Congratulations to Sean Hurt who defended his dissertation on April 10, 2018
Advisor: Rebecca Lange
One of the most important liquid components of mantle-derived carbonate melts, MgCO3, is also one of the most difficult to study experimentally. This dissertation offers constraints on the standard state thermodynamic properties of MgCO3 liquid (and FeCO3 liquid by proxy) through analysis of systematic property variations in alkali/alkaline carbonate liquids in combination with atomistic molecular dynamic (MD) simulations.
Chapter 2 presents new 1 bar density measurements on liquids in the Li2CO3-Na2CO3-K2CO3-Rb2CO3-Cs2CO3-CaCO3-SrCO3-BaCO3 system. The partial molar volumes of all eight carbonate components increase linearly along two different trends, one for the alkali carbonates and another for the alkaline earth carbonates as a function of cation volume. The two trends yield two separate estimates for the partial molar volume of CO32- in the melt, corresponding to coordination of the metal cation with the carbonate ion. The results permit the partial molar volume of MgCO3 and FeCO3 in multicomponent carbonate liquids to be calculated, if the oxygen and carbonate ion coordination with Mg2+ and Fe2+ are known. For example, in the case where Mg2+ and Fe2+ are in 6-fold coordination with both oxygen and carbonate, the estimated partial molar volumes at 1100 K are 34.4(1), and 35.1(1) cm3/mol, respectively, with a thermal expansion coefficient of 16.4(29) 10-5 K-1. If Mg2+ and Fe2+ are in 4-fold coordination, their partial molar volumes are estimated at 40.0(6) and 40.4(6) cm3/mol respectively with a thermal expansion of 22.1(17) ∙ 10-5 K-1.
Chapter 3 presents 1 bar sound speed measurements on liquids in the Li2CO3-Na2CO3-K2CO3-Rb2CO3-Cs2CO3-SrCO3-BaCO3 system using the ultrasonic frequency-sweep interferometer. The results were combined with those on Li2CO3-Na2CO3-K2CO3-CaCO3 quaternary liquids to estimate the partial molar compressibility and its temperature-dependence (i.e βT & δβT/δT ) of each of the eight components. Liquid compressibility is positively correlated with ΔVfusion (R2 = 0.99). The trend among the alkaline earth carbonate liquids suggests that of the 1-bar compressibility for the MgCO3 and FeCO3 liquids at 1500 K are much higher (18 (±6) 10-2 GPa-1) than those obtained for the SrCO3, BaCO3 or CaCO3 liquid components at 1500 K (5.4-8.8 10-2 GPa-1).
Chapter 4 introduces an empirically-derived potential set model for classical mechanical molecular dynamic simulations of MgCO3-CaCO3-SrCO3-BaCO3 liquids. The potential set model is applied to MD simulations of CaCO3-SrCO3-BaCO3 liquids; simulated volumes are modeled with a temperature-dependent 3rd-order Birch-Murnaghan equation of state. The fitted thermodynamic properties of CaCO3-SrCO3-BaCO3 melts show broad agreement with those determined experimentally in chapters 2 and 3. CaCO3, SrCO3, BaCO3 liquids display a striking uniformity in their liquid structures with Ca2+, Sr2+ and Ba2+ all in 6-fold coordination with carbonate groups.
In chapter 5, the potential set model is applied to molecular dynamic simulations of MgCO3 melts. At 1 bar, MgCO3 liquid assumes a novel open-network topology, distinct from the other alkaline earths, and is characterized by a 4-fold coordination of Mg2+ with the carbonate molecule and oxygen ion. This novel topology results in a melt that is significantly more buoyant and compressible than previous estimates have suggested. The voluminous structure of the MgCO3liquid component (and FeCO3 liquid by proxy) may well drive up carbonate melt ascent rates and prevent iron-rich ferrocarbonatite melts from stagnating or sinking in the mantle.