Por Maria Antunes Dias (Ruhr University Bochum, Germany).

Transmissão online via Microsoft Teams (pw: HS6Yg2Cz).

Abstract: Diffusion modelling is a growing field in geosciences, owing to the broad spectrum of interest in petrological studies. Among other applications, it allows to determine time information in magmatic systems, such as magma mixing events, cooling rates or crystal residence times in magma chambers.  To do so, it is desirable to have a comprehensive data set of experimentally determined diffusion coefficients, measured as a function of the relevant thermodynamic parameters. Both the experimental determination of diffusion coefficients, as well as the determination of time scales from diffusion modelling (“diffusion chronometry”), rely on the measurement of concentration profiles or maps in minerals with concentration gradients. Together with an appropriate diffusion model, it is then possible to determine timescales (or diffusion coefficients) by fitting modelled diffusion profiles to measured compositional gradients.

A solid knowledge of diffusion rates and mechanisms in orthopyroxene is extremely useful, given its ubiquity in igneous, mantle, metamorphic rocks and meteorites, often recording timescales of a variety of geological processes. Insofar, the experimental determination of orthopyroxene has been hindered by several experimental and analytical challenges. This is mostly attributed to the fact that diffusion rates in orthopyroxene are slow and therefore, the experimental concentration profiles are short and difficult to resolve. By developing and adapting known experimental and analytical methods, the diffusion rates of Fe-Mg, Li and REE in orthopyroxene were experimentally calibrated along the [001] axis as a function of T, XFe, fO2 and to a certain extent, the presence of trivalent impurities in the crystal lattice. A complex interplay between the different compositional and thermodynamic parameters was found, which allowed to discuss the diffusion mechanisms and point-defect chemistry of orthopyroxene. The direct implication of the results of this study is that previous DFe−Mg parameterizations can introduce significant errors in the derived timescales if the influence of these parameters is not considered.