Contact metamorphism of the Rove Formation by the Duluth Complex, northeastern Minnesota, resulted from the discontinuous reaction chlorite + muscovite + quartz → cordierite + biotite + H2O and the continuous reaction muscovite + biotite + quartz = cordierite + K-feldspar + H2O. Bulk compositions of hornfelses and mass-balance calculations indicate that metamorphism occurred isochemically, except for the loss of 0.6 to 2.7 wt % H2O. Calculated values of temperature and fH2O indicate that during progressive metamorphism, H2O evolved from the rocks progressively diluted a CO2-rich fluid phase. The relations among temperature, xH2O, and progress of the continuous reaction in an average bulk composition suggest that at T = 500 °C and P = 1,500 bars, chlorite + muscovite + quartz in 1 m3 of rock reacted with at least 0.5 m3 of a CO2-rich fluid, producing a fluid with xH2O = 0.15. The continuous reaction buffered the composition of this fluid to a value of xH2O = 1.0 at T = 600 °C. The relation between xH2O and the progress of the continuous reaction suggests that the volumetric fluid/rock ratio decreased from a value of 0.55 to essentially 0.0 as temperature increased from 500 °C to 600 °C. The study indicates that multivariate devolatilization reactions can be capable buffers of fluid composition. The dynamic variables—rate of temperature change, rate of H2O evolution, and rate of reaction—were determined by a heat conduction model for contact metamorphism. The calculated width of the aureole (180 m) and temperature distribution within the aureole (500 to 620 °C) are consistent with observations. The calculated reaction rates for the continuous reactions, ranging from 1.2 × 10−4 to 3 × 10−2 moles/m3/yr, are much lower than reported rates for other reactions; minimum permeabilities implied by the rate of H2O evolution, ranging from 10−12 to 5 × 10−15 cm3/sec cm3 rock, are 4 to 7 orders of magnitude lower than measured permeabilities of shales. These results strongly support the assumption of chemical equilibrium during metamorphism, even in contact-metamorphic aureoles, where the rate of temperature change is great.
- Geological Society of America