Empirical Solution Model for Alkalic to Tholeiitic Basic Magmas |
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Authors: | CAMUR M ZEKI; KILINC ATTILA I |
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Institution: | 1GENERAL DIRECTORATE OF MINERAL RESEARCH AND EXPLORATION (MTA) 06520 ANKARA, TURKEY
2DEPARTMENT OF GEOLOGY, UNIVERSITY OF CINCINNATI CINCINNATI, OH 45221, USA |
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Abstract: | Currently available models to simulate naturally occurring mineralmeltequilibria use mineral components limited to tholeiitic basaltcompositions and thus they cannot be used for alkali-rick basaltsand basanites. To expand mineralmelt equilibria calculationsto alkali-rich composition space at low pressures, we have derivedequations that describe chemical equilibria between olivinemelt,pyroxenemelt, plagioclasemelt, nephelinemeltand leucitemelt components. Excess free energies of reactionsbetween the end-member mineral and melt components at equilibriumhave been expressed as a function of melt composition, temperatureand fo2. The database used to calculate the mineralmeltexpressions consists of a total of >350 anhydrous experimentsconducted under controlled oxygen fugacity defined by the quartzfayalitemagnetite(QFM) oxygen buffer. Rocks used in these experiments range frombasanites, nephelinites and alkali olivine basalts, to tholeiiticbasalts and basaltic andesites. Using bulk compositions of startingmaterials both in this experimental database and in others thatwere not incorporated into the regression of modeled parameters,modeled equations successfully predict, at a given temperatureand fo2, compositions of multiply saturated melts as well asthe compositions of coexisting minerals. Standard deviationsof the calculated mole fractions of mineral components () areas follows: anorthite 002; forsterite 002; clinoenstatite002; enstatite 0003; nepheline 002; and leucite 001. Standarddeviations () of the calculated melt compositions in terms ofweight percent of oxides are: SiO2 096; Al2O3 132; Fe2O3023; FeO 121; MgO 084; CaO 079; Na2O 058; and K2O 069.All calculations were carried out using a non-linear NewtonRaphsonnumerical procedure. KEY WORDS: mineralmelt equilibria; alkalictholeiitic basalts; equilibrium thermodynamics
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