Abstract
Abstract We report the first calorimetric observation of the glass transition for a carbonate melt. A carbonate glass [55K2CO3–45MgCO3 (molar)] was quenched from 780 °C at 0.1 GPa. The activation energy of structural relaxation close to the glass transition was derived through a series of thermal treatments comprising excursions across the glass transition at different heating rates. Viscosities just above the glass transition temperature were obtained by applying a shift factor to the calorimetric results. These viscosity measurements (in the range of 109 Pa·s) at supercooled temperatures (ca. 230 °C) dramatically extend the temperature range of data for carbonates, which were previously restricted to super-liquidus viscosities well below 1 Pa·s. Combining our calorimetrically derived results with published alkaline-earth carbonate melt viscosities at high temperatures yields a highly non-Arrhenian viscosity-temperature relationship and confirms that carbonate liquids are “fragile.” Based on simulations, fragile behavior is also exhibited by Na2CO3 melt. In both cases, the fragility presumably relates to the formation of temperature-dependent low-dimensional structures and Vogel-Fulcher-Tammann (VFT) curves adequately describe the viscosity-temperature relationships of carbonate melts below 1000 °C.
Highlights
Melt viscosity has a fundamental control on many Earth processes, from how melts are transported through the deep mantle and crust, to its influence on effusive or explosive eruption styles at the Earth surface (McKenzie 1985; Dingwell 1996)
Physical properties are becoming well-established for multicomponent silicate systems of importance in earth sciences (Lange and Carmichael 1987; Knoche et al, 1995; Bagdassarov et al, 2000, Giordano et al 2008), carbonate melts are less well-studied in the geosciences, due to relatively rare volcanic expression at the Earth’s surface (Keller 1989; Dawson 1966)
The liquidus rises very steeply with composition. This may suggest a change in composition and the loss of CO2 at ambient pressure consistent with the observation of Eitel and Skaliks (1929) that high pressure is required to ensure melting of K2CO3 and MgCO3 constituents without decomposition but we note that any such effect is post-Tg determination
Summary
Melt viscosity has a fundamental control on many Earth processes, from how melts are transported through the deep mantle and crust, to its influence on effusive or explosive eruption styles at the Earth surface (McKenzie 1985; Dingwell 1996). In the subduction-related recycling of carbon back to Earths surface as part of the global carbon cycle (Dasgupta and Hirschmann 2010; Thomson et al 2016) Their ability to scavenge high concentrations of REE and rare metals make them increasingly important as multicommodity exploration targets for the green energy revolution (Simandl and Paradis 2018). The extreme physical and chemical properties (e.g. density, viscosity, conductivity, reactivity) which set carbonatites apart from most silicate melts, must have a strong influence on the interaction and migration dynamics of buoyant carbonatite melts throughout the lithosphere after separating from their source region (Hunter and McKenzie 1989; Minarik and Watson 1995; Brooker 1998; Hammouda and Laporte 2000; O’Leary et al 2015). The relationship between low viscosity and electrical properties of carbonate-rich melts has been linked to regions of high conductivity in the upper and lower mantle (Sifré et al 2014; Gaillard et al 2018)
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