MC-ICP-MS两步静态法测量U-Th同位素
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摘要
本研究建立了多接收电感耦合等离子体质谱(MC-ICP-MS)两步静态法测量U-Th同位素的分析步骤和数据处理流程。对于U同位素,第一步用二次电子倍增器(SEM)测量~(234)U,同时用法拉第杯测量~(233)U、~(235)U、~(236)U和~(238)U;第二步用SEM测量~(236)U,同时用法拉第杯接收~(233)U、~(234)U、~(235)U和~(238)U。对于Th同位素,第一步用SEM测量~(229)Th;第二步用SEM测量~(230)Th,同时用法拉第杯接收~(229)Th和~(232)Th。U和Th的测量均通过两个步骤的多次循环采集同位素数据。该方法的关键在于采用SEM与法拉第杯测量的~(236)U_(SEM)/~(233)U_(FC)比值(经过质量歧视校正)与~(236)U/~(233)U真值的比较以校正SEM与法拉第杯的相对增益。本研究采用两步静态法重复分析了U同位素的国际标准样品Harwell uraninite(HU-1)。测量结果表明,HU-1的δ~(234)U平均值为(-0.48±1.92)‰,误差为单次测量结果的标准偏差(±2σ,n=55)。该结果与国内外不同实验室发表的数据在误差范围内一致。此外,本研究将两步静态测试法与SEM跳峰扫描法分别用于分析次生碳酸盐岩实验室标准样品的U-Th同位素和~(230)Th/U年代(SB-530、SB-8600、SB-108K、SB-240K、SB-335K和SB-435K)。对比实验结果表明,MC-ICP-MS两步静态法测量U-Th同位素行之有效,~(234)U/~(238)U和~(230)Th/~(238)U比值的分析精度可分别达到1‰～2‰和1‰～3‰,然而该方法的可靠性还需要更多国际标准样品的检验。
This study presented a newly developed multi-static U-Th isotopic measurement method. For U isotopes, firstly, the ~(234)U was measured on the SEM and simultaneously, the signals of ~(233)U, ~(235)U, ~(236)U and ~(238)U were measured in Faraday cups. Secondly, ~(236)U was collected on the SEM, and the ~(233)U, ~(234)U, ~(235)U and ~(238)U were measured in Faraday cups. The ratio of ~(236)U_(SEM)/~(233)U_(FC)(the ratio of SEM to Faraday cup measurement corrected for mass fractionation) was used to monitor the SEM/Faraday cup relative yield. The ~(238)U/~(235)U ratio was used for instrumental mass fractionation correction by comparing the measured value with the natural value. For Th isotopes, the ~(229)Th and ~(230)Th were alternately measured on SEM and ~(232)Th 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 ~(236)U, 5-15 V for ~(238)U(equivalent to 10~(11) Ω amplifier), <10~5 cps for ~(229)Th and ~(230)Th. 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 δ~(234)U 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-108 K, SB-240 K, SB-335 K and SB-435 K. 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 δ~(234)U values range from 95 to 1 030, and ~(230)Th/~(238)U 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 ~(234)U/~(238)U and ~(230)Th/~(238)U ratios at the precision of 1‰-3‰, but its reliability needs to be further validated by more international standards.
This study presented a newly developed multi-static U-Th isotopic measurement method. For U isotopes, firstly, the ~(234)U was measured on the SEM and simultaneously, the signals of ~(233)U, ~(235)U, ~(236)U and ~(238)U were measured in Faraday cups. Secondly, ~(236)U was collected on the SEM, and the ~(233)U, ~(234)U, ~(235)U and ~(238)U were measured in Faraday cups. The ratio of ~(236)U_(SEM)/~(233)U_(FC)(the ratio of SEM to Faraday cup measurement corrected for mass fractionation) was used to monitor the SEM/Faraday cup relative yield. The ~(238)U/~(235)U ratio was used for instrumental mass fractionation correction by comparing the measured value with the natural value. For Th isotopes, the ~(229)Th and ~(230)Th were alternately measured on SEM and ~(232)Th 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 ~(236)U, 5-15 V for ~(238)U(equivalent to 10~(11) Ω amplifier), <10~5 cps for ~(229)Th and ~(230)Th. 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 δ~(234)U 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-108 K, SB-240 K, SB-335 K and SB-435 K. 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 δ~(234)U values range from 95 to 1 030, and ~(230)Th/~(238)U 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 ~(234)U/~(238)U and ~(230)Th/~(238)U ratios at the precision of 1‰-3‰, but its reliability needs to be further validated by more international standards.
