Water-rock interaction at small scales: Studies with lattice Boltzmann modeling and strontium isotopes in a potassium-metasomatized tuff

Techniques for studying small scale water-rock geochemistry in complex systems are useful for interpreting hydrologic systems at larger scales with chemical tracers. Here, chemical reactions at millimeter scales between solid and fluid are examined experimentally with strontium isotopes in the upper Lemitar tuff (ULT) from south-central New Mexico and numerically with lattice Boltzmann modeling. Lattice modeling methods, which center on simple particle dynamics whose macroscopic behavior can be described by the Navier-Stokes equation, are described in detail. A lattice Boltzmann model, previously used for hydrodynamic and reaction-diffusion models at Los Alamos National Laboratory, was extended to model sorption and dissolution-precipitation reactions in an advective fluid regime. The model was further refined to simulate reactions in compositionally heterogeneous systems such as the ULT. The ULT is an ignimbrite that has been K-metasomatized, potentially by saline brines, in a hydrologically closed basin. Microsamples of Sr isotopes of the feldspar phenocrysts and matrix of the ULT demonstrate the fluids moving through the tuff have converted plagioclase to adularia and have left sanidine unaltered. Adularia microsamples plotted on 87Rb/86Sr vs. 87Sr/ 86Sr return an age of 6.3 ± 2.7 Ma for the K-metasomatism event. Values of 87Sr/86Sr at 6.3 Ma were calculated for all the microsamples from the different ULT phases to constrain the sources of Sr in the adularia. The initial Sr isotopic composition of the microsamples indicates that during K-metasomatism: (1) the Sr in the adularia was derived from sources other than the altering plagioclase material and likely from outside of the upper Lemitar tuff, and (2) a high water to rock ratio was present based on the lack of variation of inital 87Sr/ 86Sr in the adularia from different portions of the ULT. The lattice Boltzmann model is an ideal tool for modeling such a system as demonstrated by its capabilities for modeling sorption and dissolution-precipitation reactions in complex media.