年代学-陈福坤-第五节-U-Pb.pdf

同位素地质年代学与 放射成因同位素地球化学 同位素地质年代学与 放射成因同位素地球化学 陈福坤 固体同位素地球化学实验室固体同位素地球化学实验室 中国科学院地质与地球物理研究所 铀－钍－铅定年方法铀－钍－铅定年方法 U – Th - Pb isotope system 204Pb206Pb208Pb207Pb 238U235U 205Pb 143146 92 82 142 Number of neutrons Number of protons 232Th 210Pb202Pb 120122 123124 125126128 90 234U 142 U and Th Deacy T1/2 7.1 x 108year 235U 207Pb 7α; T1/2 4.51 x 109year 238U 206Pb 8α; T1/2 1.41 x 1010year 232Th 208Pb 6α; Isotope abundance of U and Th Group Ac 锕）metal, lithophile element and trace element in the Earth Atomic abundance Atomic abundance ratio 238U/235U 137.88 234U is also radioactive, T1/2 2.47x105year α-decay 234U 0.0057 235U 0.720 238U 99.275 232Th 100 Pb Isotopes Pb produced by radioactive decay of U and Th, Increase of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb is due to U and Th decay. 238U206Pb 235U207Pb 232Th208Pb 204Pb is non-radiogenic Isotope abundance of Pb Group IV metal, chalcophile element and trace element in the Earth Atomic abundance T1/2 3x105year 202Pb 204Pb 1.4 205Pb 206Pb 24.1 207Pb 22.6 208Pb 52.3 210Pb T1/2 1.4x1017year T1/2 3x105year T1/2 22 year Both U and Th generally have a valence of 4, but under oxidizing conditions, U may have a valence of 6. U4 has an ionic radius of 0.89A and U6has an ionic radius of 0.73Ain 6-fold and 0.86Ain 8-fold coordination. Th has an ionic radius of 0.94A. 。。 。 Combination of relatively large radius and high-charge is not readily accommodated in crystal lattices of most common rock- ing minerals, so both U and Th are highly incompatible elements. This relatively immobile under most circumstances. In its reduced , U4is insoluble, but in the U6, U s the soluble oxyanion complex, UO22-, and then U can be quite mobile. U and Thcan their own phases in sedimentary rocks, uranite and thorite In igneous and metamorphic rocks, U and Th are either dispersed as trace elements in major phases or concentrated in accessory minerals Zircon ZrSiO4, U Th Monazite [La,Ce,Th]PO4, Th U Apatite Ca5PO43OH Sphene or titanite CaTiSiO4OH Pb can exist in two valence states, Pb2and Pb4. Pb2is the most common state and Pb4state is rare and restricted to highly alkaline or oxidizing solutions. The ionic radius of Pb2is 1.19Ain 6-fold coordination and 1.29Ain 8-fold coordination. 。 。 As a result of its large ionic size, Pb is an incompatible element, though not as incompatible as U and Th. The most common Pb mineral is galena PbS. In silicates, Pbsubstitutes reasonably readily for K ionic radius 1.33, particularly in potassium feldspar. 207Pb* 235Ueλ5t – 1 206Pb* 238Ueλ8t – 1 207Pb* 206Pb* 235Ueλ5t – 1 238Ueλ8t – 1 1 137.88 eλ5t– 1 eλ8t– 1 U -Pb 表面年龄 t206Pb/238Ut207Pb/235Ut207Pb/206Pb t206Pb/238Ut207Pb/235Ut207Pb/206Pb t206Pb/238Ut207Pb/235Ut207Pb/206Pb 谐和年龄（一致年龄） 不谐和年龄 反相不谐和年龄 207235 Pb / U 206238 Pb / U t207Pb/235U t206Pb/238U t207Pb/206Pb U – Pb concordia diagram 谐和年龄 207235 Pb / U 206238 Pb / U t207Pb/235U t206Pb/238U t207Pb/206Pb 不谐和年龄 U – Pb concordia diagram U – Pb concordia diagram 207235 Pb / U 206238 Pb / U t207Pb/235U t206Pb/238U t207Pb/206Pb 反相不谐和年龄 Hf ca. 1 REE appreciable U and Th 锆石 Zircon Zr[SiO4] 238U 206Pb 235U 207Pb 232Th 208Pb 200 400 600 800 1000 1200 1400 0.00010.0010.010.1110 Effective diffusion radius cm Closure Temperature C Nd, Garnet Sm, Apatite Sm, Zircon Nd, Titanite Pb, Zircon After Cherniak et al. 1997; Lee et al. 1997; Metzger et al. 1989; etc. 207235 Pb / U 206238 Pb / U Lower-intercept age Upper-intercept age U-Pb concordia diagram 有效地获得谐和的U -Pb 表面年龄的方法 1.机械磨损法（K r o g h ，19 8 2 ） 2 . 逐步酸淋滤法或称局部溶解法（M a t t i n s o n ， 19 9 4） 3. 单颗粒蒸发法（K o b e r ，19 8 6，19 8 7 ） 4. 阴极发光指导选样法（Po l l e r e t a l . ，19 9 7 ） 5 . 