electron microprobe (determination of major cation content) and mass spectrometry (delta13 C and delta18 O analysis). The siderite was interpreted as having formed from metallic iron partly protected from free oxygen access by wood tar with delta13 C = -24 0/00 and from biogenic CO2 produced from this organic matter. Inasmuch as the siderite has highly positive delta3 C values ranging from 8.05 to 15.41 0/00, we deduced that extremely isotopically heavy CO2 was generated in the process of biogenic decomposition of the organic matter. It was found that the delta18 O values of siderite (26.58 to 27.74‰ vs. VSMOW) fit very well to the recalculated curve of Becker (1971) [formula] and to published extrapolated experimental data.
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