Non‐Matrix‐Matched Determination of Th‐Pb Ages in Zircon, Monazite and Xenotime by Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry,Geostandards and Geoanalytical Research

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Non‐Matrix‐Matched Determination of Th‐Pb Ages in Zircon, Monazite and Xenotime by Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry
Geostandards and Geoanalytical Research ( IF 2.7 ) Pub Date : 2020-08-01 , DOI: 10.1111/ggr.12356
Tao Luo ₁ , He Zhao ₁ , Qiuli Li ₂ , Yang Li ₂ , Wen Zhang ₁ , Jingliang Guo ₁ , Yongsheng Liu ₁ , Junfeng Zhang ₁ , Zhaochu Hu ₁

Affiliation

Similar to the U‐Pb system, the ²³²Th‐²⁰⁸Pb decay scheme is also widely utilised in geochronology of Th‐rich accessory minerals. The matrix‐dependent systemic deviations observed between different accessory minerals and the paucity of well‐characterised reference materials for some minerals of interest (e.g., xenotime and allanite) are the major obstacles to yield accurate and precise Th‐Pb ages with laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). In this study, we investigated the effects of the addition of nitrogen, oxygen and water vapour before the ablation cell on the accuracy of Th‐Pb dating of accessory minerals (i.e., zircon, monazite and xenotime) using NIST SRM 610 glass as the calibrating reference material. Our results demonstrate that the measured ²⁰⁸Pb/²³²Th ages in zircon, monazite and xenotime were approximately 9–24% lower than their isotope dilution thermal ionisation mass spectrometry (ID‐TIMS) ages in normal ablation (without addition of additional gases). The measured ²⁰⁸Pb/²³²Th ages of zircon (Plešovice, M257 and Qinghu), monazite (44069 and Trebilcock) and xenotime (BS‐1 and MG‐1) showed excellent agreement with their respective reference values with the addition of small amounts of water vapour before the ablation cell. This may be due to the dramatically reduced biases for the down‐hole fractionation of Pb/Th intensity ratios in NIST SRM 610 and accessory minerals in the presence of water vapour. The Th‐Pb ages of zircon, monazite and xenotime were successfully analysed using NIST SRM 610 glass as the calibrating reference material with both 193 nm ArF excimer laser ablation‐quadrupole‐inductively coupled plasma‐mass spectrometry (LA‐Q‐ICP‐MS) and 213 nm Nd:YAG laser ablation‐sector field‐inductively coupled plasma‐mass spectrometry (LA‐SF‐ICP‐MS) in combination with the developed water vapour‐assisted method. This method is an effective and notably simple approach for non‐matrix‐matched analysis of Th‐Pb ages in zircon, monazite and xenotime by LA‐ICP‐MS.

中文翻译：

激光烧蚀-电感耦合等离子体质谱法测定锆石，独居石和Xenotime中的Th-Pb年龄

与U-Pb系统类似，²³² Th- ²⁰⁸铅衰变方案也被广泛用于富Th辅助矿物的年代学中。在不同的辅助矿物之间观察到的依赖于基质的系统偏差以及对某些感兴趣的矿物（例如，xenotime和尿铝石）缺乏精心设计的参考材料是阻碍激光烧蚀产生准确和精确的Th-Pb年龄的主要障碍。耦合等离子体质谱法（LA-ICP-MS）。在这项研究中，我们使用NIST SRM 610玻璃作为标定物，研究了在消融池之前添加氮，氧和水蒸气对辅助矿物（如锆石，独居石和xenotime）Th-Pb测年精度的影响。参考资料。我们的结果表明，测得的²⁰⁸ Pb / ²³²锆石，独居石和xenotime的年龄比正常消融（不添加其他气体）的同位素稀释热电离质谱（ID-TIMS）年龄低约9-24％。实测值²⁰⁸ Pb / ²³²锆石（Plešovice，M257和Qinghu），独居石（44069和Trebilcock）和xenotime（BS-1和MG-1）的年龄与消融池前添加少量水蒸气显示出与各自的参考值极好的一致性。这可能是由于在存在水蒸气的情况下，NIST SRM 610和辅助矿物质中Pb / Th强度比的井下分级的偏差大大降低了。使用NIST SRM 610玻璃作为校准参考材料，并同时使用193 nm ArF准分子激光烧蚀-四极杆-电感耦合等离子体质谱（LA-Q-ICP-MS），成功分析了锆石，独居石和xenotime的Th-Pb年龄。 213 nm Nd：YAG激光烧蚀扇区场电感耦合等离子体质谱法（LA-SF-ICP-MS）结合已开发的水蒸气辅助方法。

更新日期：2020-08-01

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