Fluid Flow during Metamorphism and its Implications for Fluid—Rock Ratios

Abstract

The study of metamorphic rocks is traditionally approached as an investigation of mineralogical changes occurring as responses to variations only in pressure and temperature. Despite the losses of large volumes of fluid during prograde metamorphism, rocks are often regarded, on the thin section scale, as systems able to buffer the chemical potentials (µ) of all components and therefore as essentially closed. There are several important reasons, apart from its simplicity, for adherence to this model. First, there is the observation that metasediments maintain their original sharp compositional discontinuities even in the amphibolite facies (e.g., Chinner, 1960). Second, low variance assemblages (implying internally buffered µ’s) are common. Finally, metasomatic changes and externally controlled activities generally lead to simple mono- or bimineralic zones such as those found in skarns or veins (e.g., Tilley, 1951; Burnham, 1959; Thompson, 1959). Such zones occur between interbedded metapelites and carbonates but are not a generally observed phenomenon in mixed pelitic/psammitic units.