HYDRODYNAMIC AND PETROLOGIC CONSTRAINTS ON METAMORPHIC CO2 DEGASSING IN THE FRANKLIN LARGE IGNEOUS PROVINCE, VICTORIA ISLAND, ARCTIC CANADA

It has been proposed that degassing of metamorphic CO₂ during orogenesis or during emplacemement of sills into sedimentary basins can affect the CO₂ budget of the atmosphere. Within the Neoproterozoic Shaler Supergroup on Victoria Island, CO₂ was produced during contact metamorphism caused by gabbro sills. The sills are a component of the widesperead ~720 Ma Franklin large igneous provice, considered to be associated with rifting and break-up of the supercontinent Rodinia. Petrology of contact aureoles and numerical simulations of CO₂-producing metamorphic reactions place constraints on the fluxes of CO₂ out of the Shaler sedimentary basin.

Gabbro sills on Victoria Island range in thickness from a few meters to ~50 m and their metamorphic aureoles are several decameters wide. Protoliths are dominated by calcite and dolomite but contain variable amounts of silicates. Incipient metamorphism is shown by tremolite. Rocks with high carbonate/silicate ratios contain the peak metamorphic assemblage diopside + phlogopite whereas rocks with equivalent amounts of carbonates and silicates contain the assemblage anorthite + garnet + diopside (± vesuvianite). Localization of reactive fluid flow close to sills is shown by narrow spatial extents of δ¹³C and δ¹⁸O shifts.

Modeling of metamorphic assemblages and reactive fluid flow was done for basin permeabilities (k) ranging from 10^–18 to 10^–14 m² and assuming initial hydrostatic conditions. The widths of contact aureoles are best reproduced when k = 10^–17 m². When k ≥10^–16 m², due to significant advective heat flow model contact aureoles are much broader than those observed. When k = 10^–17 m² and while reactions are ongoing, the maximum calculated CO₂ flux rate out of the top of the sedimentary basin is 275 mol/m²/y from the aureole of one 50 m sill. The flux rate is much smaller than rates in several present-day geothermal fields, suggesting there was only a small contribution of metamorphic CO₂ to the atmosphere. Emplacement of more sills at one time would probably not increase the flux rate because the upper sills would act is impermeable barriers to vertical fluid flow from aureoles of lower sills. The results demonstrate that rock permeability must be considered before changes in atmospheric CO₂ budget are attributed to degassing of metamorphic CO₂ from the deep crust.