磷灰石LA-ICP-MS U-Pb定年直接校准方法研究

赵令浩; 詹秀春; 曾令森; 胡明月; 孙冬阳; 袁继海

doi:10.15898/j.cnki.11-2131/td.202202260035

磷灰石LA-ICP-MS U-Pb定年直接校准方法研究

Direct Calibration Method for LA-HR-ICP-MS Apatite U-Pb Dating

摘要

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。

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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