STABLE ISOTOPE GEOCHEMISTRY OF CALCITE AND DOLOMITE VEINS FROM ORDOVICIAN LIMESTONES FROM THE NITTANY ANTICLINE, VALLEY AND RIDGE PROVINCE OF PENNSYLVANIA

Stable isotope analysis provides insight on the paleo-fluid systems and associated fluid fracture interconnectivity during deformation. 17 host rock samples and 49 vein samples were collected from Ordovician limestones from the Nittany anticline in the outer arc of the Pennsylvania Salient.

The veins are <1 to >20 mm thick and consist of one or more stages of blocky calcite and blocky dolomite. The host rock samples have δ¹³C values which span a wide range (-4.0 to 3.5 ‰), and are significantly different from typical Ordovician marine limestones. However, the δ¹⁸O values of the host rocks span a narrow range (-9.2 to -4.9 ‰), and are similar to typical Ordovician marine limestones.

The sample sites can be divided into three areas based on structural position; the southern fold plunge-out, the western overturned limb, and the Linden culmination of the Nittany anticline. The sample sites from the southern plunge-out exhibit open system behavior with vein isotopic values significantly different than the host rock. The δ¹⁸O values are 2-3 ‰ lower compared to the host rock, and the δ¹³C values show little to no change. Three veins have δ¹⁸O data 1-2‰ higher than the host. All veins are primarily northwest striking and fault veins. Sites on the western overturned limb exhibit systems that are more closed in comparison to the southern plunge-out. The δ¹⁸O values are ~1 ‰ greater and the δ¹³C values are 1-1.5 ‰ lower than the host rock. Two sites demonstrate much more open system behavior with 2-3 ‰ higher δ¹⁸O and a 1-3 ‰ lower δ¹³C values. Only two sites were sampled from the Linden culmination, which have veins similar to each other; 1.5 ‰ lower δ¹⁸O and 3.0 ‰ lower δ¹³C than the host. In summary, the vein isotopic data indicate that there was moderate fluid migration and fluid connectivity during the formation of the Nittany Anticline.

Fluid inclusion microthermometry the homogenization temperatures from fluid inclusion microthermometry indicate fluid temperatures of 141 to 168 °C, with salinities of 14.0 to 24.0 wt. % NaCl equiv. The extent of ¹⁸O fractionation between water and calcite is temperature dependent. Calculating the oxygen isotopic values of fluid in equilibrium with the calcite gives δ¹⁸O values that range from +7.9 to +13.7 ‰ (V-SMOW). This indicates basinal brines are the source of the fluids that precipitated the vein calcite.