The isotopic and chemical evolution of mineralization in septarian concretions: Evidence for episodic paleohydrogeologic methanogenesis

Detailed chemical and isotopic studies of mineralization of septarian fractures in concretions in the Devonian Marcellus Shale document a complicated paragenesis involving multiple episodes of metal-carbonate precipitation during methane fermentation. The first episode of carbonate precipitation under conditions of methanogenesis occurred with the formation of a final stage of calcite near the end of diagenesis. The second episode involved the precipitation of strontianite in voids in the septarian mineralization volume that were open to ground water. The third episode involved the replacement of barite by witherite in an external zone of each concretion.

The sequence of 4 calcites and ferroan dolomite that formed during diagenesis is marked by a pronounced enrichment in ¹³C from δ¹³C of about −14‰ for the first calcite, which lines the septarian fractures, to +13 ‰ for the final calcite, which crystallized in vugs in the ferroan dolomite. Barite precipitated concurrently with early calcite phases during the initial stages of diagenesis. Modest pyrite precipitation accompanied the first two calcite stages but ceased before the precipitation of ferroan dolomite. This sequence of mineralization is consistent with successive periods of sulfate reduction and methane fermentation.

Strontianite precipitated in open spaces in earlier mineralization that were open to external solutions through later cracks paralleling septarian fractures. The δ¹³C of the strontianite is enriched to about + 15 ‰, consistent with its precipitation during methane fermentation. The δ¹⁸O of the carbonate at −11.2 ‰ is significantly depleted compared to −5.4 ‰ for the final calcite, however, suggesting that the strontianite precipitated from a different solution than did the preceding carbonates.

Witherite replacements of barite in fractures in mineralized septarians form an external zone around a core of unaltered barite in each concretion. The solution of barite suggests sulfate reduction, and the +8.4‰ δ¹³C of the witherite suggests precipitation during methane fermentation. The −14.5‰ δ¹⁸O of the witherite is consistent with its precipitation from a third solution.

It is proposed (1) that a first period of methane fermentation occurred during burial and diagenesis in the Devonian, (2) that a second period occurred as strontium-bearing Silurian brines were displaced upward near the toe of Pleistocene glaciation, and (3) that witherite replaced barite during sulfate reduction and another period of methane fermentation in a Pleistocene mixture of ground water and glacial melt water under reducing conditions below thick ice cover. Fluid-inclusion data, oxygen-isotope values of carbonate minerals, and strontium-isotope ratios are all consistent with these suggestions.

This study demonstrates that precipitation of carbonate minerals during methane fermentation is of greater significance than previously thought. Not only is methane fermentation important in dolomite formation, but it is also an important process in the paragenesis of other metal carbonates such as strontianite and witherite.