Abstract

The kinetics of Fe2+-Mg exchange between the M1 and M2 sites (FeM22 + MgM1 = FeM12+ + MgM2) were followed by ordering experiments in an augitic clinopyroxene (Ca0.841Na0.019Mg0.888Fe0.1512+Ti0.011Al0.028Cr0.003Fe0.0543+Mn0.006)(Si1.914Al0.086) from a dike on Alicudi (Aeolian Islands, Italy). The same single crystal was annealed in silica tubes in an iron-wüstite buffered Ar atmosphere, at 700, 750, 800, 850, and 950 °C, followed by quenching after each thermal treatment, and was used for the collection of X-ray diffraction data and for microprobe analysis.

Kinetic constants (K) for each isotherm were calculated from the distribution coefficient KD [=(Fe2+/Mg)M1/(Fe2+/Mg)M2] measured during a series of experiments carried out by increasing annealing times until exchange equilibrium was achieved. Calculations of K were performed using the Ginzburg-Landau equation, the kinetic model of Mueller, and the mathematical formulation proposed by Sha and Chappell (1996) to check the responses of various kinetic models for a Ca-rich pyroxene on the same data set. The activation energy of the Fe2+-Mg exchange spans a narrow range, yielding values of 200 (±28) kJ/mol, independent of the method used to calculate K.

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