SIMS锆石U-Pb定年方法-中国科学院地质与地球物理研究所

1、SIMS 锆石 U-Pb 定年方法用于 U-Pb 年龄测定的样品(号码)用常规的重选和磁选技术分选出锆石。 将锆石样品颗粒和锆石标样 Pl &ovice (Sl ma et al., 2008)(或 TEMORA, Black et al., 2004)和Qinghu (Li et al., 2009)粘贴在环氧树脂靶上，然后抛光使其曝露一 半晶面。对锆石进行透射光和反射光显微照相以及阴极发光图象分析， 以检查锆 石的内部结构、帮助选择适宜的测试点位。样品靶在真空下镀金以备分析。U、Th、Pb 的测定在中国科学院地质与地球物理研究所 CAMECA IMS-1280 二次离子质谱仪(SI

2、MS)上进行，详细分析方法见 Li et al. (2009)。锆石标样与 锆石样品以 1:3比例交替测定。U-Th-Pb 同位素比值用标准锆石 Pl sovice (337Ma, Sl ma et al., 2008 (或TEMORA (417Ma, Black et al., 2004)校正获得，U 含量采 用标准锆石 91500 (81 ppm,Wiedenbeck et al., 1995)校正获得，以长期监测标准样 品获得的标准偏差(1SD = 1.5%,Li et al., 2010)和单点测试内部精度共同传递得 到样品单点误差，以标准样品 Qinghu(159.5 Ma, Li

3、et al., 2009) 作为未知样监测 数据的精确度。普通 Pb 校正采用实测204Pb值。 由于测得的普通 Pb 含量非常低，假定普通 Pb 主要来源于制样过程中带入的表面 Pb污染，以现代地壳的平均 Pb 同 位素组成(Stacey and Kramers, 1975)作为普通 Pb 组成进行校正。同位素比值及年 龄误差均为 1(7。数据结果处理采用 ISOPLOT 软件(文献)。参考文献Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W.,Mundil, R., Campbe

4、l, I.H., Korsch, R.J., Williams, I.S., Foudoulis, Chris., 2004.Improved 206Pb/238U microprobe geochronology by the monitoring of atrace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygenisotope documentation for a series of zircon standards. Chem. Geol., 205:115-140.Ji?iSlama, Jan

5、 Ko?ler, Daniel J. Condon, James L. Crowley, Axel GehdieM,.Hanchar, Matthew S.A. Horstwood, George A. Morris, Lutz Nasdala, NicholasNorberg, Urs Schaltegger, Blair Schoene, Michael N. Tubrett , Martin J.Whitehouse, 2008.Ple?ovice zircon A new natural reference material for U-Pb and Hf isotopic micro

6、analysis. Chemical Geology 249, 35Li, Q.L., Li, X.H., Liu, Y., Tang, G.Q., Yang, J.H., Zhu, W.G., 2010. Precise U-Pb andPb-Pb dating of Phanerozoic baddeleyite by SIMS with oxygen flooding technique.Journal of Analytical Atomic Spectrometry 25, 1107-1113.Li, X.-H., Y. Liu, Q.-L. Li, C.-H. Guo, and K

7、. R. Chamberlain (2009), Precisedetermination of Phanerozoic zircon Pb/Pb ageby multicollector SIMS withoutexternal standardization, Geochem. Geophys. Geosyst., 10, Q04010,doi:10.1029/2009GC002400.Ludwig, K.R., 2001. Users manual for Isoplot/Ex rev. 2.49. Berkeley GeochronologyCentre Special Publica

8、tion. No. 1a, 56 pp.Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution bya two-stage model. Earth Planet. Sci. Lett., 26, 207-221.Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., Vonquadt, A.,Roddick, J.C., Speigel, W., 1995. Three natura

9、l zircon standards for U-Th-Pb,Lu-Hf, trace-element and REE analyses. Geostand. Newsl. 19: 1-23.SIMS U-Pb dating methodsSamples XXX for U-Pb analysis were processedby conventional magnetic anddensity techniques to concentrate non-magnetic, heavy fractions. Zircon grains,together with zircon standard

10、 91500 were mounted in epoxy mounts which were thenpolished to section the crystals in half for analysis. All zircons were documented withtransmitted and reflected light micrographs as well as cathodoluminescence(CL) imagesto reveal their internal structures, and the mount was vacuum-coated with hig

11、h-puritygold prior to secondary ion mass spectrometry (SIMS) analysis.Measureme nts of U, Th and Pb were con ducted using the Cameca IMS-1280 SIMSat the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing.U-Th-Pb ratios and absolute abundanceswere determined relative to the s

12、ta ndard zircon 91500 (Wiede nbeck et al., 1995), an alyses of which were in terspersed with those ofunknown grains, using operati ng and data process ing procedures similar to thosedescribed by Li et al. (2009). A Iong-term uncertainty of 1.5% (1 RSD) for206Pb/38Umeasureme nts of the sta ndard zirc

13、o ns was propagated to the unknowns (Li et al.,2010), despite that the measured206Pbf38U error in a specific session is gen erallyaround 1% (1 RSD) or less. Measured compositi ons were corrected for com mon Pbusing non-radioge nic204Pb. Correcti ons are sufficie ntly small to be insen sitive to thechoice of com mon Pb compositi on, and an average of prese nt-day crustal composition (Stacey and Kramers, 1975) is used for the com mon Pb assu ming that the com monPb is largely surface con tam in atio n in troduced duri ng sample preparati on. Un certain ties on in dividual an alys