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Thermodynamic Models of Magmas

This lecture delves into the fundamental structural unit of silicates, the silica tetrahedron, and how various ions influence the properties of silicate magmas. It examines the differences between solid and liquid structures, emphasizing the role of bridging oxygens in polymerization. The MELTS model is introduced, highlighting its application in modeling complex silicate liquids, crystallization processes, and the determination of equilibrium assemblages. Key interactions among network-forming and network-modifying ions are discussed, providing insights into the behavior of magmatic systems.

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Thermodynamic Models of Magmas

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  1. Thermodynamic Models of Magmas Lecture 13

  2. Silicate Magmas Basic structural unit of silicates (solid & liquid) is the silica tetrahedron These are variously joined by shared, or bridging, oxygens, to form various structures in solids and liquids. Basic difference between solids and liquids is lack of long-range structure in the latter. Liquids structure can be studied by quenching them to glass.

  3. Liquid Structures • Bridging oxygens and joining tetrahedra results in polymerization of the melt, changing its properties. • Al3+, Ti, and Fe3+ can promote polymerization and, along with Si, are called network-forming ions. • Other ions, Ca2+, Mg2+, Fe2+, Na+, K+, and H+ tend to break up this structure and are called network modifiers.

  4. Modeling Silicate Liquids • Silicate liquids are complex solutions of many components. • Solids crystallizing from them are generally solutions themselves. • Generally these solutions cannot be treated as ideal. • Crystallization (or melting) occurs over a wide range of T (400-500˚C). • Problems are: • Decide on the components • Determine the nature of the model • Ghiorso et al. adopt a regular solution model for their MELTS model. • Determine the interaction parameters from experimental data. • The resulting program then iteratively computes free energy of the liquid plus free energy of all possible precipitating solids and calculates the equilibrium assemblage based on the principles that • the stable assemblage is the one with the lowest free energy. • The chemical potentials of components in coexisting phases are equal.

  5. MELTS Model • For network modifiers, Ghiroso chose silicate components such as CaSiO3, Mg2SiO4, Na2SiO3, KAlSiO4, etc. because mole fractions of individual oxides tend to be small numbers, reducing influence of interaction parameters. • Network formers generally just the oxides (e.g., Al2O3). • Eleven components, plus water treated separately. • Free energy of the liquid solution is: • Activity coefficients calculated as:

  6. pMELTS Predicted and actual pyroxene compositions in lavas. Predicted and actual SiO2 concentrations in experimental melts of peridotite as a function of melt percent.

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