Diffusion in common mineral phases. Part 2: plagioclase
Chemical zoning in plagioclase is ubiquitous in igneous rocks. Therefore, zoned plagioclase has been widely used in the past as an archive of magmatic history in terms of thermodynamic conditions (P, T, fH2O), to study chemical differentiation of magmas (open vs. closed system processes as inferred from trace elements and appropriate plagioclase-melt partition coefficients) and to determine time scales of magmatic processes.
Since the work of Zellmer et al. (1999) application of plagioclase for diffusion chronometry has been increasing steadily. In principle two very different time scales can be recorded in plagioclase. On the one hand, extremely long-time scales and slow cooling rates could be accessible via sluggish diffusion processes such as NaSi-CaAl interdiffusion or diffusion of high field strength elements (e.g. Ti). On the other hand, relatively mobile trace elements (e.g. Mg, Sr and in particular Li) may be used to get insights on short term processes in a magmatic system. For instance, time scales of magma residence/mingling, sub-solidus cooling rates, and ascent rates have been obtained using different diffusion coefficients.
For all of these applications since 2003 it has been considered that trace element diffusion in plagioclase is strongly coupled to the major element zoning (NaAlSi3O8(Ab) – CaAl2Si2O8(An) solid solution) according to the model of Costa et al. (2003). However, the assumptions of that model have never been tested experimentally. Secondly, diffusion rates in plagioclase are expected to depend strongly on the presence of water in the environment (fH2O), but the nature of dependency of diffusion rates on water fugacity remain poorly known.
This project aims to carry out novel experiments to address these issues and generate new diffusion data.
Investigators
Florian Pohl, Leibniz Universitaet Hannover
Harald Behrens, Leibniz Universitaet Hannover
Ralf Dohmen, Ruhr-Universitaet Hannover
References
Costa F, Chakraborty S, Dohmen R (2003) Diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase. Geochimica et Cosmochimica Acta 67:2189–2200. https://doi.org/10.1016/S0016-7037(02)01345-5
Zellmer GF, Blake S, Vance D, Hawkesworth C, Turner S (1999) Plagioclase residence times at two island arc volcanoes (Kameni Islands, Santorini, and Soufriere, St. Vincent) determined by Sr diffusion systematics. Contrib Mineral Petrol 136:345–357. https://doi.org/10.1007/s004100050543