BARIUM ISOTOPE CHARACTERIZATION OF HANOVER SHALE CARBONATE AND BARITE NODULES AND IMPLICATIONS FOR POST-DEPOSITIONAL FLUID FLOW IN THE APPALACHIAN BASIN

Calcium carbonate and barite (BaSO₄) nodules in Upper Devonian marine shales of the Hanover Formation in the Appalachian Basin, USA, record multiple changes in fluid rock interaction at the sulfate methane transition zone (SMTZ). Barite and carbonate nodules of the Hanover Shale were collected from an outcrop in western NY representing an approximate stratigraphic range of 3m. Petrographic analysis of barite nodules showed “featherlike” barite (~10 mm) radiating from a core of euhedral barite crystals. Barite rims (2-12 mm thick) encasing carbonate nodules transitioned from euhedral barite near the barite-carbonate contact to a more radial barite near the outer edge of the rim. This textural change could reflect an increase in barite supersaturation from increased sulfate or Ba concentration [Shikazono, N. 1994, Geochim. Cosmochim. Acta, 58, 2203-2213]. Barium isotope analysis was conducted on crushed carbonate nodules, barite rims on carbonate nodules, and barite nodules. δ¹³⁸Ba values (¹³⁸Ba/¹³⁴Ba normalized to NIST standard 3104a) for the carbonate nodules ranged from 0.32‰ to 0.74‰ with no obvious trends vertically along the section or correlated with Ba concentration. δ¹³⁸Ba values for the barite nodules ranged from 0.20‰ to 0.63‰ with a general increase in δ¹³⁸Ba with stratigraphic height. Within one barite nodule, the center crystals had a δ¹³⁸Ba value of 0.20‰ and the radial rim was 0.32‰ which is consistent with closed-system fractionation of the source fluid due to barite precipitation. Barite rims which encased carbonate nodules ranged from 0.37‰ to 0.44‰, distinct from the carbonate cores. Variability of δ¹³⁸Ba values across barite and carbonate samples suggests carbonate and barite precipitation occurred at different times and also suggest that the δ¹³⁸Ba composition of the fluid changed between precipitation of each horizon. Changes in barite morphology and δ¹³⁸Ba values suggest that a source of Ba rich fluid was introduced episodically, possibly alongside upward diffusing methane sourced from underlying Devonian to Ordovician shales such as the Marcellus or Utica Shales, which led to movement of the SMTZ and the formation and preservation of the barite.