Dolomite isotopic composition

Figure 1.31. Bivariable plot of oxygen versus carbon isotopic compositions of carbonates. Solid circle magnesite open circle dolomite open square calcite A oxygen and carbon isotopic compositions of igneous carbonates B oxygen and carbon isotopic compositions of marine carbonates (Shikazono et al., 1995).

Fig. 19.2. Isotopic composition (bold lines) of dolomite formed by reaction between a limestone and migrating groundwater, assuming that minerals maintain isotopic equilibrium over the simulation. Fine lines show results of simulation holding minerals segregated from isotopic exchange, as already presented (Fig. 19.1).

Since we have no direct information about the chemistry of the Fountain fluid, we assume that its composition reflects reaction with minerals in the evaporite strata that lie beneath the Lyons. We take this fluid to be a three molal NaCl solution that has equilibrated with dolomite, anhydrite, magnesite (MgCC>3), and quartz. The choice of NaCl concentration reflects the upper correlation limit of the B-dot (modified Debye-Hiickel) equations (see Chapter 8). To set pH, we assume a CO2 fugacity of 50, which we will show leads to a reasonable interpretation of the isotopic composition of the dolomite cement. 

We can predict the oxygen and carbon isotopic compositions of the dolomite produced by this reaction path, using the techniques described in Chapter 19. Figure 25.4 shows the compositions of calcite and dolomite cements in the Lyons, as determined by Levandowski et al. (1973). The calcite and dolomite show broad ranges in oxygen isotopic content. The dolomite, however, spans a much narrower range in carbon isotopic composition than does the calcite. 

Fig. 25.4. Oxygen and carbon stable isotopic compositions of calcite ( ) and dolomite ( ) cements from Lyons sandstone (Levandowski et al., 1973), and isotopic trends (bold arrows) predicted for dolomite cements produced by the mixing reaction shown in Figure 25.3, assuming differing CO2 fugacities (25, 50, and 100) for the Fountain brine. Fine arrows, for comparison, show isotopic trends predicted in calculations which assume (improperly) that fluid and minerals maintain isotopic equilibrium over the course of the simulation. Figure after Lee and Bethke (1996).

Lead, Sr, C and O isotope compositions were measured in carbonates of the dolomitic unit and in chalcocite concentrate. U-Pb and K-Ar radiometric dates were obtained from basement rocks and mafic dykes. The analysis were undertaken and The University of Sao Paulo under supervision of Professor C. Tassinari (Tassinari 2002). 

The dolomites are depleted in and O relative to normal marine carbonates, having values of S C(PDB) between -5 and - 4%o and 5 0(smow) between 23 and 25%o. This could confirm a change of isotopic composition of carbonates produced by an external fluid. 

The lead isotope ratios indicate an upper cmstal source for the lead and are consistent with mineralization at 800 Ma. Tracer isotopes in carbonates and chalcocite concentrates indicate that an external fluid altered the dolomitic unit changing its isotopic composition as well as deposited copper mineralisation. 

Fig. 3.45 Carbon and oxygen isotope compositions of some recent and Pleistocene dolomite occurrences (after Tucker and Wright 1990)...

Figure 8.16. A hypothetical trend of changes in the stable isotope composition of carbonate cements in different diagenetic environments. A- marine realm B-meteoric realm C- mixing zone D- successively deeper burial for calcite spar E-successively deeper burial for saddle dolomite. B through E are precipitated in progressively hotter waters. (After Choquette and James, 1987.)...

Hemzacek 1. M., Perry E. C., Jr., Larue D. K., and Eeng J. (1982) Sulfur isotope composition of sulfate in chert horizons of the Proterozoic (Precambrian X) Kona Dolomite, Marquette Region, Michigan, GSA Abstracts with Programs, New Orleans. 

Fig. 20. Range of temperature and isotopic composition of the pore fluids constrained for the precipitation of the analysed dolomites/ankerite (5 0 = -11.9%o to -1.5%o). Field marked for 5 0 a[er between -5 and -3%o smow (fractionation equation after Rosenbaum Sheppard, 1986).

Table 7. Sr concentration and Sr isotopic composition of selected calcites and ferroan dolomites...

The oxygen and carbon isotopic compositions of early dolomite in the central basin have 5 OpuB between about 0 and -3.5%o and between... 

Fig. 5. Oxygen and carbon isotopic composition of early-formed dolomite cements in sandstones of the central San Joaquin basin.

Fig. 6. Isotopic composition of calcite cements from North Coles Levee, South Coles Levee and Canal fields. North Coles Levee dolomite cements shown for comparison. North Coles Levee data from Schultz et al. (1989) South Coles Levee and Canal data from Table 2. Increasingly negative 5 OpDB and lower Sr ratios are correlated with higher temperatures of crystallization. Strontium ratio decrease is attributed to Sr from plagioclase alteration. Shaded box shows calculated composition of calcite in equilibrium with present pore water at Coles Levee fields based on fluid temperature and composition in Fisher Boles (1990) and Sr isotopic data in Feldman et al.( 993).

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