Date of Award
Doctor of Philosophy
A typical serpentinization reaction is of the type: Mg(,2)SiO(,4) + MgSiO(,3) + 2H(,2)O (--->) Mg(,3)Si(,4)O(,5)(OH)(,4), involving a 40% volume expansion and considerable heat generation. At all temperatures between 100(DEGREES)C and 400(DEGREES)C rates of serpentinization are geologically rapid and are controlled by the rate at which water is supplied to the reaction interface. Once serpentine begins to form, any further reaction will depend on the rate of water penetration through an existing layer of serpentine. Defects 100-1000 A wide produced by the buckling of the platy serpentine mineral, lizardite, provide the permeability.;An experimental study, at room temperatures (25(DEGREES)C-40(DEGREES)C), has shown that a thin (3-4 mm) slice of serpentinized periodotite placed in a diffusion cell exhibits a significant permeability to water, while behaving semipermeably to salt. Diffusion coefficients of water at 34(DEGREES)C range from 10('-7) to 10('-8) cm('2) sec('-1), whereas using a modified Darcy equation intrinsic permeability coefficients of 10('-11) to 10('-12) darcys are obtained.;At a typical serpentinization temperature of 200(DEGREES)C D(,H(,2)O) is estimated to be on the order of 10('-5) cm('2) sec('-1). Using the parabolic rate law, x = (2Dt)(' 1/2), such coefficients imply a mean penetration distance, x, through serpentinized peridotites in the range of 1-80 cm a('-1). It thus appears that in deep (i.e. > 5 km) groundwater or marine environments, water will penetrate massive serpentinite at an appreciable rate and hence will lead to moderately rapid reaction rates. Given the large P(,H(,2)O) gradient produced by the reaction (i.e. P(,H(,2)O) external - P(,H(,2)O) reaction interface), the process will cause large volume strains coupled to flow and reaction rates.;As the swelling pressure (stresses) of the reaction are on the order of 1 kb, refracturing will be a continuous process. Microstructures, ubiquitous to all serpentinized peridotites, show that microfracturing has occurred on all scales and is in accord with high local volume strains. In such environments serpentinization should be a continuous process, controlled only by water access to ultramafic rocks through their cover.
Macdonald, Andrew Harrington, "Water Diffusion Rates Through Serpentinized Peridotites: Implications For Reaction Induced Dynamic And Chemical Effects In Ultramafic Rocks" (1984). Digitized Theses. 1376.