U-TH-PB CHRONOLOGY OF FISH BONE IN PETROLEUM SOURCE ROCKS: AN EXAMPLE FROM THE EAGLE FORD GROUP, TEXAS

The U-Th-Pb isotope systematics of TOC (total organic carbon)-rich sedimentary rocks are investigated to test the potential for these isotope systems to record the timing of deposition and periods of diagenesis. The samples investigated for this project were derived from the well-dated Upper Eagle Ford Formation (UEFF), within the Eagle Ford Group (EFG), located in south-central Texas. The sample material analyzed in this study is dated at 90.6 Ma, based on the astro- and geochronologic age model of Eldrett et al. (2015). In situ analyses of U, Th, and Pb isotope ratios and rare earth element (REE) abundances of fishbone phosphate and carbonate calcispheres were conducted by laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Qualitative major element abundances and sample mapping were conducted by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The REE data in fish bone indicates significant local REE uptake. Thorium-lead isotope data yield an age of 92±4 Ma and indicates that the fishbones have retained their original, or very early, diagenetic Th-Pb isotope systematics, making them promising candidates for dating shale deposition. The U-Pb isotope systematics indicate that U was mobilized and greatly enriched in the fishbone after deposition. Preliminary modeling of the U and Pb isotope data points to a short period of U and uranogenic Pb re-distribution at 57 ⁺¹⁷/_-9 million years ago. This late U mobility may complicate interpretations of U isotope compositions as an indicator of redox conditions. The cause of the U mobility is still under investigation.