Paper No. 6
Presentation Time: 2:20 PM


NABELEK, Peter I., Geological Sciences, University of Missouri, 101 Geological Sciences Bldg, Columbia, MO 65211, BÉDARD, Jean H., Geological Survey of Canada, 490 rue de la Couronne, Quebec, QC G1K 9A9, Canada and RAINBIRD, Robert, Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Room 499, Ottawa, ON K1A 0E8, 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.