CONTACT METAMORPHISM AND THE RELEASE OF CARBON AND SULFUR VOLATILES: A STUDY OF THE EFFECTS OF DIABASE EMPLACEMENT

Intrusion of magma into the shallow crust can result in very rapid thermal maturation of organic-rich sedimentary lithologies, such as black shale and coal, during contact metamorphism. This can lead to the release of large quantities of CH₄, CO₂, and sulfur-bearing volatiles, and has thus been speculated as a mechanism for perturbing global climate. For example, intrusion and volcanism associated with the Central Atlantic Magmatic Province (CAMP) during the Late Triassic is thought to be a major driver of the T-J mass extinction that occurred ~ 201 Ma.

We aim here to better understand how intrusion into organic-rich facies releases volatiles and to quantify the relative contribution of carbonaceous and sulfurous species from each of the magmatic and sedimentary sources. We present geochemical datasets from Triassic lacustrine organic-rich facies within the Culpeper Basin of the Newark Supergroup (Northern Virginia), which experienced Jurassic emplacement of thick diabase sheets of the CAMP. Meta-sediments were regularly sampled along individual beds that run approximately orthogonal to contacts with the diabase, with sampling frequency ranging from the decimeter scale proximal to intrusions to the decameter scale several hundred meters from intrusions. Sulfur content in the sediments increases by approximately a factor of ten over the first five meters from the intrusion, with both carbon (δ¹³C) and sulfur (δ³⁴S) isotope data showing strong positive excursions from the initial, pre-metamorphic values revealed by distal samples. These results, coupled with petrographic constraints, thermal, and thermodynamic models, illustrate how major and accessory mineral assemblages evolved during contact metamorphism and controlled both the release of sulfur-bearing volatiles and the isotopic fractionation of this sulfur.