Carbon isotope values, saddle dolomite

Fig. 5. Stable isotopic compositions of saddle dolomite. Note the tendency toward negative carbon isotope values from siliciclastic settings. Data from Friedman (1989) could not be included because the author did not report 6 - C values, and data from Woronick Land (1985) are not included because of inadequate data presentation. Data from Wojcik et al. (1992) on saddle dolomite and ankerite are plotted in the limestone and sandstone categories, rather than the mixed category (see Table 1).

Fig. 6. Frequency histogram of 6 C values of saddle dolomite. Note that a significant portion of the samples, particularly saddle dolomite in carbonate host rocks, overlaps the carbon isotopic composition of Phanerozoic seawater (see Lohmann Walker, 1989).

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). 

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.,...

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