Carbonate minerals stability

The rainwater of Bermuda is in near equilibrium with atmospheric Pc02 = 10-3.5 atm., and contains small amounts of sea salt (0.07 wt. % seawater). The rainfall of 147 cm y1 is seasonally distributed. The rain enters the saturated zone by two main paths direct rainfall on marshes and ponds, and percolation downward from the vadose zone as vadose seepage and flow through rocks during times of soil water excess (Vacher, 1978). Total annual recharge of the saturated zone is about 40 cm y-1 (Vacher and Ayers, 1980). The residence time of the groundwater has been calculated as 6.5 years, and the average age of the sampled water as 4 years (Vacher et al., 1989). Such estimates are necessary for calculations of carbonate mineral stabilization rates, as shown in a later section. 

Because of all the data available concerning Bermudian limestone diagenesis, several authors (Lafon and Vacher, 1975 Plummer et al., 1976 Vacher, 1978 Vacher et al., 1989) have attempted to determine rates of carbonate mineral stabilization in Bermudian calcarenites and the mass transfer involved in the process. 

Pierson, B.J. Shinn, E.A. (1985) Cement distribution and carbonate mineral stabilization in Pleistocene limestones of Hogsty reef, Bahamas. In Schneidermann, N. Harris, P.M. (Eds) Carbonate Cements. Special Publication 36. Tulsa, OK Society of Economic Paleontologists and Mineralogists, pp. 153-168. 

As temperatures increase (onset between 100 and MO C), abiotic sulfate reduction by oxidation of hydrocarbons becomes kinetically important. This process leads to the release of H2S and bicarbonate to formation waters (25.81). At this time, aluminosilicate alteration to clay minerals may occur carbonate mineral stability will be a function of CO 2, Fe and S availability, as will pyrite precipitation (25.81 and Figure 4). 

We will discuss the processes determining carbonate mineral stability in each of the progressive burial zones (shallow, intermediate, and deep for a more detailed description of these zones, see Surdam et al. 1989d). 

We will determine if there are systematic and predictable chemical relationships between the vertically stacked carbonate cementation/decemen-tation zones, and the relationship between organic acids and carbonate mineral stability. 

We believe that the probability of significantly enhancing effective porosity (generating a positive porosity anomaly) during burial diagenesis is largely a function of carbonate mineral stability. 

Figure 5.12 Relative chemical stability of carbonate minerals...

Pascual J.A., Hernandez T., Garcia C., Ayuso M. Carbon mineralization in an arid soil amended with organic wastes of varying degrees of stability. Commun Soil Sci Plant Anal 1998 29 835-846. 

A basic premise of solubility considerations is that a solution in contact with a solid can be in an equilibrium state with that solid so that no change occurs in the composition of solid or solution with time. It is possible from thermodynamics to predict what an equilibrium ion activity product should be for a given mineral for a set of specified conditions. As will be shown later in this chapter, however, it is not always possible to obtain a solution of the proper composition to produce the equilibrium conditions if other minerals of greater stability can form from the solution. It shall also be shown that while it is possible to calculate what mineral should form from a solution based on equilibrium thermodynamics, carbonate minerals usually behave in a manner inconsistent with such predictions. 

Figure 6.1 shows equilibrium relations between stable (A) and metastable (B) carbonate minerals. The boundaries between stability fields are most easily obtained by consideration of the mass action equations representing mineral compatibilities in terms of the variables shown in Figure 6.1. For example, consider the phase boundaries in Figure 6.1 A the reactions and the equilibrium constants for these boundaries in terms of the ratio of activity of Ca2+ to activity of Mg2+ and PCO2 are 3s follows ...

The rate at which metastable phases dissolve or are replaced is an important problem in carbonate diagenesis. Carbonate mineral assemblages persist metastably in environments where they should have altered to stable assemblages. The question is "what are the time scales of these alterations" They are certainly variable ranging from a few thousand to a few hundreds of millions of years. Even calcites in very old limestones show chemical and structural heterogeneities, indicating that the stabilization of these phases is not complete. Unfortunately, it is difficult, but not impossible, to apply directly the lessons learned about carbonate mineral dissolution and precipitation in the laboratory to natural environments. 

Aside from mineral stabilities, the behavior of the CO2-H2O system with increasing P and T is also important to an understanding of the deep burial diagenesis of carbonate rocks. One reaction of interest, which represents the summation of K0 and Ki (see Chapter 1) for the carbonic acid system, is... 

Analysis of Ca, Mg, Fe, Mn and Sr in calcite, ankerite and siderite was performed on a JEOL 733 electron microprobe. Accelerating voltage was 15 kV sample current was 12 nA, stabilized on brass. Spot size was 10 pm. Counting time for all elements was 20 s, except for Sr, which was analysed for 60s. Detection limits are approximately 340 ppm for Mg, 450 ppm for Fe, 310 ppm for Mn and 185 ppm for Sr. Totals between 97 and 103% were accepted. Standards were carbonate minerals (calcite for Ca dolomite for Ca, Mg siderite for Fe, Mn and coral for Sr) in the standard collection at the University of Texas electron microprobe laboratory. Beam placement was guided by back-scattered electron imaging. Si was routinely counted by WDS to check for possible contamination from... 

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