Impacts of Common Lead on Apatite U-Pb Geochronology by LA-ICP-MS: Assessment and Correction Strategies

LUO Tao; WANG Hanlin; ZHU Songbai; QING Liyuan; HU Zhaochu

doi:10.15898/j.ykcs.202404070079

Rock and Mineral Analysis > 2025 > 44(1): 51-62. > DOI: 10.15898/j.ykcs.202404070079

LUO Tao，WANG Hanlin，ZHU Songbai，et al. Impacts of Common Lead on Apatite U-Pb Geochronology by LA-ICP-MS: Assessment and Correction Strategies[J]. Rock and Mineral Analysis，2025，44（1）：51−62. DOI: 10.15898/j.ykcs.202404070079

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Impacts of Common Lead on Apatite U-Pb Geochronology by LA-ICP-MS: Assessment and Correction Strategies

1.
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), Wuhan 430074, China
2.
PetroChina Tarim Oilfield Company, Korla 841000, China

More Information

Received Date: April 06, 2024
Revised Date: July 05, 2024
Accepted Date: July 10, 2024
Available Online: August 08, 2024
Published Date: August 06, 2024

Abstract

HIGHLIGHTS
(1) A systematic bias of 2.5% to 6.0% is observed when using the common lead-containing apatite standard MAD as an external standard for direct U-Pb calibration.

(2) Accurate results (with bias below 2.0%) can be obtained by correcting for common lead in the standard (using the ²⁰⁷Pb method or Tera-Wasserburg diagram method) prior to performing Pb/U elemental fractionation correction.

(3) Accurate U-Pb ages of apatite reference materials of MAD, Otter Lake, and Durango can be obtained through calibration against non-matrix-matched NIST612 glass using a water vapor-assisted laser ablation method, effectively mitigating the influence of common lead in standards on the analytical results.

