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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 钾－氩和氩－氩定年方法钾－氩和氩－氩定年方法 K-Ar and K-Ar and 40 40Ar/ Ar/39 39Ar dating Ar dating K – Ar isotope system 36Ar38Ar40Ar 40K39K 1820 19 18 2122 Number of neutrons Number of protons 41K 20 43,44,46,48Ca40Ca42Ca 19 钾 Potassium Group I, alkali metal, major element in the Earth 39K 93.26 6.73 Atomic abundance 40K41K 0.01 钙 Calcium Atomic abundance 40Ca 96.941 42Ca 0.647 43Ca 0.135 44Ca 2.086 46Ca 0.004 48Ca 0.187 Group II, alkaline metal, major element in the Earth 氩 Argon Group VIII, noble gas, trace element in the Earth Stable isotopes 36Ar, 38Ar and 40Ar Radiogenic isotope 40Ar Initial atomic abundance ratio 40Ar/36Ar 295.5 38Ar 0.07 99.59 Atomic abundance 40Ar36Ar 0.34 λe 0.581 x 10-10year-1 40K 40Ar 10.5 β-capture 40K 40Ca β- 89.5 λβ 4.962 x 10-10year-1 λ λe λβ 5.543 x 10-10year-1 T1/2 1.28 x 109year Radiogenic daughter production 40Ar* 40Ar* λe/λ40Keλt-1 Radiogenic daughter production 40Ca* 40Ca* λβ/λ40Keλt-1 The atmospheric ratio of 40Ar/36Ar is known to be 295.5. By measuring the amount of 36Ar present, we can deduce the amount of atmospheric 40Ar initially present. 40Ar 36Ar36Ar 40K λ λe 295.5 eλt-1 Because Ar is not chemically bound in lattices, the K-Arclock will generally be reset more readily than other systems. An event is that “resets” a radiometric clock is generally a thermal one. But, in the case of K-Ar, the system would be reset whenever temperatures are high enough to allow Ar to diffuse out of the rock or mineral of interest. A r d i f f u s i o n Diffusion coefficient for Argon in biotite against the inverse of thermodynamic temperature Harrison et al., 1985 Closure Temperature of Isotope System Rb-Sr system Muscovite 450-500 OC Biotite 350 OC K-Ar system Hornblende 450-500 OC Biotite 300 OC Muscovite 350-400 OC Microcline 150-250 OC U-Pb system Zircon 900 OC Garnet 800 OC Monazite 650-740 OC Sphene titanite 500-670 OC Rutile 420-380 OC Apatite 350 OC Fraction of Ar lost from a 150 cylindrical crystal as a function of temperature for various heating times. All Ar is lost in 10 Ma at 300 0C, or in 1 Ma at 360 0C. 40Ar-39Ar Dating The key of this is the production of 39Ar by a nuclear reaction on 39K, by irradiating a sample with neutrons in a reactor. 39K n, p 39Ar The production of 39Ar from 39K e the neutron energy; φe the flux of neutrons with energy e; σe the capture cross section for that energy; τ irradiation time. 39Ar 39K τ φe σede 40Ar* 36Ar λ λe 39K τ φe σede 40 Ke λt -1 C τ φeσede 1 λ λe C 40Ar* 36Ar eλt-1 39K 40K 40Ar/39Ar age spectrum produced by step heating of a biotite from a granitic gneiss. McDougall its ionic radius is 0.69. Re is one of Group VII metals and has a valence of 4; its ionic radius is 0.63. The siderophile/chalcophile nature of Re and Os makes this isotope system useful to address questions of core ation and ore genesis. Unlike other radioactive and radiogenic elements, which are incompatible ones and hence enriched in melts, Os is a highly compatible element bulk D 10 and is enriched in the residual solid. This makes Os isotope ratios particularly useful in studies of the mantle. Os is highly compatible, but Re is moderately incompatible and is slightly enriched in the melt. Partial