4:00 PM - 4:15 PM
[MGI27-09] Unraveling the controls on the silica metasomatic reactions during serpentinization using the exchange Monte Carlo method.
Keywords:serpentinization, serpentine, hydrothermal experiments, exchange Monte Carlo method, reactive-transport modeling
In this study, we conducted hydrothermal experiments (300°C, 8.6 MPa) on olivine (Ol)–quartz (Qtz)–H2O system, as analogues of crust-mantle boundary. By using tube-in-tube type hydrothermal experiments vessel, spatial distribution of minerals (reactant and product) was observed.
After the experiments, the mineralogy of the reaction products in the Ol-hosted region changed with increasing distance from the Ol–Qtz boundary, from talc to serpentine + magnetite + brucite. Talc was formed 3.2 mm from the Ol–Qtz boundary in Ol-hosted region. On the other hand, in Qtz-hosted region, no minerals were formed after the experiments.
The observed mineral distribution was modeled by reaction-diffusion equation. To model our experiments, we set eight reaction rate constants; diffusion constant for SiO2(aq) and rate constants for olivine→talc, olivine→serpentine, olivine→brucite, serpentine→talc, talc→serpentine, serpentine→brucite, and brucite→serpentine. The unknown parameters were optimized by using exchange Monte Carlo method (Hukushima and Nemoto, 1996). The surface area of minerals in the reaction-diffusion model (reactant or product) were selected by 2-hold cross validation method.
By using the exchange Monte Carlo method, the observed mineral distribution in the Ol-Qtz-H2O experiments was reproduced by numerical reactive transport model. The second Damkőhler number (DaII) of each reaction changed from DaII<<1 at 1000 h to DaII >> 1 at 2000 h reaction, suggesting that the rate-limiting processes during Si-metasomatic reaction was changed in time and space from surface-controlled reaction to transport-controlled reaction. Our experiments and kinetic analysis suggests that the dynamic changes in rate law from transport- to surface controlled reaction, and vice versa, would be responsible for the spatial-temporal evolution of the metasomatic zone at the crust-mantle boundary in the oceanic lithosphere.
Hukushima, K., and Nemoto, K., 1996, Exchange Monte Carlo Method and Application to Spin Glass Simulations: Journal of the Physical Society of Japan, v. 65, no. 6, p. 1604–1608, doi: 10.1143/JPSJ.65.1604.
Lichtner, P.C., Oelkers, E.H., and Helgeson, H.C., 1986. Interdiffusion with multiple precipitation/dissolution reactions: transient model and the steady-state limit. Geochimica Cosmochimica Acta, v. 50, 1951–1966.