130-3 A Thermodynamic and Geochemical Investigation into the Mobility of Graphitic Carbon During Orogenesis: A Field-Based Study from the Proterozoic Ottawan Orogeny, ON, Canada

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part II

Presenting Author:

Authors:

Abstract:

A significant but often overlooked carbon flux into the atmosphere comes from the metamorphism of sediments containing carbon in the form of sedimentary organic matter and graphite (Zhang et. al., 2018). This devolatilization is a relatively slow and diffuse process, which makes it challenging to quantify the amount of carbon released via surface measurements. Nevertheless, under the right conditions, such metamorphic devolatilization could be high enough to have a substantial impact on global climate and the habitability of the planet. 

This study looks at field observations made in the Flinton Group in the Mazinaw Domain, Ontario, Canada, which has a depositional age between 1180 – 1150 Ma (Mesoproterozoic) (Kinsman and Parrish, 1990; Sager-Kinsman and Parrish, 1993). We focus in particular on the Myer Cave Formation, which has a composition that is predominantly characterized by graphite-bearing pelitic schists and mixed graphite-rich dolomitic calc-silicates. The mineral assemblages commonly include quartz, feldspar, hornblende, biotite, muscovite, graphite, pyrite, calcite, dolomite, sillimanite, and occasional retrograde chlorite. This unit is believed to have undergone a single, medium to high grade metamorphic event at 976 ± 4 Ma (McCarron et. al., 2014). 

Metamorphic processes may cause graphitic carbon to be oxidized and mobilized in the form of CO₂ and released into the atmosphere. This process is often recorded in the form of a negative shift in the δ¹³C values of reduced carbon (RC), reflecting isotopic fractionation associated with devolatilization. Isotopic analysis of samples has shown fairly constant δ¹³C_RC values ranging from –19‰ to –26‰ (VPDB) across greenschist facies to upper amphibolite facies, while the initial value of protolith sediment is accepted to be around –25‰. This range of values, along with constant wt% C across increasing grade, suggests minimal mobilization of the graphitic carbon. Thermodynamic activity of CO₂ (a_CO2) vs temperature models of amphibolite facies samples show lower than expected equilibration temperatures of ~450°C at low a_CO2 conditions. The observed assemblages are also low variance, which could be an indication of rock-dominated buffering of the fluid composition. These preliminary results suggest that a low fluid:rock ratio may have contributed to relatively closed-system metamorphism and suppression of decarbonation. Further work will more precisely quantify the amount of graphite being mobilized and estimate a net flux from out of the Myer Cave formation. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9518

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