HR: 08:00h
AN: T41G-01 INVITED     [Abstracts]
TI: Geochemical Considerations Regarding the Processes Involved in Mineral Deposition in Sedimentary Rock-Hosted Veins
AU: * Morse, J W
EM: morse@ocean.tamu.edu
AF: Texas A&M University, Department of Oceanography, College Station, TX 77843
AU: Gledhill, D K
EM: dwight.gledhill@NOAA.gov
AF: NOAA/NESDISS, E/RA 3 SSMC 1, Silver Spring, MD 20910
AB: In order for mineral deposition to take place in a vein, first the opposite reaction-dissolution of the mineral must occur from some source rock to place the requisite dissolved components into solution. Then the dissolved components must be transported to the vein either by advective or diffusive means before deposition can ensue. Finally conditions must be such in the vein that a supersaturated solution is produced and conditions are favorable for the nucleation and precipitation of the vein filling mineral. Although these general principles are widely accepted, there are many fundamental questions remaining regarding the chemistry that controls these processes. The controlling parameters are far more complex than simple temperature and pressure variations that are readily dealt with by equilibrium thermodynamic models. Answers for many questions reside, at least in a substantial part, in a better understanding of mineral solubility behavior, and precipitation and dissolution kinetics in high ionic strength solutions (brines) typically found in the subsurface. (Fluid inclusions commonly indicate that vein-filling minerals have precipitated from high ionic strength solutions.) We give as an example of the chemical complexities involving mineral reactions in brines the dissolution of calcite. The good news is that the calcite dissolution reaction is close to first order at high ionic strengths. In addition, common inhibitors, such as magnesium, are not very effective in influencing the rate constant, probably as a result of surface site competition. However, the bad news is that the sensitivity of the rate constant to composition increases with increasing carbon dioxide partial pressure and becomes most strongly influenced by total ionic strength. It is hypothesized that this is the result of a depressed water activity in brines that decreases the rate of cation hydration. We also observed that the inhibitory influence of anionic brine components, such as sulfate, were strongly influenced by the concentrations of cations, such as calcium and magnesium, well beyond their impact on activity coefficients. Not unexpectedly, temperature also has a major influence on the rate constant and the relative importance of the other parameters. The multiple parameters influencing the calcite dissolution rate constant were combined into a complex polynomial expression for the dissolution rate constant. Research of similar detail is not as yet available for calcite precipitation kinetics in brines. However, based on extensive studies in seawater and related solutions, there is every reason to expect that the kinetics of precipitation from brines will be even more complex. The initiation of precipitation via nucleation is strongly dependent on solution composition. Seawater, for example, is able to sustain supersaturations of over 20 times that for equilibrium with calcite. The mineral that precipitates at higher supersaturations is the metastable calcium carbonate polymorph aragonite. Additional major complications for crystal growth are the precipitation of "foreign" ions that can influence not only solubility and reaction kinetics, but also crystal morphology.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 3630 Experimental mineralogy and petrology
SC: Tectonophysics [T]
MN: Fall Meeting 2005