Ti-in-quartz and Sr-Ba-Mg-in-feldspars diffusion chronometry: a review of available diffusion data, and a critical evaluation of applications to natural samples

Diffusion chronometry based on quartz and feldspars has the potential to place robust constraints on the timescales of magma storage, recharge, and mobilization in silicic magmas. However, widespread application of these methods has been hindered by the fact that published diffusion coefficients for Ti in quartz and for Sr, Ba, and Mg in plagioclase and alkali feldspar differ by orders of magnitude, leading to large and often irreconcilable discrepancies in inferred timescales. In this review, we first compile existing experimental diffusivities in quartz and feldspars and critically evaluate the origins of discrepancies among published datasets, including experimental design, analytical protocols, and underlying assumptions. We then review studies performed on natural samples in which diffusion timescales based on Ti gradients in quartz were compared with ones based on Sr, Ba, and/or Mg diffusion in feldspars and/or other methods. To allow proper comparison, we complement previously reported timescales with ones calculated based on more recently published diffusion coefficients. This exercise reveals that internally consistent and mutually compatible timescales can be obtained with the most recent set of diffusion coefficients. Building on these conclusions, we outline best-practice recommendations for the generation, selection, and application of diffusion coefficients that are geologically meaningful and appropriate for natural magmatic systems. We conclude by identifying key experimental gaps that must be addressed to further refine diffusion models and to establish a coherent and broadly accepted framework for diffusion chronometry in silicic magmas.