Ordering kinetics of Mg-Fe2+exchange in a Wo43En46Fs11augite

Abstract

The kinetics of Fe 2+ -Mg exchange between the M1 and M2 sites (Fe M2 2+ + Mg M1 = Fe 2+ M1 + Mg M2 ) were followed by ordering experiments in an augitic clinopyroxene (Ca 0.841 Na 0.019 Mg 0.888 Fe 2+ 0.151 Ti 0.011 Al 0.028 Cr 0.003 Fe 3+ 0.054 Mn 0.006 )(Si 1.914 Al 0.086 ) 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 K D [= (Fe 2+ /Mg) M1 /(Fe 2+ /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 Fe 2+ -Mg exchange spans a narrow range, yielding values of 200 (±28) kJ/mol, independent of the method used to calculate K.