Rep. 11, 13072 (2021). [3]        M. Rosenhauer, C. M. Scarfe, and M. Virgo, Carnegie Inst. Wash. Yearb. 78, 556 (1979). [4]        N. S. Bagdassarov, J. Maumus, B. Poe, A. B. Slutskiy, and V. K. Bulatov, Phys. Chem. Glasses 45, 197 (2004).