Characterisation of Three Sri Lankan Zircon Megacrysts as Potential Reference Materials for Microbeam U-Pb Geochronology and Hf-O-Zr Isotope Measurements |
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| Authors: | Pan Hu Tao Luo James Crowley Yuanbao Wu Chenxi Zhang Xiaoping Xia Tao Long Shaobing Zhang Zhian Bao Lei Xu Lanping Feng Wen Zhang Zhaochu Hu |
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| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, 430074 China;2. Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID, 83725 USA;3. Shandong Institute of Geological Science, Jinan, 250000 PR China;4. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 PR China;5. Beijing SHRIMP Center, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, 100037 Beijing, China;6. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026 China;7. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an, 710069 China;8. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China |
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| Abstract: | In situ U-Pb geochronology and hafnium, oxygen and zirconium isotope measurements in zircons using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and ion microprobe techniques can provide essential isotopic data to constrain geological evolutionary histories. Developing reliable zircon reference materials is the cornerstone for in situ zircon chronology and isotopic studies. In this study, the homogeneity of U-Pb ages and Hf-O-Zr isotope ratios in three Sri Lankan zircon megacrysts (SLZA, SLZB and SLZC) were investigated using multiple analytical methods. The obtained U, Th, Pb and Hf mass fractions of the SLZA zircon were 839 ± 56 μg g-1 (1s), 151 ± 15 μg g-1 (1s), 198 ± 28 μg g-1 (1s) and 8635 ± 286 μg g-1 (1s), respectively. The mass fractions of U, Th, Pb and Hf in the SLZB zircon were 1106 ± 106 μg g-1 (1s), 331 ± 61 μg g-1 (1s), 376 ± 57 μg g-1 (1s) and 9673 ± 976 μg g-1 (1s), respectively. The U, Th, Pb and Hf mass fractions determined in the SLZC zircon were 551 ± 35 μg g-1 (1s), 111 ± 8 μg g-1 (1s), 129 ± 18 μg g-1 (1s) and 7881 ± 393 μg g-1 (1s), respectively. The chemical abrasion isotope dilution thermal ionisation mass-spectrometry (CA-ID-TIMS) method yielded a Th-corrected weighted mean 206Pb/238U age of 556.94 ± 1.29 Ma (95% conf., n = 5) for the SLZA zircon, 552.90 ± 1.29 Ma (95% conf., n = 7) for the SLZB zircon and 560.83 ± 1.29 Ma (95% conf., n = 7) for the SLZC zircon. The obtained Hf isotopic compositions of the SLZA, SLZB and SLZC zircons determined with the solution MC-ICP-MS method were 0.281651 ± 0.000014 (2s, n = 10), 0.281704 ± 0.000008 (2s, n = 10) and 0.281676 ± 0.000006 (2s, n = 10), respectively. The obtained O isotopes of the SLZA and SLZB zircons measured with the laser fluorination method were 12.14 ± 0.56‰ (2s, n = 4) and 11.91 ± 0.30‰ (2s, n = 4), respectively. The Zr isotopes of the SLZA, SLZB and SLZC zircons determined with double spike TIMS analysis yielded mean δ94/90ZrSRM3169 values of -0.03 ± 0.06‰ (2s, n = 10), -0.03 ± 0.04‰ (2s, n = 10) and 0.00 ± 0.07‰ (2s, n = 8), respectively. The SLZA zircon can be used as a primary reference or quality control material for microbeam U-Pb, Hf and Zr isotope measurements because of its slight heterogeneity. The U-Pb, Hf and Zr isotopic compositions of the SLZB and SLZC megacrysts were homogeneous. The O isotopic compositions in the SLZA and SLZB zircon were slightly dispersed, indicating that these two megacrysts can only serve as secondary reference materials for microbeam O isotope measurements. |
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| Keywords: | zircon U-Pb geochronology Hf-O-Zr isotopes reference materials microbeam |
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