Earth-Science Reviews, 32 (1992) 133-135 133 Elsevier Science Publishers B.V., Amsterdam Some simple models of chemical alteration caused by the movement of metamorphic fluids in the deeper parts of the crust L.M. Cathles Cornell Unioersity, Ithaca, N Y 14853, USA (Received and accepted for publication October 1990) EXTENDED ABSTRACT Pore fluids in the crust are generally in chemical equilibrium with the common in- organic rock minerals they contact. The chemistry of the "rock-buffered" pore fluid is not very sensitive to the particular alumino- silicate minerals contacted. For this reason the temperature at which the fluid equi- librated can be inferred from analyses of fluid composition alone through the application of various geothermometers (e.g., Na-K-Ca, SiO 2 and various gas geothermometers; Hen- ley et al., 1984). Pore fluids may equilibrate at temperatures as low as 50 °C under common crustal conditions. If pore fluids are equilibrated with their host rock minerals in the deeper parts of the crust as the above discussion indicates must be the case under reasonably quiescent condi- tions, rock alteration will occur only when pore fluids move with respect to physical fields that affect their equilibrium chemistry. The most important physical fields are tem- perature, pressure, and chlorinity (C1- being a non-reactive and therefore unbuffered fluid component that controls fluid pH through charge balance). When pore fluids move through such physical gradients, the chemical flux from the rock minerals to the fluid is easily defined: Tin, Pin, Cl'in c"l-'.'"." :" [Ci]in Tout, Pout, Cl'out .... " [Ci]out Alteration flux from rock to fluid = Q ( [Ci]out - [Ci]in ) Q is the total fluid mass moved through each unit area perpendicular to the flow direction. The equilibrium chemistry, [Ci], can be calculated for a particular mineral buffer as- semblage using thermochemical data. A mod- ified version of the EQ3 data base is used here (see Cathles, 1985). Changes in mineral- ogy follow from the alteration flux through multiplication of the inverted transpose of the mineral buffer stoichiometry matrix. For a specific example, suppose pressure and salinity are constant, and the fluid moves slowly upward through a normal geothermal gradient of 25 °C km-1. For the rock mineral buffer in the table below the temperature derivative at 300 °C of the total solution CO: is > 20 times that of any other solution com- ponent for pore fluids of seawater salinity and > 200 times greater for pore fluids where CI-= 1000 ppm. In other words at tempera- tures of -300°C or greater, fluids moving down a temperature gradient through crustal rocks lose essentially only CO2; changes in all other solution components are comparatively very small. The transfer of CO2 from the fluid to the rock as the fluid cools in its upward migra- tion causes precipitation of carbonate and a cascade of other mineralogical changes that, on a weight percent basis, are usually larger than the carbonate precipitation. Carbonate precipitation has "mineralogical leverage" be- cause the Ca 2÷, Mg 2÷ or Fe 2÷ required to make the carbonate must be drawn from other (usually aluminosilicate) rock minerals, and the A13+, SiO 2 etc. from these phases must also find mineralogical homes. The musical 0012-8252/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved