High Spatial Resolution U-Pb dating of Low-Uranium Calcite Using LA-MC-ICP-MS

Significance: Carbonate minerals，especially calcite，are widely distributed and formed as primary and secondary minerals in a variety of geological environments^［1］. The U-Th-Pb dating technique is one of the most classical isotopes dating methods，applicable to almost the entire geologic time scale，and calcite can be bound to uranium during its formation，making it a potentially suitable timer for U-Pb and U-Th geochronology^［7-8］. Therefore，calcite geochronology can provide direct temporal constraints for many geoscientific applications and has a very broad application prospect. However，carbonate minerals usually have relatively low U contents，which have hindered the development of this method. With its high sensitivity and high accuracy，MC-ICP-MS has been successfully applied to high spatial resolution U-Pb dating for many high U content auxiliary minerals.LA-MC-ICP-MS with high sensitivity and accuracy has been successfully applied to high spatial resolution U-Pb dating of a wide range of minor minerals include carbonate minerals. Zhang et al ^［22］ carried out a calcite U-Pb age methodology using mixed collectors (FCs+ICs) and multiple ions counting collectors (MICs) analytical mode. And achieved accurate U-Pb age determinations for PKC-1 (uranium content of 6.2-74.18μg/g)，Duff Brown Tank，JT and ASH-15 standards for accurate U-Pb age determination by optimising the mass spectrometer and laser parameters at 33μm，90μm，75μm and 110μm beam spots，respectively. Xie et al^［21］ achieved accurate U-Pb age determinations of low-uranium (down to 0.03μg/g) calcite at 85μm beam spot conditions using a mixed collectors (FCs+ICs) mode based on optimised instrumental parameters. However，the previous U-Pb age analysis studies have not investigated the influence of the oxide yield and the guard electrode modes of the MC-ICP-MS instrument in detail. In addition，the U-Pb age analyses have only been carried out on carbonate samples with high uranium content (Duff Brown Tank，ASH-15，etc.，with ≥2 μg/g uranium) with U-Pb ages below 50μm beam spot. The spatial resolution of U-Pb dating of low uranium carbonate samples (＜1μg/g) needs to be further improved.

Methods: The samples used in the experiments were NIST glass standards NIST612 and NIST614^［28］，and the calcite reference materials WC-1，PTKD-2，LD-5，and TARIM，listed in Table 1. NIST612 was used to optimise the instrumental parameters，NIST614 and WC-1 were used as standards to calibrate ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios，and PTKD-2，LD-5，and TARIM were unknown samples with low uranium contents. Glass and calcite samples were fixed in epoxy resin to prepare the target and polished with 7000 grit sandpaper until the sample surface was exposed. Sample surfaces were polished smooth with 0.3μm Al₂O₃ polishing powder and ultrasonically cleaned with ultrapure water prior to each test.

　　The Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS，ThermoFisher Scientific) and the GeoLas Pro excimer laser ablation system (LA，Coherent) were used in this experiment. In order to achieving the highest sensitivity，multiple ions counting collectors (MICs) analytical mode was used to receive all U and Pb elements，and the specific cup configuration and instrumental parameter settings are shown in Table 2.

　　The data were processed using the two-step calibration method of the previous work^［29］. The raw data were first imported into the Iolite software^［30］ and processed using X_U-Pb_Geochro4. After blank subtraction，the ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios of the calcite standard WC-1 and the unknown samples were calibrated using NIST 614 as an external standard sample and the uncertainty of the ratios was derived with a single-point internal precision of 2 SE. The ²³⁸U/²⁰⁶Pb ratio of the unknown sample was further corrected with matrix-matched WC-1，and the ²⁰⁷Pb/²⁰⁶Pb，²³⁸U/²⁰⁶Pb of WC-1 was plotted on a Tera-Wasserburg plot using IsoplotR^［31］，and the ratio of the obtained lower intercept to the reference lower intercept (26.43) is the correction factor. This correction factor includes matrix effects，laser induced fractionation and fractionation between U/Pb elements associated with ICP and is used to further correct the ²³⁸U/²⁰⁶Pb ratio of the unknown sample. Finally，the ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios of the samples resulting from the two-step correction were plotted on a Tera-Wasserburg plot using IsoplotR to calculate the lower intersection age and age uncertainty (2SE).

Data and Results: The effects of the experimental parameters (including the cone combination，the amount of nitrogen introduced，whether the guard electrode is open，and the Ar carrier gas flow rate) on the U and Pb sensitivity and the plasma state (oxide yield，UO/U) were systematically discussed in order to realize the best analysis instrument parameters. The Jet+X cone combination and the introduction of N₂ had significant sensitising effects on both U and Pb. The Jet+X cone combination combined with the introduction of N₂ increased the sensitivities of U and Pb to a maximum of 7.4 and 5.8 times，respectively，under GE on conditions. See Fig.1 and Fig.2 for details. The high-sensitivity Jet+X cone combination and the introduction of N₂ have obvious sensitizing effects on U and Pb. The sensitivity of U and Pb was enhanced up to 7.4 and 5.8 times under the GE on mode，the Jet+X cone combination and with the introduction of N₂. In which the sensitizing effect of the high-sensitivity cone combination on Pb is more obvious，and that of the introduction of N₂ on U is more obvious.

　　Fig.3 shows the effect of the instrumental conditions on the oxide yield，the oxide yield (UO/U) in the GE on mode is usually higher than that in the GE off condition，and the oxide yield becomes rapidly higher with the increase of the Ar carrier gas flow rate. In addition，the optimisation of the experimental conditions is aimed at increasing the sensitivity and decreasing the oxide yield，in other word，the conditions with the highest sensitivity are chosen at lower oxide yields. Fig.4 shows the maximum signal intensity of ²⁰⁶Pb for different instrumental conditions for oxide yields (UO/U) within 1% and the Ar carrier gas flow rate corresponding to the maximum signal intensity. The best experimental conditions were achieved using the Jet+X cone combination，N₂ flow rate of 8 mL/min and the GE on mode，corresponding to an Ar carrier gas flow of 0.9 L/min.

　　Under optimised experimental conditions，the same depth to diameter ratios (aspect ratio) of the ablation crater is guaranteed，and the “down-hole” fractionation behavior of WC-1 is similar under different laser spot sizes and frequencies. Based on the optimized experimental parameters，the method was validated using three low-uranium (0.04-0.63μg/g) calcite U-Pb dating reference materials. High-uranium WC-1 was used for ²³⁸U/²⁰⁶Pb correcting，the spot size and frequency were altered on the premise of crater aspect ratios of the standard and samples keeping consistent. In our experiment，the calcite samples LD-5，PTKD-2 and TARIM yield a lower intercept age of 73.20±0.56Ma，152.7±2.5Ma and 206.2±1.3Ma，respectively，which were in agreement with the calibrated results from the ID-TIMS/ID-MC-ICP-MS within uncertainties.