CONTACT METAMORPHISM OF COAL: IMPLICATIONS FOR THE LARGE-SCALE RELEASE OF 12C-ENRICHED METHANE

Petrological and geochemical alteration of intruded coal has been documented worldwide. Increased vitrinite reflectance (R_o) has been observed across a distance typically equal to 1-2x intrusion thickness. Volatile matter (VM) (including CH₄) release may be associated with these contact aureoles, which in turn may be related to important shifts in atmospheric composition such as that inferred by the negative δ¹³C excursion associated with the Toarcian oceanic anoxic event. Assuming intrusions result in the release of isotopically depleted volatile matter, then coals within the alteration haloes should show heavier δ¹³C. An intruded Pennsylvanian-age coal from the Illinois Basin, USA, was studied as a test case to evaluate the extent of geochemical alteration and isotopic enrichment adjacent to an intrusion.

Fifteen coal samples from the Springfield (No. 5) coal in southern Illinois were analyzed. R_o increases as the intrusion is approached (from 0.70% to 4.94%), whereas VM decreases (from ~35% to ~13.5%). Despite the high rank attained adjacent to the intrusion, δ¹³C becomes only slightly heavier toward the contact, changing from −24.9‰ to −24.3‰. The extent of isotopic shifts associated with intrusions may be a function of the size of the intrusion, pre-intrusion rank, thermal conductivity, permeability and heating duration, and the style of heat transfer (convection vs. conduction). Electron microprobe data for individual macerals (and coked equivalents) show an increase in the carbon content for vitrinite, increasing from ~80% to over 95%, with the most rapid increase occurring below vitrinite reflectances of <2%. Oxygen decreases from a maximum of over 10% to less than 0.5% at the coal/dike contact. Less pronounced changes in composition were seen in the inertinite macerals: C increases from 90 to over 95%, whereas O decreases from over 5% to less than 0.5%. These shifts in isotopic and geochemical composition will be discussed in terms of maturation under contact metamorphic conditions and the possibility of significant methane release associated with this and other intrusions.