Determination of time-integrated metamorphic fluid fluxes from the reaction progress of multivariant assemblages

Abstract

An expression is derived for the calculation of time-integrated metamorphic fluid fluxes in two or more dimensions in rocks undergoing multivariant reactions under conditions of varying pressure, temperature and angle of flow. This calcuation requires knowledge of mineral assemblages, modes and compositions, which are obtained from isobaric T-XCO2 pseudosections constructed using the program THERMOCALC and compared with those observed in east central Vermont. THERMOCALC is capable of reproducing peak mineral assemblages, modes, compositions and the observed reaction sequences within the system KCaNaFMASCH for two kyanite grade pelitic carbonate rocks from a Barrovian style regional metamorphic terrain in east central Vermont, U.S.A. Calculation of fluid fluxes for decarbonation reactions under conditions of horizontal, layer-parallel flow produces time-integrated fluid flux figures of the order of 108 moles m−2. Allowance for possible cross-layer flow from adjacent dehydrating pelites reduces this figure significantly, with episodic cross-layer fluxes of the order of 105 moles m−2 being capable of driving the observed decarbonation. Chlorite bearing carbonate protoliths would have initially dehydrated with increasing temperature, a process requiring down-temperature fluid flow to produce the assemblages currently observed.