Hydrothermal alteration of granite rock cores: experiments and kinetic modelling

The kinetics of water-rock interactions at elevated temperatures is key for understanding the dynamic evolution of porosity and permeability in natural and industrial systems. The implementation of rate data in numerical models simulating reactive transport is crucial to reliably predict subsurface fluid flow. The vast majority of data are constrained by single phase powder experiments inhering unrealistically high surface areas and hampering consideration of microstructural effects on reaction progress. We present experimental results of batch experiments conducted at 200 °C for up to 60 days on a quartz-monzodiorite and pure water that bridge the gap between powder experiments and complex natural systems. The effect of reactive surface area was modelled by using bulk-rock powders, intact, and thermally cracked rock cubes. Fluid composition was monitored (ICP-MS) and solid products were analysed after each experiment (SEM, EMPA). The evolution of fluid and solid compositions was compared to a self-coded geochemical transport model accounting for dissolution, nucleation, growth and reactive surface area. Experimental and modelling results indicate a fast increase of Na, Ca, K and Si in the fluid related to kinetically controlled dissolution of feldspar (plg and kfs) and quartz. Maximum concentrations of Al, Mg, and Fe are reached within two days followed by a rapid decrease induced by secondary mineral precipitation. The amount and type of secondary phases strongly depend on the host substrate indicating that local fluid composition and substrate surface are the controlling parameters. Observed reaction rates differ strongly between powder and rock cube experiments due to differences in reactive surface area. Combining kinetic data, gained by modelling the experimental results, with new information on substrate-precipitate relationship will aid large scale-transport models to realistically predict chemo-mechanical changes and fluid flow in subsurface systems.