A Stable Isotope Study on the Origin of Permineralized Peat Zones in the Herrin Coal

Calcite permineralization of peat stages in the Herrin (No. 6) Coal Member at the Delta Mine is typical of the "coal balls" which are encountered in this major Pennsylvanian seam of the Illinois Basin. The $$\delta^{13}C$$ values of composite samples and discrete calcite generations range from +0.8 to -23.9 permil (rel. PDB), indicating variable proportions of transported inorganic carbon (and calcium) and of dissolved $$CO_{2}$$ species released by local decomposition of organic matter. The variations in $$\delta^{13}C$$ between successive calcite generations within and between coal-ball zones indicated wide temporal and spatial fluctuations in the dominant source of carbon at this site. Oxygen isotope compositions are more uniform ($$\delta^{18}C$$ = -4.6 to -9.3 permil rel. PDB) suggesting precipitation (or subsequent recrystallization) in water of predominant meteoric origin. The isotopic data are most compatible with contemporaneous permineralization during peat accumulation in independent subsystems from the lowermost coal-ball zone upward. The isotopic composition of cleat calcite suggests that it was precipitated much later. Several mechanisms of inorganic carbonate and calcium transport into or within the peat swamp are considered.