Siderite isotopic data

On the basis of the isotopic data obtained, the hypothesis of chemogenic deposition of cherty iron sediments of complex composition in sea basins seems to be most applicable. Only by such a process was it possible for compounds to form which already differed sharply in oxygen isotopic composition—oxides (goethite or hematite, magnetite) and carbonates (siderite, sideroplesite)—in the original iron sediment. 

In other districts, for instance in the KMA, reduction of oxide sediment to magnetite and siderite by organic matter probably was of great ore-forming importance (Plaksenko, 1966). Isotopic data on this district are still insufficient for definite conclusions in two out of three analyses (see Fig. 96) the S 0 of coexisting oxides are similar, and in one the magnetite oxygen is... 

The REE patterns of the two siderites (Nos 15 and 16 in Fig. lOA) are alike. Along with the O, C and Sr isotope data (see above), this confirms that the chemical and isotopic compositions of siderite were controlled by fluid composition at a regional scale. 

Carothers, 1992 Spiro et al., 1993 Morad et a ., 1994), The mineral is most suitable for the study of pore-water evolution during sediment subsidence because, unlike other carbonate minerals, siderite probably does not undergo recrystallization and isotope re-equilibration during burial diagenesis, as it has no unstable precursors or polymorphs (Curtis etai, 1975 Gautier, 1982 Pearson, 1985 Curtis Coleman, 1986). Therefore, stable isotope data of siderite cements can provide a powerful tool for the interpretation of diagenetic events in geological... 

Table 3. Carbon and oxygen isotope data of the Tirrawarra Sandstone siderite cements. A good match is observed between measured (6 0 5as) and calculated oxygen isotope values determined from image analysis results.

The isotope data reflect the varying proportions of the different generations of siderite cement ... 

Figure 10. Comparison of isotopic fractionations determined between Fe(II)aq and Fe carbonates relative to mole fraction of Fe from predictions based on spectroscopic data (Polyakov and Mineev 2000 Schauble et al. 2001), natural samples (Johnson et al. 2003), DIR (Johnson et al. 2004a), and abiotic formation of siderite under equilibrium conditions (Wiesli et al. 2004). Fe(II)aq exists as the hexaquo complex in the study of Wiesli et al. (2004) hexaquo Fe(II) is assumed for the other studies. Total cations normalized to unity, so that end-member siderite is plotted at Xpe = 1.0. Error bars shown reflect reported uncertainties analytical errors for data reported by Johnson et al. (2004a) and Wiesli et al. (2004) are smaller than the size of the symbol. Fractionations measured on bulk carbonate produced by DIR are interpreted to reflect kinetic isotope fractionations, whereas those estimated from partial dissolutions are interpreted to lie closer to those of equilibrium values because they reflect the outer layers of the crystals. Also shown are data for a Ca-bearing DIR experiment, where the bulk solid has a composition of q)proximately Cao.i5Feo.85C03, high-Ca and low-Ca refer to the range measured during partial dissolution studies (Johnson et al. 2004a). Adapted from Johnson et al. (2004a).

Fig. 8. Histograms of siderite carbon isotopic compositions (A) Middle and Upper Jurassic reservoir siderite data combined (B) Middle Jurassic reservoir siderite data (C) Upper Jurassic reservoir siderite data.

Table 3 shows that the measured and calculated bulk-rock 6 0 values generally exhibit a good correlation (r = 0.82). Minor discrepancies between the two data sets are probably due to small errors in the estimation of the volume of individual cement generations in some samples. The bulk-rock 6 C composition of each sample is also controlled by the relative proportion of each generation of siderite cement, and the same mathematical formula as shown above (substituting 6 C for 6 0) can be applied. However, the correlation coefficient for calculated 6 C values (r = 0.64) is lower than the one for calculated 6 0 compositions. We are uncertain about the reason(s) for this phenomenon, but believe it may be due to the fact that 6 in a cement, at any given pore-water isotope composition, is a strongly temperature-dependent variable. 

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