Ordering kinetics of Mg-Fe2+ exchange in an En46Fs11Wo43 augite

The kinetics of Fe2+-Mg exchange between the M1 and M2 sites (Fe-M2(2+) + Mg-M1 = Fe-M1(2+) + Mg-M2) 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-wustite buffered Ar atmosphere, at 700, 750, 800, 850, and 950 degreesC, 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 [= (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.