Argon diffusion in plagioclase and implications for thermochronology

Despite being the most abundant mineral in the Earth's crust and one commonly used for 40Ar/39Ar geochronology, plagioclase has been little studied with respect to Ar diffusion kinetics and remains unexploited in most thermochronologic studies. We present results of 39Ar and 37Ar diffusion experiments and 40Ar/39Ar geochronology on both plutonic and volcanic plagioclase encompassing a variety of compositions and microstructural states. Single 300-1200 μm neutron- irradiated plagioclase crystals were wrapped in platinum or niobium packets and incrementally heated (in most cases cyclically) with a diode laser between 400 and 1600 °C. Temperature was controlled and monitored with an optical pyrometer, calibrated to ± 10 °C (1σ). We calculated diffusion coefficients (D/a2) from fractional release data assuming a spherical geometry. From least-squares regression of the low temperature values (<1000 °C; the first ~10-30% of the total 39Ar and 37Ar released), we quantified the diffusion parameters Ea and D0/a2. Like K- feldspar, microstructure, grain size, and compositional zoning heavily influence argon diffusion in plagioclase, highlighting the importance of single-crystal analyses. Over 15 experiments with different heating schedules yield Ea of 30-70 kcal/mol, positively correlated with D0/a2, which varies between -1.0 and 20.0. Arrhenius plots reveal a range of behaviors interpretable in terms of either single diffusion domains, multiple diffusion domains, fast track diffusion pathways, and/or microstructures evolving during the heating experiment. These diffusion parameters correspond to closure temperatures, Tc, of 225-375 °C for cooling rates of 5-100 °C/Ma. Our results indicate that plagioclase can be used to constrain thermal histories in almost all geologic settings, including those lacking alkali feldspars (e.g. meteorites and mafic/ultramafic intrusions). When multiple single-crystal analyses are considered in conjunction, detailed low-temperature thermal histories can be quantified. We illustrate this with analyses of plagioclase grains from the Bushveld Complex, South Africa.