INSIGHTS INTO BARIUM DIAGENESIS USING IN SITU S ISOTOPE MEASUREMENTS OF BARITE AND PYRITE IN MARCELLUS SHALE

Devonian Marcellus Shale (ca. 384 Ma) drill core from Beaver Meadows, NY shows ellipsoid-shaped grains with compositions that range from pure barite to intermediate barite-pyrite to pure pyrite over a depth of a mm to cm in the sample. Mineralogical characterization correlated with in-situ S isotope analysis (Secondary Ion Mass Spectrometry, or SIMS) was performed to better understand the diagenetic conditions under which these grains formed. Grain sizes as well as the replacement texture present in grains containing barite and pyrite suggest pyrite replacement of diagenetic barite. The median δ³⁴S of the coexisting barite and pyrite are 63.82 ± 0.45‰ and 1.69 ± 0.34‰, respectively. The median Δ_{sulfate-sulfide}= 62.13‰ is greater than the 2-46‰ fractionation associated with bacterial sulfate reduction. Mass balance calculations indicate that an additional sulfur source with δ³⁴S= −16.4‰ must mix with the sulfate to produce the lighter coexisting pyrite and maintain the grain’s volume. Seventy-seven percent of the coexisting pyrite’s sulfur derives from this source with the remaining 23% coming from the replaced barite. Based on our results, we suggest the following model for the formation of the observed barite-pyrite grains in Marcellus Shale: 1) dissolution of biogenic barite below the sulfate-methane transition zone (SMTZ) due to the depletion of pore water sulfate, 2) Ba²⁺ transport and reprecipitation of barite (authigenic) at the SMTZ, 3) burial of authigenic barite and partial dissolution below the SMTZ, 4) movement of the SMTZ (possibly due to variable methane flux) below the depth of the partially dissolved barite, and 5) precipitation of pyrite in the pore volume created by barite dissolution. The proposed model suggests that the ellipsoid-shaped grains containing barite and pyrite form at the SMTZ. The preservation of authigenic barite fronts suggests the presence of a SMTZ, and thus oxic or suboxic conditions in the water column during early diagenesis.