Pyrite metamorphism in the Devonian Hunsruck Slate of Germany: Insights from laser microprobe sulfur isotope analysis and thermodynamic modeling

The Devonian Hunsruck Slate of Alday, Rhenish Massif, Germany, contains an exceptional spectrum of pyrite textures, recording progressive recrystallization and remobilization of sedimentary/diagenetic pyrite during very low grade metamorphism. Sedimentary/diagenetic pyrite is very fine-grained and forms distinct enrichments, including thin layers, elongate concretions, and pyritized fish coprolites. These pyrites show abundant idioblastic overgrowths, which are very coarse-grained and are always located at the interface between the sedimentary/diagenetic pyrite and the slate matrix. Progressive recrystallization resulted in the formation of porphyroblasts and an almost complete obliteration of the original internal structures. A detailed, texturally-resolved sulfur isotope study of the different pyrite types has been carried out, using an in situ laser microprobe technique. The 8345 values of sedimentary/diagenetic pyrite, idioblastic overgrowths, and porphyroblasts are very homogeneous and lie in the range of +7.9 to +12.8 permil (mean: +10.6%), +8.6 to +12.4 permil (mean: +10.8%), and +8.3 to +12.1 permil (mean: +10.3%), respectively. The sulfur isotope data for each population and the isotopic relationships on the sample scale indicate that metamorphic recrystallization has not resulted in a significant modification of the original isotopic composition. Calculation of phase relations and modeling of fluid-rock equilibria place important constraints on the conditions and mechanisms of pyrite remobilization during metamorphism. The calculations show that pressure variations have no detectable influence on pyrite solubility, indicating that pressure solution was not important in the metamorphic mass transfer of pyrite. The effects of temperature and oxidation state are more pronounced, and infiltration of fluids originating from deeper crustal levels could have caused the remobilization of pyrite. Modeling of the isotopic fractionation shows that remobilization driven by interaction with such fluids having a higher temperature and oxidation state would have resulted in a significant negative shift of the 8345 values, which is not consistent with the observed isotopic pattern. We envisage an alternative model that involves fluid-assisted reduction of the grain surface area (and surface energy) per unit volume of pyrite as the dominant mechanism driving recrystallization and porphyroblast growth. The difference in surface energy between the fine-grained sedimentary/diagenetic and the idioblastic pyrite resulted in establishment of a small-scale chemical potential gradient, which evolved along a self-enhancing path during progressive idioblastic growth. The results of our modeling have implications for the preservation of primary sulfur isotopic signatures and emphasize the importance of constraining the history of fluid-rock interaction for a meaningful interpretation of sedimentary/diagenetic processes in pyrite-rich metasediments.