引文
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[2] SHEN C C,WU C C,CHENG H,LAWRENCE EDWARDS R,HSIEH Y T,GALLET S,CHANG C C,LI T Y,LAM D D,KANO A,HORI M,SPOTL C.High precision and high-resolution carbonate 230Th dating by MC-ICPMS with SEM protocols[J].Geochimica et Cosmochimica Acta,2012,99:71-86.
[3] IVANOVICH M,HARMON R S.Uranium-series disequilibrium:applications to earth,marine,and environmental science[M].Clarendon Press,Oxford University Press,1992.
[4] BOURDON B,HENDERSON G M,LUNDSTOM C C,TURNER S P.Uranium-series geochemistry[J].Reviews in Mineralogy and Geochemistry,2003,52:656.
[5] BROECKER W S,THURBER D L.Uranium-series dating of corals and oolites from Bahaman and Florida Key limestones[J].Science,1965,149(3 679):58-60.
[6] LAWRENCEEDWARDSL,CHEN J H,WASSERBURG G J.238U-234U-230Th-232Th systematics and the precise measurement of time over the past 500,000 years[J].Earth and Planetary Science Letters,1987,81(87):175-192.
[7] LUO X Z,REHKAMPER M,LEE D C,HALLIDAY A N.High precision 230Th/232Th and 234U/238U measurements using energy-filtered ICP magnetic sector multiple collector mass spectrometry[J].International Journal of Mass Spectrometry and Ion Processes,1997,171(1/2/3):105-117.
[8] GOLDSTEIN S J,STIRLING C H.Techniques for measuring uranium-series nuclides:1992-2002[J].Reviews in Mineralogy and Geochemistry,2003,52(1):23-57.
[9] SHEN C C,LAWRENCE EDWARDSR,CHENG H,DORALE J A,THOMAS R B,MORAN S B,WEINSTEIN S E,SDMONDS H N.Uranium and thorium isotopic and concentration measurements by magnetic sector inductively coupled plasma mass spectrometry[J].Chemical Geology,2002,185(3):165-178.
[10] HELLSTROM J.Rapid and accurate U/Th dating using parallel ion-counting multi-collector ICP-MS[J].Journal of Analytical Atomic Spectrometry,2003,18(11):1 346-1 351.
[11] FIETZKE J,LIEBETRAU V,EISENHAUER A,DULLO C.Determination of uranium isotope ratios by multi-static MIC-ICP-MS:method and implementation for precise U- and Th-series isotope measurements[J].Journal of Analytical Atomic Spectrometry,2005,20(5):395-401.
[12] MAKISHIMA A,CHEKOL T A,NAKAMURA E.Accurate determination of 234U/238U and 230Th/232Th for U-Th disequilibria studies by MC-ICP-MS with simple bracketing[J].Journal of Analytical Atomic Spectrometry,2007,22(11):1 383-1 389.
[13] MASON A J,HENDERSON G M.Correction of multi-collector-ICP-MS instrumental biases in high-precision uranium-thorium chronology[J].International Journal of Mass Spectrometry,2010,295(1/2):26-35.
[14] ANDERSEN M B,STIRLING C H,POTTER E K,HALLIDAY A N.Toward epsilon levels of measurement precision on 234U/238U by using MC-ICPMS[J].International Journal of Mass Spectrometry,2004,237(2/3):107-118.
[15] POTTER E K,STIRLING C H,ANDERSEN M B,HALLIDAY A N.High precision Faraday collector MC-ICPMS thorium isotope ratio determination[J].International Journal of Mass Spectrometry,2005,247(1/3):10-17.
[16] STIRLING C H,LEE D C,CHRISTENSEN JN,HALLIDAY A N.High-precision in situ 238U-234U-230Th isotopic analysis using laser ablation multiple-collector ICPMS[J].Geochim Cosmochim Acta,2000,64:3 737-3 750.
[17] ROBINSON L F,HENDERSON G M,SLOWEY N C.U-Th dating of marine isotope stage 7 in Bahamas slope sediments[J].Earth and Planetary Science Letters,2002,196(3/4):175-187.
