Simultaneous Determination of U-Pb Age and Trace Elements of Zircon by Laser Ablation Sector Field Inductively Coupled Plasma-Mass Spectrometry

(1) Simultaneous determination of zircon U-Pb age and multiple trace element contents by LA-SF-ICP-MS can affect the stability of U-Pb isotope signals and ratios, leading to an increased uncertainty in single-analysis U-Pb dating results. However, it has no effect on the accuracy of the concordant ages due to multi-point analysis, and the precision is slightly reduced.

(2) Simultaneous measurement of zircon U-Pb ages and trace element contents using LA-SF-ICP-MS, yields zircon U-Pb age deviations of less than 1% and deviations in key trace elements such as REE, Ti, Hf, Pb, Th, and U of less than 10%.

(3) Solely determining zircon U-Pb ages by LA-SF-ICP-MS effectively reduces uncertainty introduced by single-analyses isotope ratio measurement, improving dating precision (2σ<2%) and accuracy (RSE<0.5%).

Significance: Zircon (ZrSiO₄) is a common accessory mineral in both terrestrial and extraterrestrial rocks. It serves as the most frequently utilized mineral in determining the age, origin, and thermal history of rocks through U-Pb geochronology, primarily due to its high closure temperature, resistance to alteration, high uranium content, and minimal incorporation of common lead during its crystallization. Apart from age determination, zircon also provides various geochemical information, including trace elements (Ti, REEs), O isotopes, and Zr isotopes. By combining all these data, it assists in constructing metamorphic P-T-t paths, which helps in inferring the mineral growth history, making it an integral component of “Petrochronology” research^[20]_. Natural zircon, however, generally exhibits small grains and often possesses intricate internal structural features. Hence, higher analytical spatial resolution and acquiring more in-situ elemental/isotopic information within limited analytical space is crucial.　　Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a routine technique for zircon U-Pb dating. The sector field mass spectrometer (SF-ICP-MS) possesses high sensitivity, offering possibilities for high spatial resolution U-Pb dating of zircon^[28-29] and ultra-low uranium mineral content^{[21-25, 30]}. However, the usage of the magnetic sector mass analyzer results in slower scanning speeds^[27]. Consequently, in the reported zircon U-Pb dating by LA-SF-ICP-MS, typically, only U-Pb-ages-related isotopes are detected, and simultaneous quantitative analysis of key trace elements cannot be conducted.　　This study optimized instrument parameters and enhanced signal stability to enable simultaneous determination of U-Pb dating alongside quantification of key elements such as Ti, REEs, and Hf. Seven commonly employed zircon U-Pb standard samples were measured to assess the feasibility of the method and its influence on U-Pb dating results.

Methods: The experiments were carried out at the Key Laboratory of Elemental Microanalysis and Morphology of the China Geological Survey using an ESL NWR 193UC ArF excimer laser and the ELEMENT Ⅱ sector field inductively coupled plasma-mass spectrometer (SF-ICP-MS, ThermoFisher Scientific, USA).

　　Helium gas was used as the carrier gas^[39], while 1mL/min of nitrogen was introduced to boost instrument sensitivity^[40]. A signal homogenization device with higher aerosol diffusion space was utilized to improve signal stability. Laser spot size and frequency were 25μm and 8Hz, respectively, with a laser energy density of 3J/cm². SF-ICP-MS was employed in low-resolution mode (M/ΔM=300), with sensitivity of U about 5000cps/(μg·g^-1), and signal stability RSD of 1% to 2%, oxide production rates (ThO⁺/Th⁺) below 0.2%. U-Pb isotopes and key trace elements were measured, including ²⁹Si, ⁴⁹Ti, ⁸⁹Y, ⁹¹Zr, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵¹Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, ¹⁷⁵Lu, ¹⁷⁸Hf, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U. The measuring time for ²⁰⁶Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U was 10ms, ²⁰⁷Pb was 20ms, and the remaining elements were 5ms each, resulting in a total measuring time of 0.87s per reading, with an effective analysis time of 71%. Since most zircons contain no, or extremely low, common lead and single-collector mass spectrometry it makes it challenging to accurately measure ²⁰⁴Pb for common lead correction. ²⁰²Hg and ²⁰⁴Pb were not detected to improve the proportion of effective analysis time. Detailed instrument operating condition parameters are provided in Table 1.

　　The 91500 zircon and NIST610 were used as standard materials for U-Pb isotope ratios and trace element quantitative analysis, respectively. For every 10 unknown sample analyses, a set of standard samples was inserted to correct for fractionation effects. Each spot analysis consisted of a 20s background collection and 40s sample data acquisition and 30s flushing. Raw data were reduced offline using GLITTER 4.0 software package^[41]. All samples shared the same signal interval as the standard zircon 91500 for analysis. Age calculations and Concordia diagram construction were performed by Isoplot/Ex version 2.23^[42]. The uncertainty for all isotope ratios and age values in this study were reported at a 2σ level.