ABSTRACT

Apatite is a widespread U-bearing mineral in igneous, metamorphic, and sedimentary rocks. U-Pb geochronology of apatite can provide significant information for constraining magmatic evolution processes and tracing provenance. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a crucial technique for in situ U-Pb age analysis of apatite. However, the lack of suitable matrix-matched apatite reference materials and the inevitable presence of common lead in reference materials are major obstacles restricting high-precision determination of apatite U-Pb ages. This study investigates the impact of common lead on LA-ICP-MS apatite U-Pb dating results. The significant systematic biases (6%−30%) in both measured lower intercept ages and initial lead compositions are observed when calibrating against MAD apatite without common lead correction. However, accurate apatite U-Pb ages (within 2% systematic bias) can be obtained by correcting for common lead in MAD using the ²⁰⁷Pb method or Tera-Wasserburg plot method prior to Pb/U fractionation calibration. Furthermore, a vapor-assisted laser ablation method is employed in conjunction with NIST612 glass as an external standard to accurately analyze apatite U-Pb ages. This non-matrix matched method eliminates the need to consider the influence of common lead in the reference materials. Novel high-precision LA-ICP-MS apatite U-Pb dating methods are established with both matrix-matched and non-matrix-matched analyses, which greatly promote the application of apatite U-Pb geochronology in Earth science. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202404070079.
- laser ablation inductively coupled plasma-mass spectrometry,
- apatite,
- U-Pb dating,
- common lead correction,
- non-matrix-matched analysis
BRIEF REPORT
Significance: Apatite is a ubiquitous accessory mineral occurring in diverse geological environments^[1]. It typically contains U ranging from a few to several hundred ppm or more^[2-3]. Consequently, apatite U-Pb dating is widely employed to constrain the timing of significant geological processes, including diagenesis, mineralization, and fossil formation^[4-8]. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) is one of the important techniques for conducting in situ U-Pb age analysis of accessory minerals. Matrix-matched standards are crucial for obtaining accurate results in LA-ICP-MS U-Pb geochronology. However, apatite from diverse geological origins typically incorporates common lead, and the standards reported for in situ U-Pb geochronology of apatite likewise contain variable amounts of common lead. The measured Pb/U ratios are a mixture of radiogenic and common lead components when the apatite standards contain variable amounts of common lead. In previous apatite U-Pb geochronology studies, the ²⁰⁷Pb method was commonly employed to correct for common lead in MAD standards, which contain minimal common lead (approximately 1% age discordance). However, this approach is unsuitable for analyzing standards with higher common lead contents. As in situ U-Pb dating techniques for apatite continue to advance and the number of microanalysis laboratories expands, the availability of apatite MAD standards with low common lead content is diminishing. To obtain accurate and precise LA-ICP-MS apatite U-Pb results, different common lead correction methods (²⁰⁷Pb method or Tera-Wasserburg plot method) are performed to the apatite standard prior to Pb/U elemental fractionation correction in this study. Moreover, a water vapor-assisted method is proposed to determine accurate apatite U-Pb ages with NIST612 glass as the external standard. This non-matrix-matched analytical approach alleviates the lack of apatite U-Pb dating standards and effectively mitigates the impact of common lead in apatite standards on analytical results.
Methods: Experiments were performed with Agilent7900 quadrupole inductively coupled plasma-mass spectrometer (Agilent Technologies, Tokyo, Japan) coupled with a Coherent 193nm excimer nanosecond laser ablation system (Geolas HD, MicroLas Gttingen, Germany). Detailed experimental parameters are presented in Table 1. Apatite reference materials MAD, Durango, Otter Lake, and NIST612 glass were analyzed for U-Pb dating. For matrix-matched apatite U-Pb dating analysis, MAD was used as an external standard and Durango, and Otter Lake served as monitors to evaluate data accuracy. To obtain accurate and precise apatite U-Pb results, different common lead correction methods (²⁰⁷Pb method or Tera-Wasserburg plot method) were performed to the MAD standard prior to Pb/U elemental fractionation correction. The calibration procedures for the ²⁰⁷Pb method were as follows: (1) Subtract the background from the U and Pb signals for each measured MAD; (2) Perform common lead correction for each measured MAD; (3) Use the common lead-corrected MAD samples for Pb/U fractionation correction. These specific correction steps can be implemented using the VizualAge UComPbine function in the Iolite software. The principle of the Tera-Wasserburg plot method is based on the Tera-Wasserburg concordia diagram. The processes are as follows: (1) Calculate the intersection point of the discordia line constructed from all measured MAD with the X-axis of the Tera-Wasserburg concordia diagram; (2) Calculate the intersection point of the discordia line passing through the recommended age of the MAD standard with the X-axis. The ratio of these two intersection points is the Pb/U fractionation correction factor; (3) Correct the Pb/U ratios of unknown samples using the fractionation correction factor. The ²⁰⁷Pb/²⁰⁶Pb ratios of the unknown samples were directly corrected using a standard with homogeneous Pb isotopic composition (such as NIST glass). A water vapor-assisted method was proposed for non-matrix-matched apatite U-Pb age determination with approximately 4mg/min of water vapor introduced into the laser ablation cell. Apatite U-Pb ages were determined using NIST612 glass as the external standard.
Data and Results: The U-Pb age results of apatite Otter Lake and Durango by calibration against MAD without common lead correction are presented in Fig.2. The initial lead compositions of apatite standards are anchored with the Pb isotopic compositions from Stacey and Kramer’s model. The obtained lower intercept ages of Otter Lake and Durango show systemic bias of 2.5% and 6% relative to their reference ages, respectively. The results indicate that significant systematic biases in measured lower intercept ages are observed when calibrating against MAD apatite without common lead correction.
　　Fig.3 presents the U-Pb results for Otter Lake and Durango, calibrated against apatite MAD after common lead correction using the ²⁰⁷Pb method. The obtained lower intercept age for Otter Lake is less than 1% younger than the reference value. Moreover, the lower intercept age of Durango shows a deviation of −1.7% with the initial lead composition anchored in the Tera-Wasserburg concordia diagram. The results of Otter Lake and Durango using MAD as the external standard with common lead correction via the Tera-Wasserburg plot method are presented in Fig.4. The lower intercept ages of Otter Lake and Durango are 929.0±7.1Ma (2σ, MSWD=1.2) and 29.3±0.5Ma (2σ, MSWD=1.0), showing deviations of 1.7% and 6.6% from their recommended values, respectively. The larger deviation for Durango apatite may be attributed to its higher common lead content and the insufficient spread of the measured Pb/U ratios. Fig.5 shows the U-Pb results of apatite MAD, Otter Lake, and Durango using NIST612 glass as the external standard with the addition of water vapor within the laser ablation cell. The obtained lower intercept ages from the Tera-Wasserburg diagrams for apatite MAD, Otter Lake, and Durango are 474.7±2.7Ma (2σ, MSWD=1.6), 934.8±2.1Ma (2σ, MSWD=2.1), and 31.2±1.2Ma (2σ, MSWD=1.6), respectively. These ages show good consistency with their recommended values within the analytical uncertainties, respectively.

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