U-Th Isotopic Measurements by MC-ICP-MS with Multi-Static Method

被引：0

作者：

Li C.-H. ^{[1
,2
]}

Huang M.-J. ^{[1
,2
]}

Liao Z.-B. ^{[1
,2
]}

Kong X.-G. ^{[1
,2
]}

Shao Q.-F. ^{[1
,2
]}

机构：

[1] Key Laboratory of Virtual Geographic Environment, School of Geography Science, Nanjing Normal University, Nanjing

[2] Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing Normal University, Nanjing

来源：

Journal of Chinese Mass Spectrometry Society | 2019年 / 40卷 / 03期

关键词：

!sup]230[!/sup]Th/U dating; Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS); Secondary carbonate; U-Th isotopes;

D O I：

10.7538/zpxb.2018.0105

中图分类号：

学科分类号：

摘要：

This study presented a newly developed multi-static U-Th isotopic measurement method. For U isotopes, firstly, the 234U was measured on the SEM and simultaneously, the signals of 233U, 235U, 236U and 238U were measured in Faraday cups. Secondly, 236U was collected on the SEM, and the 233U, 234U, 235U and 238U were measured in Faraday cups. The ratio of 236USEM/233UFC (the ratio of SEM to Faraday cup measurement corrected for mass fractionation) was used to monitor the SEM/Faraday cup relative yield. The 238U/235U ratio was used for instrumental mass fractionation correction by comparing the measured value with the natural value. For Th isotopes, the 229Th and 230Th were alternately measured on SEM and 232Th in a Faraday cup. Th isotopic measurements were carried out immediately after the U measurements from a same sample. The correction factors of SEM/Faraday cup relative yield and instrumental mass fractionation determined in U measurements were used for Th isotopic data corrections. The ideal ion beam intensities for measurements are 3-6 mV for 236U, 5-15 V for 238U (equivalent to 1011 Ω amplifier), <105 cps for 229Th and 230Th. The U and Th isotope data are usually acquired 200-400 cycles taking 20-30 min. To test the reliability of this method, the U isotopic standard Harwell uraninite (HU-1) was repeatedly measured and yielded a mean δ234U value of (-0.48±1.92)‰ (±2σ, n=55), which is in agreement with the data previously published by different laboratories. In this study, the multi-static method was applied to six in-house secondary carbonate standards, SB-530, SB-8600, SB-108K, SB-240K, SB-335K and SB-435K. The repeated measurements suggest that the six standards have U content of 500-3 000 ng/g and Th content of 0.1-1.0 ng/g. Their δ234U values range from 95 to 1 030, and 230Th/238U ratios range from 0.01 to 1.33. Their error weighted mean ages are (0.53±0.01), (8.61±0.01), (107.6±0.2), (239.4±0.8), (332.8±1.9) and (437±5) ka, respectively. These results were confirmed by the comparative study using the SEM method carried out in the Institute of Geology and Geophysics, Chinese Academy of Sciences. This study demonstrates that the newly developed U-Th isotopic measurement method can routinely measure the 234U/238U and 230Th/238U ratios at the precision of 1‰-3‰, but its reliability needs to be further validated by more international standards. © 2019, Editorial Board of Journal of Chinese Mass Spectrometry Society. All right reserved.

引用

页码：209 / 221

页数：12

共 40 条

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