　　Two methods were used to determine zircon U-Pb ages. Method 1: Only U-Pb-related isotopes were measured, including ²⁰²Hg, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U. Single measuring time was 0.306s per reading, and within the 40s sampling time, data for 134 sets of element signal intensities were collected. Method 2: Simultaneous determination of U-Pb ages and trace elements content, including Ti, REE, Hf, and U-Pb isotopes ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, ²³⁸U, etc. Single measuring time was 0.857s, and within the 40s sampling time, effective signal intensity data for 47 sets were obtained.

Data and Results: The U-Pb dating results are shown in Fig.1 and Table 2, while the measured trace element contents are presented in Fig.3 and Table 3.

　　(1) U-Pb dating results comparison

　　Fig.1 provides a visual comparison of precision and accuracy of the results obtained by these two dating methods. It shows that the dating results from Method 1 exhibit smaller errors, and the data points are more concentrated.

　　Using LA-SF-ICP-MS with solely U-Pb-related isotope detection, the dating results show an error of 1.5% and 1.3% for ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U, respectively. The consistency of single-point ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ages is good, with RSD values ranging from 0.2% to 0.9% and 0.5% to 1.3%, respectively. In contrast, when employing Method 2, the errors for ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ages increase slightly to 1.9% and 1.7%, respectively. The dispersion of single-point ages increases as well, with RSD values for ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ages ranging from 0.4% to 1.4% and 1.2% to 3.3%, respectively. Particularly, the RSD value for ²⁰⁷Pb/²³⁵U ages notably increases.

　　To investigate potential causes for the variability in precision between the two methods, the RSD of the signal intensities of ²⁰⁶Pb, ²⁰⁷Pb, and ²³⁸U/²⁰⁶Pb signal ratios of zircon 91500 obtained under the same experimental conditions were statistically analyzed. For Method 1, the RSD values for the collection of ²⁰⁶Pb, ²⁰⁷Pb signals, and ²³⁸U/²⁰⁶Pb signal ratios were 13%, 13%, and 5.2%, respectively. Meanwhile, for method 2, the RSD values for ²⁰⁶Pb, ²⁰⁷Pb signals, and ²³⁸U/²⁰⁶Pb isotope signal ratios were 14%, 19%, and 9.3%, respectively. Simultaneously collecting multiple elements extended the mass spectrometer’s single-scan time, which to some extent affected the stability of the detected element/isotope signals and their ratios, especially for low-content isotopes like ²⁰⁷Pb. The fluctuation characteristics of isotopic signals and ratios corresponded to the changes in U-Pb dating results. Considering factors during data processing, age uncertainty calculation methods, and influencing elements^[43-46], it can be inferred that the impact of simultaneous multi-element detection on the stability of isotope signal intensity might be the primary cause for the increased final dating result errors.

　　Generally, the precision of LA-ICP-MS zircon U-Pb dating results is considered to be within 1% to 2%, while the accuracy of the measured age can reach 1% or better^[43-46]. The experimental results in this study indicate that although simultaneous multi-element detection can increase the range of changes in single-point zircon U-Pb ages, the precision of the data is still better than 2%. Furthermore, simultaneous multi-element detection does not affect the accuracy of the sample’s concordia age and the ²⁰⁶Pb/²³⁸U weighted average age. For method 1, the relative deviations of concordia ages and ²⁰⁶Pb/²³⁸U weighted average ages for each sample compared to TIMS ages are less than 0.5%, while for method 2, these figures are less than 1.0% and 0.7%, respectively.

　　(2) Quantitative results of trace elements

　　The results of trace element analysis in zircon samples are shown in Table 3. The detection limits for REE range between 11ng/g and 73ng/g, with the majority being below 20ng/g. Among these, the detection limits for La and Pr are 20ng/g and 13ng/g, respectively. Ti has the highest detection limit, reaching up to 302ng/g, while Pb, Th, and U have the lowest detection limits at 15ng/g, 4ng/g, and 2ng/g, respectively.

　　Si and Zr were employed as internal standard elements for quantitative analysis. All quantitative results of zircon samples using Si as the internal standard element show slightly higher values overall than those calculated using Zr as the internal standard element (Fig.2b). For the 91500 and SA01 samples, the analysis results using Si as the internal standard exhibit relative errors of less than 10% compared to the recommended values in literature [35] and [48]. However, when using Zr as the internal standard, the relative errors range between 10% and 20%. Furthermore, due to significant differences in Zr content between zircon and the selected standard samples, prolonged flushing is required after zircon analysis to decrease the background Zr levels in the instrument under conditions of high aerosol diffusion space. This prolonged flushing might impact the accuracy of Zr detection in the standard samples.