CHEMICAL-TRANSPORT AND DISSOLUTION PRECIPITATION OF CRYSTALLINE SOLUTION DURING HYDROTHERMAL CONVECTION

A mathematical formalism is developed to compute the aqueous species t ransport coupled to reactions forming crystalline solutions during hyd rothermal circulation. The formalism takes into account that, during c onvection in a fracture network at temperatures from 0 degrees C to 20 0 degrees C, dissolution/precipitation reactions between the fluids an d crystalline solutions do not reach a ''true'' equilibrium at the loc al fluid temperature; rather a ''pseudo-equilibrium'' is reached local ly either with the dissolving or with the last precipitated crystallin e solution. These assumptions permit the explicit solutions of the mas s transfer equations during simple convective loops. Two examples of r eaction associated with convective flow are given: (1) O-16 and O-18 p artitioning between quartz and an aqueous fluid and (2) compositional variations in the celestite-barite (Sr,Ba)SO4 solid solution. Computat ions show that after several convective cycles, an asymptotic precipit ation regime is reached which is independent of the initial compositio n of the fluids percolating in the fracture network. Also, for most cr ystalline solutions, the compositions of the precipitated solids in th e asymptotic precipitation regime are not affected by the fact that th e ''pseudo-equilibrium'' is reached with the dissolving or with the la st precipitated crystalline solution. Thus, explicit relations are der ived giving the composition of the precipitated products as a function of the convective fluid temperature and the reacting crystalline solu tion. These relations are suggested as possible geothermometers to stu dy paleohydrothermal systems.