In-situ LA-MC-ICPMS U-Pb dating method for low-uranium carbonate minerals 低铀碳酸盐矿物的LA-MC-ICPMS微区原位U-Pb定年方法

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