Timing magma transit in the earth using crystal clocks: re-evaluating element diffusion in minerals

The booming field of diffusion chronometry allows geoscientists to extract the timing and duration of subsurface magmatic processes that occur prior to volcanic eruptions. The technique relies on modeling step-wise, concentric chemical gradients that form within magmatic minerals as they grow or get perturbed by new incoming magma prior to volcanic unrest. These chemical ‘tree rings’ are smeared with time by element diffusion, so that the amount of time between perturbation and eruption can be recovered if the mobility (diffusivity) of elements is calibrated in the lab at magma temperatures. This project aims to resolve recently uncovered discrepancies between widely-used element diffusivities obtained in simplified systems (e.g., mineral-mineral couples) and those obtained in melt bearing systems (mineral-melt couples). The new experiments carried out during a STUDIUM-supported sabbatical in 2024-2025 confirmed that the presence of melt is responsible for important differences in element mobilities for olivine, perhaps via the presence of H2O. Diffusivities in plagioclase, by contrast, are not influenced by melt or H2O, implying that current community practices are robust. The underlying mechanisms by which these differences in element behavior appear are still being investigated, and new tools recently tested (hyperspectral cathodoluminescence) may hold important clues as to the presence and distribution of point defects in these minerals.