Determining the redox conditions of pore fluids during the deposition of organic-rich sediments is an integral part of reconstructing the depositional environment of source rocks. Multiple geochemical tools including degree of pyritization and trace metal enrichment are utilized to evaluate the presence of oxygen and sulfide during deposition. In this publication the measurement of dolomite Fe-content was investigated as a potential tool for depositional environment reconstruction. In anoxic sediments, sulfate reducing bacteria (SRB) metabolize organic matter by reducing sea-water sulfate then release bicarbonate and H2S into pore fluids as byproducts. Pyrite and carbonate minerals, especially dolomite, will precipitate from the interaction of SRB byproducts with ions readily available in the pore system (Ca, Mg, Mn, and Fe). Consequently, the composition of organogenic dolomite will reflect the relative availability of these ions in sediments. Dolomite precipitated in Fe-limited systems (high degrees of pyritization or low detrital input) will be Mg-rich. Conversely, dolomite precipitated in sulfide-limited systems (moderate to low degrees of pyritization or high detrital input) will be Fe-rich.
The viability of Fe substitution in dolomite as a measure of depositional iron availability was tested in the Marcellus Formation. A core from northern Pennsylvania was analyzed using powder X-ray diffraction, X-ray fluorescence, and SEM imaging. The amount of Fe substitution in dolomite was quantified from the unit cell dimensions using X-ray diffraction. Samples with the lowest Fe content in dolomite contained the most total organic carbon (TOC), biogenic silica in thin section, and the highest enrichments of Mo. Samples containing Fe-rich dolomite correlated with moderate TOC, lacked biogenic silica cement, and were moderately enriched in Mo. Preliminary results indicate Fe content in organogenic dolomite does reflect Fe availability during precipitation, however, the absolute amount of Fe substitution is as of yet uncalibrated with degree of pyritization measurements.
The preservation of organic matter in black shale deposits is a result of the combination of multiple depositional mechanisms including rate of primary productivity, flux of detrital material, and redox conditions in sediment pore fluids (Aurthur and Sageman, 1994; Werne et al., 2002; Sageman et al., 2003; Bohacs et al., 2005). The relative importance of each of these mechanisms towards the ultimate preservation of organic matter has been studied using geochemical tools including degree of pyritization, morphology and size-distribution of pyrite, trace metal enrichment, and studies of the stable isotopes of both light and heavy elements (Raiswell et al., 1987; Wilkin et al., 1996; Murphy et al., 2000; Tribovillard et al., 2006; Duan et al., 2010). Detailed analyses of mineralogy are often excluded from geochemical studies despite the presence or absence of certain minerals is the result of chemical reactions occurring during deposition and early diagenesis.
