Ordering kinetics of Mg-Fe2+ exchange in a Wo(43)En(46)Fs(11) augite

The kinetics of Fe2+-Mg exchange between the M1 and M2 sites (Fe-M2(2+) + M g-M1 = Fe-M1(2+) + Mg-M2) were followed by ordering experiments in an augit ic clinopyroxene (Ca0.841Na0.019Mg0.888Fe0.1512+ Ti0.011Al0.028Cr0.003Fe0.0 543+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 b y quenching after each thermal treatment, and was used for the collection o f X-ray diffraction data and for microprobe analysis. Kinetic constants (K) for each isotherm were calculated from the distributi on coefficient K-D [= (Fe2+/Mg)(M1)/(Fe2+/Mg)(M2)] measured during a series of experiments carried out by increasing annealing times until exchange eq uilibrium was achieved. Calculations of K were performed using the Ginzburg -Landau equation, the kinetic model of Mueller, and the mathematical formul ation 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 20 0 (+/-28) kJ/mol, independent of the method used to calculate K.