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Constraining Thermal Histories in Carbonates and Marine Shales: the Conodont AHe Thermochronometer

Rachel Landman
Department of Geological Sciences, University of Colorado-Boulder, Boulder, Colorado, USA
[email protected]

Apatite (U-Th)/He (AHe) thermochronology is one of the major tools used to decipher the thermal history of rocks in the upper kilometers of the earth’s crust. The method is sensitive to temperatures from ~30-90 °C, and as a result this tool has also been used to predict hydrocarbon maturation in basins through the reconstruction of time-temperature paths. However, this technique is limited to rocks in which crystalline apatites of sufficient quality occur. Carbonates and shales contain neither apatite nor any other established thermochronometer, and thus the thermal histories for these lithologies cannot be constrained using current thermochronometric methods. This project will explore the potential of conodont AHe thermochronology to solve this problem.

Conodonts are apatite microfossils 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 can be interpreted in the same manner as conventional AHe. This project will build on their work with two calibration studies of the conodont AHe thermochronometer. These studies will compare conventional AHe dates with conodont AHe dates from nearby lithologies that have experienced identical thermal histories. Two calibration sites, in the White River Uplift of Colorado and the Sangre de Cristo Range in New Mexico, will allow a range of thermal histories to be tested. If successfully calibrated, conodont AHe will open carbonate and marine shale terranes across the world to low-temperature thermochronometry.

AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects