Improved Inter‐calibration of Faraday Cup and Ion Counting for In Situ Pb Isotope Measurements Using LA‐MC‐ICP‐MS: Application to the Study of the Origin of the Fangshan Pluton, North China
Improved Inter‐calibration of Faraday Cup and Ion Counting for In Situ Pb Isotope Measurements Using LA‐MC‐ICP‐MS: Application to the Study of the Origin of the Fangshan Pluton, North China
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DOI：
10.1111/j.1751-908x.2015.00336.x
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发表时间：
2015-12
影响因子：
3.8
通讯作者：
Wen Zhang;Zhaochu Hu;Lu Yang;Yongsheng Liu;K. Zong;Haijin Xu;Haihong Chen;Shan Gao;Lei Xu-Lei
中科院分区：
文献类型：
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作者：
Wen Zhang;Zhaochu Hu;Lu Yang;Yongsheng Liu;K. Zong;Haijin Xu;Haihong Chen;Shan Gao;Lei Xu-Lei
开发了一种激光烧蚀多收集器电感耦合等离子体质谱（LA‐MC‐ICP‐MS）方法，以使用法拉第杯和离子计数器（FC-IC）的组合来获得低Pb材料（< 10 μg g g−1）中的Pb同位素比率的精确且准确的测量。使用IC收集低丰度204 Pb（~ 1.4%）。使用NBS 981标准溶液交叉计算FC-IC增益并研究IC的信号响应特性。在1小时内观察到FC-IC增益的显著、连续和线性降低，但可以使用校准品-样品-校准品交叉进样方法校正该漂移。此外，观察到本研究中使用的IC的非线性响应，并通过非线性校正算法进行校正，该算法通过测量一系列重量法制备的NBS 981标准品溶液（NIST SRM 981）建立。与传统的安排相比，使用新设计的X撇渣锥和射流样品锥提高了铅同位素的信号强度的1.9倍。与仅使用法拉第杯相比，发现使用组合FC-IC阵列在204 Pb强度< 8 mV时将20 xPb/204 Pb的测量重复性（RSD）提高约一个数量级。八个天然玻璃和NIST SRM 612参考材料玻璃（作为校准材料）进行了测量，以评估铅同位素测定的新协议。在2s的测量不确定度范围内，分析结果与参考值一致。对于MPI‐DING ATHO‐G（总Pb为5.67 μg g−1）、KL 2 ‐G（总Pb为2.07 μg g−1）和ML 3B ‐G（总Pb为1.38 μg g−1），20 xPb/204 Pb的典型准确度为首选值的0.09%，精确度< 0.33%（2 RSD）。用该方法测定了方山岩体中花岗闪长岩中的长石和镁铁质微粒包体中的Pb同位素比值。整个寄主花岗闪长岩中长石的铅同位素组成表明，在边缘带，长石为放射成因，逐渐变弱。对于MMEs，铅同位素组成的辉长岩是高度可变的，并与整个主机花岗闪长岩重叠。对于单颗粒长石，Pb同位素组成和微量元素的强烈边缘-核-边缘变化被解释为通过岩浆混合产生的。这些结果表明，房山岩体经历了幔源镁铁质岩浆与长英质岩浆的混合作用，镁铁质岩浆流入的比例随着时间的推移而减少。
A laser ablation multi‐collector inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) method was developed to obtain precise and accurate Pb isotopic ratio measurements in low‐Pb materials (< 10 μg g−1) using a combination of Faraday cups and ion counters (FC–IC). The low abundance 204Pb (~ 1.4%) was collected using an IC. A NBS 981 standard solution was used to cross‐calculate the FC–IC gain and to investigate the signal response characteristics of the IC. A significant, continuous and linear decrease in the FC–IC gain was observed within 1 hr, but this drift could be corrected using the calibrator‐sample‐calibrator bracketing method. In addition, a non‐linear response of the IC used in this study was observed and corrected by a non‐linear correction algorithm, which was established by measuring a series of gravimetrically prepared NBS 981 standard solutions (NIST SRM 981). Compared with the conventional arrangement, the use of the newly designed X skimmer cone and Jet sample cone improved the signal intensities from Pb isotopes by a factor of 1.9. Compared with only Faraday cups, using a combination FC–IC array was found to enhance the measurement repeatability (RSD) of 20xPb/204Pb by approximately one order of magnitude when the 204Pb intensity was < 8 mV. Eight natural glasses and the NIST SRM 612 reference material glass (as a calibration material) were measured to evaluate the new protocol for Pb isotope determination. The analytical results were in agreement with the reference values within 2s measurement uncertainties. For MPI‐DING ATHO‐G (5.67 μg g−1 total Pb), KL2‐G (2.07 μg g−1 total Pb) and ML3B‐G (1.38 μg g−1 total Pb), the typical accuracies of 20xPb/204Pb were 0.09% of preferred values with precisions of < 0.33% (2RSD). The Pb isotope ratios in feldspars from granodiorite and within mafic microgranular enclaves (MMEs) from the Fangshan pluton, North China, were measured using the present method. The Pb isotopic compositions of feldspars from the whole host granodiorite show that that are radiogenic in the margin zone and gradually become less radiogenic. For the MMEs, the Pb isotopic compositions of feldspars are highly variable and overlap with those of the whole host granodiorite. For single‐grain feldspar, the strong rim‐core‐rim variations of the Pb isotopic compositions and trace elements are interpreted to have been generated via magma mixing. These results suggest that the Fangshan pluton underwent magma mixing of mantle‐derived mafic magmas with felsic magmas, and the proportion of the mafic magma influx decreased over time.