机械晶区分离法（Se r g e e v e t a l . ，19 9 7 ） 6. 离子探针微区分析（Co mp s t o ne t a l . ，19 8 4） Zircon dating U-Pb isotope dilution Pb-Pb evaporation in-situ U-Pb dating e.g., SHRIMP Chemical ProcedureChemical Procedure Zircon U-Pb isotope dilution Zircon UZircon U- -PbPb isotope dilutionisotope dilution 1.1.Decomposition of Decomposition of zircon grainszircon grains Vessel Bombe HF Chemical ProcedureChemical Procedure 2. Separation2. Separation of U and of U and PbPb HClHCl Zircon U-Pb isotope dilution Zircon UZircon U- -PbPb isotope dilutionisotope dilution resinresin 6.2 N HCl PbPb SampleSample solutionsolution ZrZr 3.1N3.1N HClHCl U U U U H2O UPbUPb resinresin 6.2 N HCl PbPb SampleSample solutionsolution ZrZr 3.1N3.1N HClHCl U U U U H2O U U H2O UPbUPb Sample load 1 ml, 3.1N HCl Schematic Zr distribution Sample wash 1 ml, 3.1N HCl Pb off 1 ml, 6.2N HCl U off 1 ml, H2O 90 U 10 U 58 Pb 32 Pb 10 Pb Ion exchange elution curves of Zr, Pb and U time Krogh, 1973 Chemical ProcedureChemical Procedure Separation of U and Pb HBr 2.8-2.5 Ga; 1.8-1.0 Ga s. margin 1.8 Ga; 1.7-1.5 Ga; 0.8-0.6 Ga n.-w. margin 0.5-0.4 Ga s. margin N. China BlockS. China Block North China Block E. Proterozoic Archean E. Proterozoic Archean 1.8 Ga Fr o m G u oJ-H 120 E 100 E 20 N 30 N Shanghai Hongkong Yangtze Block Cathaysia Block Grenville-age mobile belt South China 820-800 Ma plutonia and volcania e.g. Li et al. 2001 780-760 Ma gneisses e.g. Hacker et al. 1998 Laurentia Australia South China Yangtze Cathaysia Greater India East Antarctica Rift zone At 1.0 - 0.7 Ga 0 0 30 30 Li et al. 1995 North China Block E. Proterozoic Archean E. Proterozoic Archean 1.8 Ga Beihuaiyang Low-Grade Zone, the Dabie UHP Belt, E. China The Dabie UHP zone Beihuaiyang low-grade zone N. Dabie gneiss and migmatite S. Dabie UHP zone Susong HP zone Qinling-Dabie Orogenic Belt The Qinling – Dabie Orogenic BeltThe Qinling – Dabie Orogenic Belt Zhang et al. 1996 Northern suture Southern suture QILING DABIE Two-stage model Qinling ocean Triassic Northern Suture SCBNCBNQSQ Northern Qinling Southern Qinling Rifting Ordovician - Permian SCB NCB North China Block Proto-Tethyan ocean Qinling ocean South China Block Late Proterozoic - Cambrian SCB NCB Northern Suture Southern Suture Late Triassic SCBNCB NQSQ Ultrahigh-pressure eclogite Two major models proposed for the Dabie-SuLu pattern * Indentation model * Crustal-detachment model Indentation model Yin 2 Rowley et al. 1997; 3 Xue et al. 1997; 4 Ames et al. 1996; 5 Hacker et al. 2000 6 Chen et al. 2003 Zircon ages Hacker et al. 2000 31 00 31 20 116 00 116 30 Cretaceous Fault Mozitang Xiaotian - Suture Hacker et al. 1998 Cretaceous basin Northern Dabie gneisses and migmatites Basement rock Luzhengguan gneiss Foziling schist Cretaceous syenite to tonalite Cretaceous volcanics Metasediments 0 Ma1000200030004000 0 2 4 6 Cratonization of the NCB Cratonization of the SCB Break-up Rodinia Crystallisation ages of metagranites Zircon 207Pb/206Pb age spectrum of metasediments, the low-grade Beihuaiyang zone, Dabie Sources for the metasediments * Young magmatism 0.6-0.4 Ga * Yangtze-affinity 1.0-0.7 Ga * North-China-affinity 1.8 Ga What dose this mean for the tectonic scenario Qinling ocean Prior to Late Triassic Northern Suture Yangtze Cathayian Blocks North-China affinity 1.8 Ga Yangtze affinity 1.0 - 0.7 Ga Young magmatism 0.5 - 0.3 Ga South China Block North China Block NQSQ Potential sources for the low-grade Beihuaiyang zone Foziling schists Mass transfer Nd isotope of gneisses and schists the low-grade Beihuaiyang zone 341 -25 -20 -15 -10 -5 0 5 εNd 780 Ma Metagranite Dabie gneiss NW Yangtze sediment Metasediment -25 -20 -15 -10 -5 0 5 εNd 400 Ma 2341 谢谢谢谢