FLUID-ROCK REACTIONS DURING METAMORPHISM AT MIDCRUSTAL CONDITIONS

A simplified model of the regional metamorphic process demonstrates that the highest grade experienced by a rock is produced at the highest temperature while the maximum pressure generally occurs earlier. Whether isograd reactions are surface reaction- or fluid transport rate-limiting, at mid-crustal conditions, depends on how fluid is channelized while flowing toward the earth's surface. Fluid is generally present locally during prograde metamorphism, being produced by sliding equilibria devolatization reactions. High-grade mineral assemblages produced at mid-crustal conditions do not react to lower-grade assemblages as temperature and pressure decrease during uplift because of the lack of a fluid phase. A fluid phase is required for reaction to transport reactant material to the site of mineral growth. The last fluid-producing reaction occurs at the highest grade, and then the fluid escapes by hydrofracturing with at least same flow velocity component toward the earth's surface. The speciation and composition of a typical aqueous chloride-rich fluid in equilibrium with a potassium-rich quartzo-feldspathic rock at the peak metamorphic conditions of 500 degrees C and 2 kbar is calculated, and the implication this has for the textural fabric in a rock is discussed. This fluid is determined to be somewhat acidic to neutral and contains a low concentration of Al. From an energy standpoint, there appears to be coupling of surface reaction rates with transport and nucleation rates to minimize the entropy change associated with these irreversible processes.