2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 7
Presentation Time: 3:00 PM

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


BOWMAN, John R., Department of Geology and Geophysics, University of Utah, 135 S 1460 E, Rm 719 WBB, Salt Lake City, UT 84112, jrbowman@mines.utah.edu

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 500o to 550oC) 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 (qD < 1 x 10-11 m3 m-2 s-1) and porosity (n < 0.001) at 500o to 550oC.  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 (qT) 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 = 575o to 625oC) 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 (nr) to be at least equal to the rate of porosity creation (nc) 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 (nr 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).