榍石LA-ICP-MS U-Pb定年技术研究
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国家自然科学青年基金(编号41203022)及国家地质实验测试中心基本科研业务费(编号2012CSJ05)资助


In-situ U-Pb Dating of Titanite by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry(LA-ICP-MS)
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    摘要:

    榍石具有较高的U含量和封闭温度,是一种重要的适用于U-Pb定年的副矿物。然而,基体效应与普通Pb校正成为制约榍石LA-ICP-MS U-Pb定年发展的主要因素。本文以206Pb/238U为例,采用基体归一化因子(FAVG)评估了锆石与榍石U-Pb定年标准的基体效应,结果显示榍石FAVG几乎都大于1.20,而锆石FAVG明显都小于1.20,表明锆石与榍石U-Pb定年标准存在显著的基体效应。以BLR-1榍石标准为外部校准标样,OLT1榍石获得的谐和年龄1014.9±4.8 Ma(95%置信水平,n=23,MSWD=0.32)与SHRIMP谐和年龄1017.1±3.6 Ma及ID-TIMS谐和年龄1014.6±1.3 Ma在误差范围内一致,同时获得206Pb/238U、207Pb/235U、207Pb/206Pb加权平均年龄在误差范围内与其谐和年龄一致;而以91500锆石为外部校准标样,OLT1榍石获得的206Pb/238U加权平均年龄为891.3±9.5 Ma(n=23,MSWD=5.3),与ID-TIMS测得的谐和年龄相比偏低约12%。经207Pb法校正后,TCB榍石获得的206Pb/238U加权平均年龄1015.6±6.2 Ma(n=16,MSWD=0.84)与ID-TIMS谐和年龄1018.1±1.7 Ma在误差范围内一致。本研究表明,采用基体匹配的榍石标准为外部标样,利用LA-ICP-MS对榍石进行U-Pb定年也能获得与ID-TIMS相一致的年龄,精度(2RSE)小于2%。

    Abstract:

    Titanite(CaTiSiO5) has relatively uranium contents(1~1000 ppm) and high Pb closure temperature(650~700℃), making it an important and ideal accessory mineral for U-Pb dating. However, development of titanite U-Pb dating has been limited by matrix effect between U-Pb dating standards and titanite samples and common-lead correction by LA-ICP-MS. Taking 206Pb/238U as an example, matrix normalisation factors(FAVG) were used to evaluate the matrix effect between zircon and titanite standards for U-Pb dating in this paper. The results showed that the FAVG values of titanite reference materials were almost larger than 1.20, while the FAVG values of zircon reference materials were almost less than 1.20, which indicated the matrix effect between zircon and titanite standards could not be ignored. OLT1 and TCB titanite standard were dated by a 213 nm Nd: YAG laser ablation system coupled with an ELEMENT2 ICP-MS using BLR-1 titanite U-Pb dating standard as external reference materials. The concordia age of OLT1 was 1014.9±4.8 Ma(95% conf.,n=23,MSWD=0.32),which agreed well with the concordia age 1017.1±3.6 Ma obtained by SHRIMP and 1014.6±1.3 Ma obtained by ID-TIMS. In addition, the weighted mean 206Pb/238U、207Pb/235U、207Pb/206Pb ages of OLT1 were generally consistent with the concordia age obtained by ID-TIMS. However, the weighted mean 206Pb/238U age of OLT1 was 891.3±9.5 Ma(n=23,MSWD=5.3) obtained by 91500 zircon as external standard, which was 12% lower than the concordia age dated by ID-TIMS. Using BLR-1 as external reference material, the weighted mean 206Pb/238U age of TCB titanite was 1015.6±6.2 Ma(n=16,MSWD=0.84) after common lead corrected by 207Pb method, which was consistent with the reference age of 1018.1±1.7 Ma determined by ID-TIMS. This study showed that U-Pb ages of titanite dating by LA-ICP-MS could be well consistent with the ages obtained by ID-TIMS when using matrix-matched titanite reference material as external standard, with the precision less than 2%(2RSE).

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袁继海,孙冬阳,赵令浩,范晨子,胡明月,詹秀春.2016.榍石LA-ICP-MS U-Pb定年技术研究[J].地质学报,90(8):2059-2069.
YUAN Jihai, SUN Dongyang, ZHAO Linghao, FAN Chenzi, HU Mingyue, ZHAN Xiuchun.2016. In-situ U-Pb Dating of Titanite by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry(LA-ICP-MS)[J]. Acta Geologica Sinica,90(8):2059-2069.

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  • 收稿日期:2016-08-23
  • 最后修改日期:2016-08-23
  • 录用日期:2016-08-24
  • 在线发布日期: 2016-08-24
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