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Thermodynamic Models of Magmas

Thermodynamic Models of Magmas. Lecture 13 . 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.

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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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