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

Abstract

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 (aSiO2) in the system. The partitioning of Mg between plagioclase and clinopyroxene was determined over a temperature range of 1100–1200°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 (DMgPl) of Mg in plagioclase under these conditions to be determined. Both KMgPl/Cpx (defined as KMgPl/Cpx=CMgPl/CMgCpx) and DMgPl decrease with temperature and increase with aSiO2. Isothermal data for different XAn in plagioclase show a linear increase of lnKMgPl/Cpx with increasing XAn, but DMgPl appears to be insensitive to XAn. 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+2034XAn)/(lnKMgPl/Cpx-1.6-lnaSiO2).Application of this geothermometer to experimental data from this study reproduces the experimental temperatures within ±20°C. Diffusion of Mg in plagioclase is described byDMgPl[m2s-1]=1.25×10-4[m2s-1]·exp(-320,924[J mol-1]/(RT))·(aSiO2)2.6.