Reduction of mercury background on ICP-mass spectrometry for in situ U-pb age determinations of zircon samples
Reduction of mercury background on ICP-mass spectrometry for in situ U-pb age determinations of zircon samples
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DOI：
10.1039/b504153h
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发表时间：
2005-07
影响因子：
3.4
通讯作者：
T. Hirata;T. Iizuka;Y. Orihashi
中科院分区：
文献类型：
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作者：
T. Hirata;T. Iizuka;Y. Orihashi
对三颗显生宙锆石（300 mL）进行238U-206Pb精确测年，降低了Pb和u的信号强度。对仪器的汞信号强度和元素灵敏度进行了一系列测量，结果表明，当使用200 mL活性炭时，获得了最高的信噪比（分析物信号强度最高，汞信号最小）。通过使用木炭过滤器过滤Ar补充气体，可以将204Hg的信号强度降低到<100 s−1，这几乎比未使用Hg-trap时观察到的信号强度低一个数量级。木炭过滤装置可以显著降低204Pb强度测量的不确定度，从而提高了非放射性成因Pb （common-Pb）校正后206Pb和207Pb强度数据的可靠性。这对提高年轻锆石U-Pb年龄数据的准确性和精密度具有重要意义。我们测量了3颗显生宙锆石（≤600 Ma）的U-Pb年龄，锆石坑尺寸为18 ~ 20 μm。结果表明，斯里兰卡（SL13）和鱼峡谷凝灰岩（FCT）锆石的238U-206Pb年龄分别为566±26 Ma （2SD, n = 10）和27.6±1.8 Ma (2SD, n = 13)，与前人的研究结果吻合良好。单峰火山火成岩杂岩的238U-206Pb年龄（108.8±3.9 Ma）明显高于普遍接受的年龄（98.7±0.6 Ma）。这一差异的可能原因将在文中讨论。结果表明，利用LA-ICPMS对20 μm烧蚀坑显生宙锆石定年是可行的。
Precise 238U–206Pb ages for three Phanerozoic zircons ( 300 mL) resulted in lowering the signal intensity of Pb and U. A series of measurements for Hg signal intensity and elemental sensitivity of the instrument revealed that the highest S/N ratio (minimal Hg signal with highest signal intensity of analytes) was obtained when 200 mL of activated charcoal was used. By filtering the Ar make-up gas using the charcoal filter, the signal intensity of 204Hg could be reduced down to <100 s−1, which is almost an order of magnitude lower than that observed without the Hg-trap. With the charcoal filter device, the uncertainty on the measured 204Pb intensity could be reduced remarkably, and this resulted in improvements in reliability of the 206Pb and 207Pb intensity data after the correction of non-radiogenic Pb (common-Pb). This is especially important to improve the accuracy and precision of the U–Pb age data for young zircons. We have measured U–Pb ages of three Phanerozoic zircons (≲600 Ma) from 18–20 μm pit sizes. The resulting 238U–206Pb ages for zircons from Sri Lanka (SL13) and Fish Canyon Tuff (FCT) were 566 ± 26 Ma (2SD, n = 10) and 27.6 ± 1.8 Ma (2SD, n = 13), respectively, showing excellent agreement with the previously reported values. The 238U–206Pb age for Mount Dromedary Igneous Complex (108.8 ± 3.9 Ma) was significantly older than the widely accepted value (98.7 ± 0.6 Ma). The possible cause of this discrepancy will be discussed in the text. The data obtained here demonstrate that the dating of Phanerozoic zircons from 20 μm ablation pits using LA-ICPMS is a real possibility.