2020 | Constraining the timescales of magmatic processes
Chemical diffusion in silicate minerals and melts has been extensively used in an inverse manner to re- trieve valuable information with respect to the timescales of geological processes. A fundamental element in the numerical modelling of chemical diffusion in minerals is the choice of particular boundary condi- tions. Boundary conditions are essential for the accurate evaluation of mass exchange between a mineral and its surrounding matrix. The boundary conditions are commonly constrained by equilibrium thermody- namics experiments. This is because the compositions of coexisting minerals and melts are highly sensi- tive to, inter alia, pressure, temperature and oxygen fugacity (P-T-fO2). Such a dependence has been used for many years for the derivation of thermodynamic formulations (e.g. geothermometers, geobaro- meters) that can be applied to deduce P-T-fO2 conditions in natural systems. In reality, all thermodynamic formulations are calibrated from experimentally produced minerals whose compositions are assumed to be in equilibrium with that of other coexisting phases (minerals and melts). In order to ensure that equilib- rium was attained, most experiments are conducted at elevated temperatures where the kinetic inhibition of solid-state diffusion and mineral reaction is negligible. On the one hand, experiments are needed at conditions where global thermodynamic equilibrium is not always guaranteed considering the typical ex- perimental timescales. Furthermore, equilibrium mineral compositions can be affected by quenching ef- fects during cooling and decompression of the experimental assembly after the experiment. These practi- cal problems increase the uncertainty in the calibration of geochronometers for magmatic/volcanic pro- cesses via diffusion modelling. In this project, we propose a combination of experimental and modelling approaches for accurate and precise quantification of equilibration time scales in magmatic systems. Mineral-mineral and mineral-melt pairs from nominally equilibrium experiments at controlled T-P-fO2-time conditions will be investigated to unravel diffusion-induced grain-boundary effects. The data will be utilized for diffusion and thermodynamic modelling in an inverse manner in order to retrieve the information regarding the equilibri- um partition and the diffusion coefficients of specific elements/isotopes at various conditions. When cali- brated on a series of representative experiments, the developed model will be tested and applied to a number of natural magma system examples (e.g., Iceland, Kamchatka, Italy).