HYDROGEN, CARBON, AND NITROGEN ISOTOPIC RESPONSES TO INSTANTANEOUS HEATING ALONG A COAL SEAM NEAR TWO DIKE INTRUSIONS IN ILLINOIS

The near-instantaneous and short-range thermal alteration of organic matter in sediments near magmatic intrusions more closely resembles artificial maturation experiments in the laboratory than natural long-term geologic maturation of organic matter upon burial. Our study investigates chemical and H, C, N-isotopic changes in a high volatile bituminous coal seam (Springfield Coal Member, Pennsylvanian age) near two dike intrusions in an Illinois underground coal mine. Vitrinite reflectance values R_o directly adjacent to a small and a large dike reach 3.71% and 5.03%, respectively. The coal close to the larger dike expresses stronger and more systematic consequences of heating.

Thermal stress in coal caused hydrogen and nitrogen loss near intrusions and a reduction of isotopically exchangeable hydrogen in kerogen from 5 - 6 % of total hydrogen in unaltered coal kerogen to 2 - 3% at contacts, which is mostly due to the thermal elimination of functional groups (e.g., -OH, -COOH, -NH₂). In contrast to all previously published data on D/H ratios in thermally matured organic matter, our study shows that the more mature kerogen near the two dike contacts is further D-depleted, which is attributed to a thermal elimination of D-enriched organic hydrogen in functional groups and the thermal drying of coal prior to the onset of cracking reactions, thereby precluding D-transfer from relatively D-enriched waters into kerogen. D-depletion of organic hydrogen with increasing maturity is highly unusual and testifies to hydrologic isolation of the affected coal when magma sub-vertically intersected and dried the coal seam prior to peak thermal overprinting.

A maximum in organic nitrogen concentration in kerogen at a distance of ~3.5 m from the larger dike indicates that reactive nitrogenous compounds were pyrolytically liberated at higher temperature closer to the contact, migrated horizontally along the coal seam away from the contact, and chemically recombined with coal kerogen at intermediate distance and temperature. A strong δ¹³C_kerogen vs. δ¹⁵N_kerogen correlation across 5.5 m of coal next to the larger dike results from the dike/coal environment functioning as a chromatographic flow-through reactor along a dynamic thermal gradient, although minor variations in maceral composition may contribute to the correlation.