Mg in plagioclase: Experimental calibration of a new geothermometer and diffusion coefficients

The temperature-sensitive exchange of Mg between plagioclase (Pl) and clinopyroxene (Cpx) has been studied experimentally, accounting for different anorthite-contents in plagioclase (XAn) and various silica activities (aSiO(2)) in the system. The partitioning of Mg between plagioclase and clinopyroxene was determined over a temperature range of 1100-1200 degrees C, using plagioclase single crystals of different compositions (XAn = 0.5-0.8), surrounded by different clinopyroxene-bearing matrix powders to account for different silica activities from 0.55 to 1.0. The experimental design also allows the diffusivity (D-Mg(Pl)) of Mg in plagioclase under these conditions to be determined. Both K-Mg(Pl/Cpx) (defined as K-Mg(Pl/Cpx) = C-Mg(Pl/ Cpx) = C-Mg(Cpx)) and D-Mg(Pl) decrease with temperature and increase with aSiO(2). Isothermal data for different X-An in plagioclase show a linear increase of ln K-Mg(Pl/Cpx) with increasing X-An, but D-Mg(Pl) appears to be insensitive to X-An. The partitioning data allow a new geothermometer to be calibrated, which may be widely applicable to terrestrial and extraterrestrial rocks where plagioclase and clinopyroxene coexist: T [K] = (9219 + 2034X(An))= (ln K-Mg(Pl/Cpx) - 1.6 - ln a(SiO2)). Application of this geothermometer to experimental data from this study reproduces the experimental temperatures within +/-20 degrees C. Diffusion of Mg in plagioclase is described by D-Mg(Pl) [m(2) s(-1)] = 1.25 x 10(-4) [m(2)s(-1)].exp(-320,924[J mol(-1)]/(RT)) . (a(SiO2))2.6. (C) 2013 Elsevier Ltd. All rights reserved.