Mafic Recharges over Millennia Trigger an Eocene Silicic Supereruption in Southern Tibetan Plateau: Evidence from Multi-Element Diffusion Chronometry in Quartz and Sanidine

Abstract The frequency and magnitude of silicic supereruptions are controlled by magmatic processes in the crustal plumbing system, particularly the timescales/fluxes of magma recharge in the upper crustal reservoir(s). Titanium (Ti) diffusion speedometry in quartz is a commonly used method for timescale estimation in such systems. However, it is controversial due to various Ti diffusion coefficients and the effect of the initially curved (non-step) diffusion profile. By combining the diffusion of Ti and Aluminum (Al) in quartz crystals from an early Eocene supereruption (the crystal-rich Pangduo ignimbrite, southern Tibetan Plateau), we assessed varying Ti diffusivities and removed the effect of initially curved profile. This was compared with Barium (Ba) diffusion timescales in sanidine crystals. We also introduced a heat conduction model for constraining possible recharge scenarios. Quartz crystals exhibit zoning textures, with Ti content in the rims being 12 to 60 μg/g higher than in the cores. This suggests crystallization temperatures in the rims are 25°C to 125°C higher than those in the cores or an increase in TiO2 activity (by up to 0.5). Resorption textures around the quartz cores indicate partial dissolution, suggesting reheating was at least partly responsible for the Ti increase in the rims. Combined Ti and Al diffusion in quartz, alongside Ba in sanidine, suggests that the reheating duration triggered by magma recharge spans at least millennia. Pulsatory thermal disturbances align with the millennial reheating path recorded in the crystals. Our study, therefore, proposes a novel and feasible approach to combining Ti and Al diffusion in the same quartz grain, together with Ba diffusion in sanidine, to accurately constrain the recharge duration before silicic supereruptions.