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

A mathematical formalism is developed to compute the aqueous species transport coupled to reactions forming crystalline solutions during hydrothermal circulation. The formalism takes into account that, during convection in a fracture network at temperatures from 0 degrees C to 200 degrees C, dissolution/precipitation reactions between the fluids and crystalline solutions do not reach a ''true'' equilibrium at the local fluid temperature; rather a ''pseudo-equilibrium'' is reached locally either with the dissolving or with the last precipitated crystalline solution. These assumptions permit the explicit solutions of the mass transfer equations during simple convective loops. Two examples of reaction associated with convective flow are given: (1) O-16 and O-18 partitioning between quartz and an aqueous fluid and (2) compositional variations in the celestite-barite (Sr,Ba)SO4 solid solution. Computations show that after several convective cycles, an asymptotic precipitation regime is reached which is independent of the initial composition of the fluids percolating in the fracture network. Also, for most crystalline solutions, the compositions of the precipitated solids in the asymptotic precipitation regime are not affected by the fact that the ''pseudo-equilibrium'' is reached with the dissolving or with the last precipitated crystalline solution. Thus, explicit relations are derived giving the composition of the precipitated products as a function of the convective fluid temperature and the reacting crystalline solution. These relations are suggested as possible geothermometers to study paleohydrothermal systems.