Saddle dolomite precipitation

Many authors reported paragenetic relationships that allow the relative timing of saddle dolomite precipitation within a given rock unit to be determined. There is general agreement among these... 

The 6 0 values of saddle dolomite range from -20.1 to -1.6%o, with a mean of -9.4 2.9%o (n = 838 Fig. 7). No relationship exists between the oxygen isotopic composition of saddle dolomite and host-rock lithology, suggesting that saddle dolomite precipitation occurred over a similar temper-... 

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.)...

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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