Direct Calibration Method for LA-HR-ICP-MS Apatite U-Pb Dating
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摘要:
磷灰石作为含铀副矿物在各种类型地质样品中广泛存在，其U-Pb封闭温度~500℃，是良好的热年代学研究对象。但是磷灰石相对较低的铀含量和较高普通铅含量以及缺少基体匹配标准样品等问题限制了磷灰石LA-ICP-MS U-Pb定年技术的发展和应用。本文采用激光剥蚀高分辨电感耦合等离子体质谱(LA-HR-ICP-MS)针对Madagascar磷灰石样品MAD2进行U-Pb定年分析，探讨其U-Pb同位素均一性及直接用作磷灰石LA-ICP-MS U-Pb定年标准样品的可行性。结果表明：该样品U、Pb含量均值分别为23.8×10-6和13.5×10-6，颗粒内207Pb/206Pb和206Pb/238U比值均一性较好，加权平均值分别为0.0943±0.0006和0.0794±0.0004，可以用于直接校准磷灰石LA-ICP-MS U-Pb同位素分析过程中的元素分馏效应，无需普通铅校正。以MAD2为标准样品，结合207Pb普通铅扣除法，测定了不同年龄磷灰石样品U-Pb年龄，结果为：McClure Moutain(521±5Ma)、Tory Hill-apt(1021±16Ma)、Durango(30.7±1.5Ma)、房山岩体闪长岩磷灰石(~131Ma)，各样品年龄测定值与推荐值在误差范围内一致，表明本文建立的LA-ICP-MS U-Pb定年方法和同位素比值校准方案的可行性和准确性。本文采用的校准和数据处理方案有效地降低了磷灰石LA-ICP-MS U-Pb定年数据处理难度，有利于方法的推广和应用。
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关键词:
- 磷灰石 /
- U-Pb定年 /
- LA-ICP-MS /
- 标准样品 /
- Madagascar磷灰石(MAD2)
Abstract:BACKGROUND Apatite is a common U-bearing accessory mineral with a U-Pb closure temperature of ~500℃, making U-Pb dating of apatite a potentially valuable thermochronometer. However, its low U concentration, high common lead concentration and lack of reference material has limited widespread application to LA-ICP-MS dating.
OBJECTIVES To develop a technique for U-Pb dating of apatite using laser ablation sector field inductively coupled plasma-mass spectrometry (LA-HR-ICP-MS).
METHODS The U-Pb isotope ratio in apatite samples was determined by LA-HR-ICP-MS, with apatite MAD2 as the external standard to correct U-Pb and Pb-Pb elemental fractionation directly without a common Pb correction.
RESULTS Long-term U-Pb analysis of Madagascar apatite sample (MAD2) showed homogeneous distribution of U, Pb and U-Pb isotope ratios, with average contents of U and Pb ~23.8×10-6 and ~13.5×10-6, respectively and weighted average 207Pb/206Pb and 206Pb/238U ratios of 0.0941±0.0006 and 0.0794±0.0004, respectively. Taking MAD2 apatite as a reference mineral, combined with 207Pb-correction method, the ages of apatite samples, McClure Mountain (521±5Ma), Tory-Hill-apt (1021±16Ma), a Durango (30.7±1.5Ma) and Fagnshan diorite apatite (~131Ma) can be determined accurately.
CONCLUSIONS The Madagascar apatite sample (MAD2) can be used to calibrate apatite U-Pb isotope ratio measured by LA-ICP-MS directly, without common-Pb correction, similar to the calibration strategy in zircon U-Pb dating. The method greatly reduces the difficulty of data processing during apatite U-Pb dating by LA-ICP-MS, which is conducive to the wide application of the method.
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Key words:
- apatite /
- U-Pb dating /
- LA-ICP-MS /
- reference material /
- Madagascar apatite (MAD2)
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表 1 LA-ICP-MS仪器参数和工作条件
Table 1. Instrumental setup and operating conditions
高分辨电感耦合等离子体质谱
(Thermo Scientific Element XR)激光剥蚀系统
(NWR 193ArF准分子激光器)参数 工作条件 参数 工作条件 RF功率 1400W 波长 193nm 冷却气(Ar)流速 16L/min 脉冲时间 15ns 辅助气(Ar)流速 0.9L/min 激光斑束 25μm, 30μm, 40μm 样品气(Ar)流速 0.98L/min 激光频率 10Hz 分辨率 低(M/ΔM=300) 激光能量 7mJ 扫描模式 E-Scan 剥蚀模式 点剥蚀 扫描质量
积分时间202Hg(16ms), 204Pb(16ms),
206Pb(24ms), 207Pb(24ms), 208Pb(16ms), 232Th(16ms), 238U(24ms)载气(He) 流速 0.87L/min 接收器模式 Counting: 202Hg, 204Pb, 207Pb, 208Pb Analog: 206Pb, 232Th, 238U 剥蚀时间 40s 
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