THERMAL EVOLUTION OF THE ANTICOSTI BASIN, EASTERN CANADA: AN EMPIRICAL CALIBRATION OF THE CONODONT (U-TH)/HE THERMOCHRONOMETER

Advances in our understanding of the systematics of thermally controlled diffusion of radiogenic helium in apatite and zircon have resulted in the increased application of the (U-Th)/He thermochronometer in sedimentary basin analysis. Unfortunately, these methods are of limited use in regions dominated by carbonate sedimentary rocks, as the accessory minerals required for the analyses are not common in the strata. Conodonts may represent a solution to this problem. An initial investigation into the application of (U-Th)/He analysis on conodonts showed that the method behaves similarly to the (U-Th)/He system in magmatic apatite. Our study resolves to do a first-of-its-kind empirical investigation into the effects of temperature on conodont (U-Th)/He age, sampling from the subsurface of the Lower Paleozoic Anticosti Basin, Eastern Canada. In this study, ten samples were collected from two different boreholes. While sampling primarily targeted conodont-rich lithologies, material for apatite and zircon (U-Th)/He thermochronology (AHe, ZHe) was also collected where available. (U-Th)/He analysis of the crystalline basement (1700 m depth) yield a mean AHe age of 29 ± 2 Ma whereas apatite from the Mingan Formation sandstone (1520 m depth) record ages between 40-64 Ma. Existing AFTA thermal modeling from the basement suggests two thermal events: a 120°C thermal maximum at c. 280 Ma, and a secondary heating episode between 100-60 Ma. Preliminary conodont (U-Th)/He ages exhibit scatter: a few single conodont ages (26-78 Ma) were in line with our existing borehole AHe data. Other age populations (97-113 Ma; 195-291 Ma) in our data are more likely associated with thermal resetting during the possible heating events in the basin. Interestingly, several conodonts yielded ages between 425-464 Ma and have likely not experienced significant post-depositional helium loss. To better understand this complex age distribution, future analyses will use x-ray computed microtomography to assess the effect of morphology, volume and microstructure on (U-Th)/He age. Ultimately, we believe that a well-constrained study with a well-characterized dataset will explain many of the complexities associated with the (U-Th)/He system in conodonts, and help to develop a powerful tool for thermochronology in carbonate basins.