Combined Hydrodynamic and Thermodynamic Model of a Convective Hydrothermal System.
1. The Marker Method of Modeling

(C) 1997 A. V. Tutubalin, D. V. Grichuk

Moscow State University
Department of Geology
Vorob'evy gory, Moscow, 119899 Russia

A combined model of a convective hydrothermal system in a mid-ocean ridge was developed. The modeling technique includes two stages of calculations. At the first stage, paths are calculated for the motion of markers (point-sized portions of the solution with the given initial coordinates); temperatures, pressures, and filtration rates are found for each part of these paths. At the second stage, compositions of metasomatic mineral associations are calculated within each part of the marker paths, as well as the composition of the equilibrium hydrothermal solution. Temperature and pressure data are directly included in the thermodynamic calculations, whereas the information concerning filtration rates is used to estimate the intensity of interaction between the solution and the host rock. Calculations within the hydrodynamic part of the model were based on a numerical solution of the Darcy and heat transfer equations by the finite elements method. At the thermodynamic stage, the technique of a flow-type stepwise reactor was used, and the local equilibrium between the solution and the rock was calculated at each step of this reactor.

The results of modeling showed that the hydrothermal system is characterized by formation of a convective cell with a wide region of descending motion and narrow region of ascending flow in the fracture zone above the intrusive body. Near the upward branch, a region of circular flow appears; however, this region is nonstationary and degenerates with time. The position of isotherms in the system is subparallel to the contact with a magma chamber. The rock--water ratios increase in the contact part of the system and reach maximum values above the top of the magma chamber. The calculated distribution of metasomatic minerals agrees with the one observed for ophiolites. The time required for a portion of sea water to pass through the convective system was estimated: for a system as a whole, it is of the order of 1000 years; for the part with an active water--rock interaction, it amounts to tens of years.