Dolomites carbon isotope ratios

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. 

Figure 7.9. Summary of carbon and oxygen isotope ratios in sedimentary dolomites. (After Land, 1980.)...

Fig. 9. Compilation of Sr isotope ratios of saddle dolomite. Horizontal arrows indicate studies that reported only a range of values for saddle dolomite occurrences in carbonate rocks. The total range of Phanerozoic seawater Sr/ Sr ratios is shown for comparison (Smalley et at.,...

The overall trend calculated in carbon isotopes is toward heavier isotope ratios for carbonate in solution after sulfide oxidation and neutralization. Dolomite dissolution introduces heavier carbon ((5 C = -17oo i )- Calcite precipitation reverses this trend by preferentially removing = 11.8 to 27oo Ht urc, where e, is the isotopic fractionation between calcite precipitated (p) and (s) the solution), but the mass transfer for this step tended to be small. The effect of CO outgassing above pH = 5.5 is to make the in solution heavier by preferentially removing light carbon = 0.5 to -9.07oo where is the isotopic... 

A generally positive covariation between 6 0 and S C is apparent across the data set as a whole (Fig. 10). However, covariations between 6 0 and 6 C and other parameters, e.g. carbonate content, trace element content or trace element ratios, are absent or very weak. Oxygen isotopic values for both early calcite and early dolomite, combined with temperature constraints inferred from IGVs, are consistent with precipitation from mostly 0-depleted fluids (Fig. 11). Ferroan dolomite and late... 

The Sr isotope database for saddle dolomite consists of 106 analyses plus a few minimum and maximum values for datasets that were not published in full. Note that none of the sandstone studies reported Sr isotope data, and only four studies of carbonate-sandstone mixtures contain such data (Table 1). As a consequence, the current dataset is of limited significance. Nevertheless, the data depicted in Fig. 9 reveal some interesting aspects. With a single exception, all of the saddle dolomites are more radiogenic than 0.708 and about one-third of the dolomites show ratios > 0.710. Values in excess of 0.7092 reflect precipitation from Sr-enriched pore waters rather than initial (syndepositional) seawater (e.g. Smalley el al., 1994). The current Sr isotope database is biased toward samples from lower Palaeozoic formations (Table 1), a period characterized by relatively high marine Sr/ Sr ratios ( 0.7082-0.7092). 

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