Experimental study on controls on fluid chemistry and permeability evolution during serpentinization reactions

Serpentinization plays a key role in hydrothermal processes and structural deformation at slow and ultraslow mid-ocean ridges, where it is commonly associated with the formation of oceanic core complexes and detachment faults. Many details of how serpentinization reactions progress at small scales and the links between the reactions and changes in permeability and stresses are poorly understood. To advance our understanding of the evolution of permeability and fluid chemistry accompanying serpentinization reactions and provide a framework for developing more realistic models at a larger scale, we performed a series of high-temperature permeability experiments on fractured solid ultramafic rock samples that reasonably simulate serpentinization in natural settings. Experimental conditions were 260°C, 50 MPa confining pressure, and 20×2 MPa pore pressure. Ultramafic rock types containing different proportions of olivine and pyroxene were tested, to investigate the effect of mineral assemblage on fluid-rock interaction and permeability. Samples were cylindrical cores of 18 mm diameter and 23 mm length that were split axially to form a well-mated tensile fracture, jacketed in a 0.5 mm thick teflon liner and inserted into a 0.4 mm thick annealed silver jacket. A 7.5 mm thick layer of the same rock, crushed and sieved (0.18-1.0 mm size range) was placed on the inlet side of the sample to produce a coarse-grained gouge. The gouge layer provided a heated fluid reservoir with which the deionized water, used as pore fluid, could equilibrate before entering the fracture. Routinely, about 1 cm³ of water was pumped through the sample each day and collected (without dropping sample pore pressure) for chemical analysis. Pore fluid flow was in one direction and the pore pressure change and flow rate were recorded to determine permeability. In most samples, the initial fracture permeability at 260°C was between 10^-15 and 10^-18 m², and decreased by about 3 orders of magnitude in approximately 2 weeks. The behavior of Mg, Fe, and Si in solution varied with the composition of the host rock and with time. Two dunite samples showed decreases in the concentration of dissolved Mg and increases in Fe and Si towards the end of the experiments, whereas in harzburgite the concentrations of Mg, Fe, Si and Ca in solution all decreased over time. These experiments show that serpentinization reactions have a rapid effect on permeability and that the volume expansion observed in natural systems might be independent of serpentinization and result of tectonic processes. Thin section analysis of the samples from before and after the experiments shows that experiments reasonably simulate the natural setting observed in IODP samples that show pervasive, small-scale fracturing.