[18] PIETRUSZKA A J,CARLSON R W,HAURI E H.Precise and accurate measurement of 226Ra-230Th-238U disequilibria in volcanic rocks using plasma ionization multicollector mass spectrometry[J].Chemical Geology,2002,188(3):171-191.
[19] HOFFMANN D L,PRYTULAK J,RICHARDS D A,ELLIOTT T,COATH C D,SMART P L,SCHOLZ D.Procedures for accurate U and Th isotope measurements by high precision MC-ICPMS[J].International Journal of Mass Spectrometry,2007,264(2/3):97-109.
[20] BALL L,SIMS K WW,SCHWIETERS J.Measurement of 234U/238U and 230Th/232Th in volcanic rocks using the Neptune MC-ICP-MS[J].Journal of Analytical Atomic Spectrometry,2008,23(2):173-180.
[21] POURMAND A,TISSOT F L H,ARIENZO M,SHARIFI A.Introducing a comprehensive data reduction and uncertainty propagation algorithm for U-Thgeochronometry with extraction chromatography and isotope dilution MC-ICP-MS[J].Geostandards and Geoanalytical Research,2013,38(2):129-148.
[22] SHAO Q,PONS-BRANCHU E,ZHU Q P,WANG W,VALLADAS H,FONTUGNE M.High precision U/Th dating of the rock paintings at Mt.Huashan,Guangxi,southern China[J].Quaternary Research,2017,88(1):1-13.
[23] 廖泽波,邵庆丰,李春华,孔兴功,汪永进.MC-ICP-MS标样-样品交叉测试法测定石笋样品的230Th/U年龄[J].质谱学报,2018,39(3):295-309.LIAO Zebo,SHAO Qingfeng,LI Chunhua,KONG Xinggong,WANG Yongjin.Measurement of U/Th isotopic compositions in stalagmites of 230Th/U geochronology using MC-ICP-MS by standard-sample bracketing method[J].Journal of Chinese Mass Sepctrometry Society,2018,39(3):295-309(in Chinese).
[24] FRANK N,TURPIN L,CABIOCH G,BLAMART D,TRESSENS-FEDOU M,COLIN C,JEAN-BAPTISTE P.Open system U-series ages of corals from a subsiding reef in New Caledonia:implications for sea level changes,and subsidence rate[J].Earth & Planetary Science Letters,2006,249(3/4):274-289.
[25] HIESS J,CONDON D J,MCLEAN N,NOBLE S R.238U/235U Systematics in terrestrial uranium-bearing minerals[J].Science,2012,335(6 076):1 610-1 614.
[26] RUSSEL W A,PAPANASTASSIOU D A,TOMBRELLO T A.Ca isotope fractionation on the Earth and other solar system materials[J].Geochimica et Cosmochimica Acta,1978,42(8):1 075-1 090.
[27] KAUFMAN A,BROECKER W S.Comparison of 230Th and 14C ages for carbonate materials from Lakes Lahontan and Bonneville[J].Journal of Geophysical Research,1965,70:4 039-4 054.
[28] EDWARDS R L,GALLUP C D,CHENG H.Uranium-series dating of marine and lacustrine carbonates[J].Reviews in Mineralogy and Geochemistry,2003,52:393-405.
[29] RICHARDS D A,DORALE J A.Uranium-series chronology and environmental applications of speleothems[J].Reviews in Mineralogy and Geochemistry,2003,52(1):407-460.
[30] 王立胜,马志邦,程海,段武辉,肖举乐.MC-ICP-MS测定铀系定年标样230Th年龄[J].质谱学报,2016,37(3):262-272.WANG Lisheng,MA Zhibang,CHENG Hai,DUAN Wuhui,XIAO Jule.Determination of 230Th dating age of uranium-series standard samples by multiple collector inductively coupled plasma mass spectrometry[J].Journal of Chinese Mass Spectrometry Society,2016,37(3):262-272(in Chinese).
[31] WANG Y J,CHENG H,EDWARDS R L,AN Z S,WU J Y,SHEN C C,DORALE J A.A high-resolution absolute-dated Late Pleistocene monsoon record from Hulu Cave,China[J].Science,2001,294(5 550):2 345-2 348.
[32] WANG Y J,CHENG H,LAWRENCEEDWARDS R,HE Y Q,KONG X G,AN Z S,WU J Y,KELLY M J,DYKOSKI C A,LI X D.The Holocene Asian monsoon:links to solar changes and North Atlantic climate[J].Science,2005,308(5 723):854-857.
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