Hydrochemical impact of a mantle plume recorded by petrology, geochemistry, and U-Pb geochronology of a calcite vein within the Ottawa-Bonnechere graben, Ontario, Canada

Petrology, geochemistry, fluid inclusions and U-Pb dating of a wide (30 cm) calcite vein in Upper Ordovician (Sandbian) limestone characterize a hydrochemical microcosm of changing fluid sources and carbon pathways related to burial, then uplift during the Phanerozoic along the Ottawa-Bonnechere graben, central Canada. U-Pb dating of the host limestone produces an age younger than its chronostratigraphic age suggesting the impact of diagenesis, otherwise well-defined petographically. An interred calcite fragment dated as Cambrian may highlight a platform source in this part of the graben hitherto unrecognized stratigraphically. The vein was initiated with extension across a high-angle narrow (2 cm) fault gouge allowing incursion of hot (80-125 degrees C) basinderived (Mg, Ca)-Cl-2 brine recorded by a saddle dolomite-dedolomite-calcite succession, not isotopically datable. Microbial methanogenesis occurred in initial narrow structural recesses giving way to recycled sedimentary bicarbonate with fracture widening. The oldest datable vein calcite (122 +/- 5 Ma) consists of microcrystalline geopetal cumulates of vadose meteoric origin demarcating renewed carbonate formation following a period of near-surface alteration, dissolution, and microfracturing of the prior vein calcite during uplift. Initial Pb-207/Pb-206 ratios are consistent with average crust (0.819 +/- 0.003), and delta O-18 signatures identify a meteoric signature similar to the host rock. Subsequent interlayering of isopachous calcite and pyrite, with an age of 105 +/- 3 Ma for the calcite, defines onset of a meteoric phreatic environment. The role of microbial sulphate reduction is characterized isotopically. Initial Pb-207/Pb-206 ratios scatter below the crustal average (0.84-0.76) that, along with a meteoric delta O-18 signature, suggest incursion of a new fluid reservoir. A subsequent Sr, Mn, Ba-bearing-magnesium calcite marks the beginning of sustained phreatic meteoric dilution for the remaining vein history. Stable (C, O) isotopes document varying temperature and bicarbonate mixtures of sedimentary and bacterial (sulphate reduction) origins associated with a meteoric fluid. This calcite has a mean age of 94 +/- 2 Ma and displays the widest range of initial Pb-207/Pb-206 ratios, which scatter down to 0.65. Stable and radiogenic isotopic compositions suggest a meteoric setting largely influenced by a crustal fluid reservoir of long residence time and-or transport resulting in leaching of radiogenic minerals. The final vein calcite, also of meteoric origin, has an Sr-87/Sr-86 ratio similar to that of the initial basin brine, yet with C-13 signatures that characterize a source of oxidized methane. U-Th series analysis delimits an age older than 0.5 Ma. This stage of vein calcite likely arises through meteoric fracking of Ordovician source rocks during continued basin inversion. In summary, initial brine influx followed by a shallow-crustal meteoric realm document structurally focused fluid migration with and following early Mesozoic graben exhumation. The phreatic meteoric regime of Early Cretaceous age spans a similar to 20 Ma period coincident with transient magmatism arising from near passage of the Great Meteor Hotspot. Regional paleohydmlogy appears to have been influenced for similar to 10 Ma after plume passage.