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Constraining Thermal Histories in Marine Carbonates and Black Shales: A Pilot Conodont (U-Th)/He Thermochronometry Study in the Illinois Basin

Abstract

Apatite (U-Th)/He (AHe) thermochronology is one of the major tools used to decipher time-temperature paths for rocks in the upper crust. The method is sensitive to temperatures from 30-90 °C depending on apatite chemistry. As a result, AHe can be used to predict hydrocarbon maturation in basins through the reconstruction of thermal histories. However, the technique is limited to rocks in which crystalline apatites occur. While apatite is common in granitoid, some metamorphic, and coarse clastic sedimentary rocks, most carbonates and shales contain neither apatite nor any other established thermochronometer. As a result, quantitative time-temperature paths cannot be reliably constrained in basins dominated by these lithologies. Conodont AHe thermochronology has the potential to solve this problem. Conodonts are biomineralized structures composed of microcrystalline apatite that are common in Paleozoic marine sediments. A pilot study by Peppe and Reiners (2007) suggested that conodont apatite retains helium across the same temperature range as crystalline apatite, implying that conodont AHe data could be interpreted in the same manner as conventional AHe. Here we further explore the conodont AHe thermochronometer through application to three samples from a core in the central Illinois basin. Our initial dataset consists of 19 conodont AHe dates. Most analyzed conodonts have Th/U ratios that range from 14–140. Five yielded Th/U ratios >5 and displayed anomalous dates up to 392 Ma, suggesting that mobility of U and Th during diagenesis is a factor affecting the conodont AHe system. Sample reproducibility of the remaining data is 20–35%. Conodont dates (n=5) from a marine black shale in the Late Pennsylvanian Patoka Formation (208 m depth) and a marine black shale (n=10) in the Late Pennsylvanian Modesto Formation (252 m depth) yield overlapping mean dates of 42 ± 15 Ma and 39 ± 8 Ma, respectively. Four analyses from a marine limestone in the Mid-Pennsylvanian Shelburn Formation (262 m depth) yield a mean date of 126 ± 41 Ma. Whether conodont chemistry or other factors are responsible for this older date relative to the shale samples is under investigation. All conodonts have CAI values of 1 to 1.5, indicating a maximum burial temperature of =90 °C. Since all samples have AHe dates significantly younger than depositional age, this further implies that the closure temperature for the conodont AHe system is no higher than that of the conventional AHe system.