Potential of Conodont (U-Th)/He Thermochronometry to Resolve Shallow Crustal Thermal Histories in Carbonates and Shales

Abstract:

Acquisition of thermochronology data in carbonates and shales has traditionally been elusive owing to the absence of dateable minerals of suitable size for analysis. Development of such a thermochronometer is highly attractive owing to the widespread occurrence of these lithologies – including in large sections of sedimentary basins where thermal histories are relevant for hydrocarbon exploration – as well as the possibility of coupling this tool with other geochemical proxies in these units. Conodont (U-Th)/He thermochonometry has the potential to fill this niche. Conodonts are toothlike bioapatite structures common in Paleozoic through early Mesozoic marine carbonates and shales. They are well-known to the petroleum industry through use of their color alteration index (CAI) as a semi-quantitative indicator of peak temperature. Moreover, the only published conodont He study showed intriguing promise (Peppe and Reiners, 2007).

Here we present a conodont (U-Th)/He thermochronology dataset from the Illinois Basin that displays both the potential of this tool and the outstanding questions that remain in its development. We acquired conodont He data for 7 drillcore samples of Pennsylvanian marine black shale and limestone. The conodonts have CAI values of 1-1.5, indicating maximum burial temperatures of ≤ 90 °C. First, the conodonts yield He dates substantially younger than their depositional age, indicating that the ≤ 90 °C temperatures were sufficient to cause He loss, and therefore implying that the conodont He closure temperature is no higher than the conventional apatite He system (70-80 °C). This result is compatible with conodont laboratory diffusion experiments. Second, most of our conodont dates are < 90 Ma, consistent with regional apatite He data for basement drillcore samples and suggesting that the conodonts and apatites record similar thermal histories. Finally, several samples contain dates that range from 100 to 250 Ma. The least reproducible samples are characterized by wide ranges of conodont [U] and Th/U ratios, as well as negative correlations between date and [U]. The patterns suggest that U loss generated the anomalously old conodont dates. Work is underway to ascertain whether U mobility occurred during diagenesis or was induced by the acid-based separation process.