OBSERVATIONAL CONSTRAINTS ON CONTACT METAMORPHIC DECARBONATION DURING EMPLACEMENT OF THE CENTRAL ATLANTIC MAGMATIC PROVINCE, NORTH AMERICA

Large Igneous Provinces (LIPs) are associated with dramatic changes to the Earth System, relating to at least three of the “Big Five” mass extinctions and many lesser biotic crises. LIPs have large volumes and short emplacement times causing a rapid flux of carbon to the surface, inducing climate change and creating biotic stress. While these greenhouse gases are associated with volcanism, carbon is also devolatilized from sediment as dikes and sills interact with already deposited host rock. As sediment is heated in high-grade metamorphic contact aureoles, nearly 100% of the sedimentary carbon may be released through decarbonation (Heimdal et al., 2020, 2021).

While numerous projects have modeled the metamorphic carbon flux, we aim to apply an observation-based approach to track and quantify the release of methane and CO₂ from heated sediment. Here, we consider the metamorphic carbon flux due to the emplacement of the Central Atlantic Magmatic Province (CAMP), coincident with the end-Triassic mass extinction. Samples were collected from three Triassic basins spanning from Florida to Connecticut. We present preliminary results from Florida Geological Survey borehole W-1789. Drilled in the north Florida basement, it intersects Paleozoic organic-rich shales and two small (~10 m) CAMP sills. Isotopic measurements of d¹³C_org and total organic carbon (TOC) along with d¹³C_carb and percentage of calcite record upward transport of carbon from the bottom to the top of a sill with evident reprecipitation and capture as calcite. Raman spectroscopic and reflectance measurement studies on primary organic matter, preserved in chitinozoa, reveal samples reached moderate metamorphic temperatures and underwent partial graphitization. Phase equilibria modeling in Theriak-Domino further demonstrates the importance of metamorphic fluids in heating and carbon release, with temperatures in excess of 440°C and an activity of methane of at least ~0.05 recorded by the observed mineralogy. Despite abundant evidence for contact metamorphism, we see minimal carbon loss, suggesting models of sediment-degassing may overestimate the resultant flux from small-scale intrusions.