Fluid-Rock Reactions during Metamorphism at Mid-Crustal Conditions

Abstract

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 some flow velocity component toward the earth's surface. The speciation and compo...