In-situ X-ray Tomography and Chemical Tracer Experiments Examining Hydrothermal Alteration of Peridotite: Pore Scale Studies with Implications for Water-Rock Interaction Models

The porosity and permeability of rocks is impacted heavily by reactions between minerals and through-going fluids. The resulting dissolution of minerals that are unstable at the temperature, pressure, and chemical conditions at which the fluids and rocks are interacting and the precipitation of more stable mineral species in the pores, fractures, and grain boundaries between minerals controls where and how much porosity a rock has and how easily fluids can flow through it. This research uses novel, flow-through, hydrothermal, reactor vessels where changes in the porosity and permeability of rocks and the precipitation and dissolution of minerals can be observed and measured in real time. Results of these experiments will be analyzed using a mathematical technique (i.e., Lattice-Boltzmann approach) that provides a better simulation of the fine-scale processes than mathematical techniques presently used in water-rock interaction studies that depend solely on continuum conditions. Intact cores of classic seafloor peridotites, rocks that commonly host or are the source of sulfide-rich, hydrothermal deposits on the seafloor, will be used in the experiments; and alteration of the original mineralogy will be tracked using the stable isotopes of Ca, Mg, and Si, which will document reactions and reaction rates of carbonate and silicate minerals. Broader impacts of the work include improving infrastructure for science by developing new experimental and mathematical methods to advance studies of water-rock interaction and how the porosity and permeability of geological materials change as rocks and sediments react with waters flowing through them. The research has significant potential applications in improving our understanding of the behavior of carbon sequestration reservoirs, the migration of pollutants, and the conditions of nuclear waste disposal. The project also involves undergraduate students from the National Science Foundation Research Experience for Undergraduates (NSF-REU) program in cutting-edge science; trains a graduate student and a postdoc; and helps to develop and verify theoretical and model simulations use to predict the chemical and physical property evolution of natural rocks and fluids.

This research focuses on understanding the hydrolysis and carbonation of mantle peridotite by through-going aqueous fluids, an important process affecting the geochemical and geophysical properties of the ocean crust at mid-ocean ridges and in subduction zones. Reactions will be examined using coupled experimental, analytical, and theoretical approaches that emphasize the time series monitoring of the chemical and physical evolution of peridotite-fluid systems using a novel flow-through hydrothermal reactor and transparent reaction cells that are coupled to an X-ray Computed Tomography instrument that allows changes in mineral dissolution and precipitation processes to be examined in real time and in high resolution. This allows direct investigation of changes in the 3D architecture of the rock on a fine scale during water-rock interaction. Chemical tracers using non-traditional stable isotopes (Si, Mg and Ca) will be used to constrain mineral dissolution and precipitation rates and indicate changes in mineral surface area. The experiments and their results will be modeled using a Lattice-Boltzmann multicomponent, multiphase, fluid flow and solute transport computer code with the results being used to develop more accurate reservoir-scale modeling approaches using continuum-scale simulators. Goals of the research are to investigate the feedback between fluid-mineral reactions and associated pore-space geometry changes in peridotites that result from processes like fluid flow, advection of chemical species, and reaction locations and reaction rates. Samples will consist of cores of seafloor peridotite.