U-PB CARBONATE DATING OF EARLIEST DIAGENETIC CEMENTS WITHIN PALEOSOL CARBONATE NODULES: PRELIMINARY WORK AND FUTURE OPPORTUNITIES

Recent advances in U-Pb dating via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) demonstrate the potential for dating carbonate materials with a precision of ca. 3-10% (2s). Pedogenic carbonates that precipitate in fossil soils, paleosols, are important proxy records of past physical, biological, and chemical surface processes. Little attention has been given to LA-ICP-MS U-Pb carbonate dating of paleosols to establish maximum, or near, depositional ages. Here, we present preliminary work and U-Pb carbonate ages obtained from the earliest diagenetic cements lining cracks within carbonate nodules in paleosols from the early Permian eastern shelf of the Midland Basin (USA) and the Late Triassic Ischigualasto Fm of Argentina.

Our age results, while in stratigraphic order, do not represent depositional ages. Instead, we interpret the dates as the timing of first-generation carbonate precipitation within syndepositional cracks in carbonate nodules that formed as the carbonates passed through the freshwater vadose to the upper phreatic zone. Detailed petrographic characterization prior to U-Pb data acquisition is key to the success of this dating approach and the age interpretation of the ablated material. Based on crystal morphology, we hypothesize these earliest cements crystallized within hundreds to thousands of years after soil deposition. Thus, the obtained ages approximate the age of pedogenesis since the duration between these events falls within the uncertainties of the dating method. The high spatial resolution of the LA-ICP-MS permits the identification of cogenetic zones with widespread U and radiogenic Pb concentrations within the pedogenic carbonates that can be subsequently analyzed by ID-TIMS. This coupled approach is an opportunity for dating depositional and diagenetic records, lacking or with sparce volcanic deposits, to higher precision.