LIMITS TO FLUID FLUXES, REACTION RATES, AND POROSITY DURING INFILTRATION-DRIVEN CONTACT METAMORPHISM OF CARBONATE ROCKS

Periclase (Pe) and wollastonite (Wo) reaction fronts developed in marbles in the inner parts of many contact aureoles are typically narrow (< 1m).  If experimentally-determined reaction rates (on the order of k = 10^-13 mol cm^-2 s^-1 at 500^o to 550^oC) for several devolatilization reactions (e.g., Schramke et al., 1987; Heinrich et al., 1989; Kerrick et al., 1991) are utilized in a model of CO₂ reaction and transport, development of narrow reaction fronts requires upper limits to Darcy flux (q_D < 1 x 10^-11 m³ m^-2 s^-1) and porosity (n < 0.001) at 500^o to 550^oC.  Petrologic and stable isotope analyses of infiltration-driven metamorphism of carbonate rocks in a number of contact aureoles have established that time-integrated fluid fluxes (q_T) range from 10² to 10⁴ m³ m^-2 (summarized in Ferry et al., 2002).  Hydrodynamic analyses for the Alta, UT and Notch Peak, UT aureoles suggest that the timescales of hydrothermal circulation necessary to produce the observed locations of isograds and widths of ¹⁸O/¹⁶O depletion zones are relatively short, 5³ to 5⁴ yrs (Cook et al., 1997; Cui et al., 2003).  If these timescales are representative, then the estimates of total flux based on study of natural systems require Darcy fluid fluxes from 6 x 10^-11 to 6 x 10^-8 m³ m^-2 s^-1. To produce narrow reaction fronts at these Darcy fluxes, the reaction-transport model results require reaction rates as high as 10^-10 mol cm^-2 s^-1.  These reaction rates would be compatible with the upper end of experimental results at the higher temperatures (T = 575^o to 625^oC) characterizing periclase reactions in the inner zones of many contact aureoles.  However these rates are 10¹ to 10² greater than the experimental results of Tanner et al. (1985) for the reaction calcite (Cc) + quartz (Qtz) = Wo + CO₂ over this same temperature range.   These estimates of reaction rates in turn constrain rates of porosity reduction (n_r) to be at least equal to the rate of porosity creation (n_c) in order to maintain low porosity (n < 0.001) during prograde reaction.  Resulting rates of porosity reduction (via compaction and/or creep) would be significantly greater than those (n_r approx. equal to 10^-7 s^-1) deduced by Zhang et al. (2000) from their experimental studies on the reaction calcite (Cc) + quartz (Qtz) = Wo + CO₂ at conditions of high P (H₂O): P (Total).