Experimental serpentinization of intact dunite cores

Abstract:

Serpentinization in ultramafic-hosted hydrothermal systems, such as Lost City, produces relatively cool and alkaline fluids that support diverse ecosystems. To simulate serpentinization in such systems, we conducted single-pass, flow-through experiments on dunite cores cut out of a sample from Jackson County, North Carolina. Experimental seawater prepared using laboratory-grade reagents and standards was pumped through a core at 150ºC and 150 bar pore-fluid outlet pressure at a flow rate of 0.01 ml/min. An additional experiment will be conducted at 200ºC. At 150ºC, permeability decreased by 2.3 times with reaction progress over the course of the 36 day experiment. Fluid-rock reaction generally produced CO₂, H₂, CH₄, and CO concentrations of 100 µmol/kg, up to 40 µmol/kg, 2 µmol/kg, and less than 1 µmol/kg, respectively. Outlet fluid chemistry was relatively stable, except for initial peaks in Al, Ba, Fe, Mn, and Si. pH of outlet fluids increased with reaction progress, but it was always lower (6.9-7.4) than the initial seawater (7.8). X-ray computed tomography scans were/will be collected for both pre- and post-experimental cores. The combination of flow-through experiments on solid, intact rock cores cut out of natural samples and X-ray tomography permits visualization and quantification of mineralogical changes and flow path evolution during serpentinization. This approach further permits physical and chemical processes to be documented on a fine scale to better understand feedbacks between chemical reactions and flow fields, with implications for ultramafic-hosted hydrothermal systems.