Assessment On the Trace Element Control on Apatite Fission-Track Annealing, Anadarko Basin, OK, USA.

Abstract

Apatite fission-track is a low-temperature thermochronometer with a partial annealing zone of 110 – 70 ± 10 °C, which is well established through laboratory and natural annealing studies. Because of its temperature sensitivity and ubiquitous occurrence of apatite, the fission-track method has been widely used to resolve a range of tectonic, geomorphic, and sedimentary questions. The precise temperature sensitivity of fission-track annealing is a function of a minerals nominal geochemistry. Apatite, however, often yields a highly variable major- and trace-element geochemistry due to changes in parent melt, co-genetic mineral growth, and subsequent metamorphic and hydrothermal alteration. As a result, the nominal ~120 – 60°C apatite partial annealing zone (APAZ) represents a bulk estimate and may not correspond to individual apatite composition. Previous workers have highlighted the role of halogens such as Cl, light rare earth elements (REEs), and kinetic parameters such as Dpar on annealing. In this study, we explore the role of apatite trace and rare earth elements (REEs) on annealing applied to borehole sandstone samples collected from the Anadarko Basin, Oklahoma, U.S.A. The Anadarko Basin contains apatites sourced from both metamorphic and igneous rocks from the proximal Wichita Igneous Province and distal Appalachian uplifts and has been relatively tectonically quiescent across Mesozoic-Cenozoic time. We present apatite major, trace, and REE geochemistry paired with single-grain fission-track ages and confined-track lengths from seven samples, collected from ~3500 – 900 meters (41 – 102°C). We observe a negative relationship between younger AFT single-grain ages and higher light REE (LREE) concentrations in the more shallowly buried samples (1500 – 970m). Apatites that contain elevated REE concentrations (>20000 ppm) are more susceptible to annealing at lower PAZ temperature (~70 – 50°C). The more deeply buried samples (~3500 – 2900m) show no systematic correlation between single grain AFT ages and LREE concentrations. We suggest that this may be a function of prolonged residence within the PAZ, which would overprint any influence of LREE concentration on apatite annealing. Based on these observations, we suggest that apatites with elevated LREE concentrations are sensitive to annealing at lower PAZ temperatures. We do not observe a correlation between eU and single-grain AFT across all samples. However, we do observe a positive relationship between eU and REE concentrations. We suggest that correlations between eU and single-grain AFT age are likely reflecting a proxy for lower APAZ sensitivity caused by greater LREE concentrations.