Magnesium diffusion in plagioclase: Dependence on composition, and implications for thermal resetting of the 26Al–26Mg early solar system chronometer

Abstract

Experimental data are reported on Mg diffusion in plagioclase crystals with a range of anorthite content (xAn), at temperatures between 800 and 1150 °C. Oriented and polished single crystals of anorthite (xAn=0.93), labradorite (xAn=0.67), andesine (xAn=0.43) and oligoclase (xAn=0.23) were each embedded in powered source material enriched in natural MgO or 25MgO and suspended in a furnace at constant temperature. Diffusion profiles in quenched samples were measured from the polished surface using SIMS depth profiling. The diffusion coefficient does not depend significantly on the Mg concentration gradient, and little anisotropy is observed between the b and c directions in labradorite. Diffusion coefficients increase systematically with decreasing xAn, and the entire data set is described by lnD=(−6.06±1.10)−(7.96±0.42)xAn−287±10 kJ/molRT, where R is the gas constant, T is absolute temperature, and the diffusion coefficient D is in m2/s. 26Al–26Mg ages in albite-rich plagioclase are much more easily reset than in anorthite, with closure temperatures up to 120–150 K lower.