In-situ LA-MC-ICPMS U-Pb dating method for low-uranium carbonate minerals

The in-situ U-Pb isotopic geochronology of carbonate minerals has some broad application prospects, not only for the recording of paleoclimate, paleoenvironment, paleoecology, ancient human caverns, calcium plate and marine coral, but also in carbonate vein dating associated with fault activity, neotectonics, paleoearthquake and hydrocarbon reservoirs. However, the development of this method is limited by many factors, such as, very few samples satisfying essential dating conditions, low-uranium carbonates having ultra-low concentrations; and lack of ideal mineral standards. Currently, this technology is still in development in both China and Australia, with only four laboratories in the world having reported results which are mostly for high-uranium (>0.1 ppm) carbonate samples. In this study, Nu Plasma II multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) with ASI RESOlution SE laser ablation system (LA) is used to refine an in-situ U-Pb isotopic dating method for low-uranium carbonate minerals. Here, NIST614 international glass standard is used for Pb/Pb isotopic fractionation calibration, and carbonate mineral standard is used for U/Pb element fractionation calibration. The age of the lower intersection obtained by using the Tera-Wassenburg (TW) U/Pb-Pb/Pb diagram represents the U-Pb age of the carbonate mineral. The improvements made in this study are mainly completed on the following aspects. Firstly, pre-screening with LA-ICP-MS allows for rapid selection of suitable samples, which have large variations in U/Pb and Pb/Pb to construct a good TW reverse isochron or their common Pb is so low that measuring points can fall near the concordant line. Secondly, 10 Ω preamplifier at the highest quality end of the Nu Plasma II MC-ICPMS (10 times more sensitive than a versatile 10 Ω preamplifier) and an ETP (electron multiplier) discrete dynode multiplier dedicated to static measurement of U isotope were installed. The high sensitivity Faraday cup measurement is used when the U content is sufficiently high (e.g. >0.1 ppm); while the discrete dynode multiplier is used if the U content is low (e.g. <0.1 ppm). Thirdly, a potential carbonate standard AHX-1a (calcite) has been characterized through cross-calibration with the international standard ASH15 (flowstone) and attained the recommended age of 209.8±1.3 Ma (2S, N=21, MSWD=2.7). This result is a weighted average age of 21 consecutive independent calibrations of different parts of the sample mount over a period of six months, with approximately 50-150 sets of data points at each time, for a total of approximately 2000 data points. In order to evaluate the accuracy of the recommended age (209.8 Ma) of AHX-1a, we have performed simultaneous measurement of AHX-1a and WC-1 (marine calcite cement) in different sessions, treating AHX-1a as the known standard and using the age of 209.8 Ma to correct element fractionation of WC-1. The dating results are highly consistent with previous results within the error range, which demonstrates that the recommended age (209.8 Ma) of the potential standard AHX-1a is accurate and reliable. Compared with the reported standards ASH15 and WC-1, AHX-1a is more homogeneous and wider application potential. However, in view of the fact that the original sample size of AHX-1a is small, which makes it not suitable for isotope dilution calibration method before its spatial distribution characteristics of U-Th-Pb are completely delineated. Therefore, AHX-1a is temporarily used only as an in-house standard in our laboratory. Currently, we are continuing to search for and calibrate other potential standards with similar characteristics to AHX-1a, but of a larger sample size. To continuously characterize and identify suitable carbonate reference materials, further calibration using isotope dilution is planned.