Hematite isotopic data

Figure 2. Isotopic effects of congruent partial dissolution of hematite. Within the 2a error of the analyses, there is no significant Fe isotope fractionation over wide ranges of percent dissolution. Gray bars denote bulk composition (2a) of the two hematite grains. Data from Skulan et al. (2002) and Beard et al. (2003a).

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. 

Figure 12. Extent of dissolution and re-precipitation between aqueous Fe(III) and hematite at 98°C calculated using Fe-enriched tracers. A. Percent Fe exchanged (F values) as calculated for the two enriched- Fe tracer experiments in parts B and C. Large diamonds reflect F values calculated from isotopic compositions of the solution. Small circles reflect F values calculated from isotopic compositions of hematite, which have larger errors due to the relatively small shifts in isotopic composition of the solid (see parts B and C). Curves show third-order rate laws that are fit to the data from the solutions. B. Tracer experiment using Fe-enriched hematite, and isotopically normal Fe(lll). C. Identical experiment as in part B, except that solution Fe(lll) is enriched in Te, and initial hematite had normal isotope compositions. Data from Skulan et al. (2002).

Figure 19. Plot of oxygen isotope fractionation factor between Fe(lll) oxides (hematite and goethite) and water at low temperatures. Data sources Zheng (1991, 1998) -...

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