Thermodynamic and geochemical investigation into the fate of graphitic carbon and sulfides during orogenesis, from a field-based approach of the Proterozoic Ottawan orogeny, ON, Canada

Metamorphic degassing has turned out to be an often-overlooked flux when considering the global carbon and sulfur cycles. Sediments containing organic matter and sulfidic minerals like pyrite undergo the very slow and diffuse process of metamorphic decarbonation and desulfidation. This makes it very difficult to quantify the amount of carbon and sulfur being released into the atmosphere via surface measurements, yet both of these fluxes can have profound effects on planetary habitability over time. The release of carbon and sulfur into the atmosphere also have opposite effects on the global temperature and operate on different timescales. While CO₂ with a residence time of ~ 4 years in the atmosphere (Harde, 2017) has an overall warming effect, any SO₂ released has a net cooling effect and stays in the troposphere for anywhere between a few hours to 14 days. Depending on the rate of devolatilization, the net effect on habitability will shift.

This study focuses on the Flinton Group, which is a part of the Mazinaw Domain in Ontario, Canada and was deposited between 1180 – 1150 Ma (Kinsman and Parrish, 1990; Sager-Kinsman and Parrish, 1993). Special emphasis is given to the Myer Cave Formation for this study, the lithology of which is defined by sulfidic and graphitic schists and pelites, calcitic and dolomitic marbles and marble clast breccias (Easton, 2006). The mineralogies of the schists can be characterized by quartz, dolomite, calcite, feldspars, muscovite, biotite, hornblende, sillimanite, graphite, pyrite and pyrrhotite along with traces of retrograde chlorite. Unlike the rest of the region that has undergone two phases of metamorphism, the Flinton Group seems to have been subjected to a single medium to high grade event at 976 ± 4 Ma (McCarron et. al., 2014).

The oxidation of graphitic carbon and its release into the atmosphere in the form of CO₂ is recorded as a negative shift in the δ¹³C values of reduced carbon (RC)/graphite. We find that the δ¹³C_RC values in the Flinton Group remain fairly consistent within the range of –19‰ to –25‰ on the Vienna Pee Dee Belemnite (VPDB) scale. Assuming a protolith with  δ¹³C_RC  ~ –25‰, this suggests minimal mobilization of graphite across greenschist and amphibolite facies. Thermodynamic models show much earlier equilibration in temperature and activity of CO₂ (a_CO2) space. The sulfur isotope to be analyzed is the stable isotope ³⁴S, which on the Vienna Canyon Diablo Troilite (VCDT) scale may show enriched values of ~10‰ to 40‰ or even more depending on the mass-based fractionation as well as the source of the deposits. Thermodynamic models in temperature vs sulfur fugacity (f_S2)  space will also bring to light the equilibration conditions and mobility of sulfur.