Moreover, for specific rock compositions, many of these reactions are not 'seen' by a particular bulk composition. However, petrogenetic grids, especially complex ones with lots of reactions, can be difficult to interpret, and most mineral assemblages in real rocks are higher variance. Univariant reactions in a petrogenetic grid are extremely useful in providing bounding constraints on the stability of mineral assemblages. Such an optimal approach allows PT, their uncertainties, and a range of diagnostics for outlier identification, to be calculated in a computationally inexpensive way. These essential correlations should be included in any thermobarometry calculations. As a consequence, the equilibria are constrained to move in a more or less highly correlated way because the equilibria involve overlapping subsets of the end-members. These displacements are mainly made by varying the activities of the end-members of the minerals, in proportion to their uncertainties. In finding a PT of formation, there is an implied displacement of the equilibria to coincide with this PT. Rock-forming minerals, thermobarometry may involve combining many equilibria to find the PT of formation of a rock. With the existence of thermodynamic data for a wide range of end-members in This composition line goes through the composition used in Fig. T-x pseudosection in KFMASH (+mu+q+H2O), for a composition line along which FeO:MgO varies, with X = FeO/(FeO+MgO), and Al 2O 3 = 41.89, FeO + MgO = 45.48, and K 2O = 12.63 (in mol%). The main non-linear equations involved are the "equilibrium relationships": the relationships for balanced chemical reactions between the end-members of phases that are in equilibrium with each other:įig. In the application of equilibrium thermodynamics to the calculation of phase diagrams, there are two approaches that can be followed: one based on the minimization of Gibbs energy, the other being a derivative equivalent based on the solution of sets of non-linear equations. For the latter there exists a program - drawpd - that allows manually assembled THERMOCALC output to be drawn in postscript. The mineral equilibria problems that can be addressed with THERMOCALC include inverse modeling ones (geothermometry/barometry using average PT), and forward modeling ones (calculating phase diagrams for model systems). It has two main components: the application itself, and the internally-consistent thermodynamic dataset it uses. THERMOCALC is thermodynamic calculation software for tackling mineral equilibria problems. Julie Baldwin, University of Montana Dexter Perkins, University of North Dakota and Dave Mogk, Montana State University What is it? Advanced Modeling Programs: Introduction to the THERMOCALC Mineral Equilibria Modeling Software
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