melting appears to produce an increase in the Re/Os ratio by a factor of 102. Therefore, the range of Os isotope ratios in the Earth is enormous compared to other radiogenic elements. Average Re/Os ratio chondrite0.08 mantle peridotite0.08 basalts 8.9 Average 187Os/188Os ratio chondrite0.128 cosmic fluxes 0.13 mantle 0.127 crust 1.2-1.3 modern see water 8 Esser and Turekian, 1993; Allgre and Luck, 1980 Geochronologicalapplications of Re-Os are limited due to the very low concentrations of Os in most minerals. Re-Os geochronology has applied on dating the ation of iron meteorites, platinum group metal ores, and some ultramafic rocks. Re is strongly concentrated in molybdenite MoS2 and some copper-sulfides. Hence, the Re-Os system can be suitable for dating certain types of sulfide ore deposits. Re-Os isochron for a komatiite from Monro Township. Walker et al.,1988 187Os/188Ossam- 187Os/188Oschon 187Os/188Oschon x 100γOs Schematic evolution of Os isotope ratios in the mantle and crust. 187Os/188Os evolution in the mantle γOsvalues of peridotite of the subcontinentallithosphere, upper oceanic mantle and oceanic island basalts. average 10average -2 average -10 Os isotope composition of seawater over the last 80 Ma Peuker-Ehrenbrink et al., 1995 R e - O s 同位素分析的样品分解 1 . 碱熔法 Na 2 O 2 , Na O H 确保难熔矿物分解和样品与稀释剂同位素平 衡; 实验流程长，流程本底高 2 . 酸溶法 本底低，但不能确保样品和稀释剂同位素平 衡，挥发性O s O 4 易逸出，实验重复性差 R e - O s 同位素分析的样品分解 3. 卡洛斯管高温高压熔样（王水或反王水） 本底低，完全溶解样品，确保样品和稀释剂 同位素平衡，溶样时保持密闭，装样和取样 时保持低温环境，可防止挥发性O s O 4 逸出 卡洛斯管装样 和取样示意图 样品 乙醇 干冰 天然气喷灯 卡 洛 斯 管 卡洛斯管高温 高压溶样 卡 洛 斯 管 样品 钢套 O s 分离纯化 1 . 溴提取法 O s O 4 H Br O s Br 6 2 - 溶液层 液溴层 59 oC O s 分离纯化 2 . 微蒸馏法 电热板 OsO4 HBr 样品 H2SO4 CrO3 R e 分离纯化 －离子交换树脂分离纯化 Fission track dating （裂变径迹） From J.M. Bird A fraction of U atoms undergo spontaneous fission rather than alpha decay. The sum of the masses of the fragments is less than that of the parent U atom this difference reflects the greater binding energy of the fragments. The missing mass has been converted to kinetic energy of the fission fragments. The energy is deposited in crystal lattices through which the fission fragments pass by stripping electrons from atoms in the crystal lattice. The ionized atoms repel each other, disordering the lattice and producing a small channel and a wider stressed region in the crystal. The damage is visible as tracks seen with an electron microscope operating at magnifications of 50,000 x or greater. The stressed region is more readily attacked and dissolved by acid; so by acid etching the tracks can be enlarged to the point where they are visible under the optical microscope. Etching procedure for fission track dating Mineral Etching solution Temperature Duration oC Apatite 5 HNO325 10-30 s. Epidote37.5M NaOH159 150 min. SpheneConc. HCl90 30-90 min. Muscovite 48 HF 20 20 min. Volcanic glass 24 HF 25 1 min. Zircon 100M NaOH270 1.25 h. Number of tracks is a function of time and the U content of the sample Fs λƒ/λα 238U eλαt– 1 λfis the spontaneous fission decay constant, the best estimate for which is 8.46 x 10 -17yr-1. About 5 x 10-7U atoms undergo spontaneous fission for every one that undergoes α-decay. Analytical procedures 1. Determining fission track density involves a relatively straightforward procedure of polishing and etching a thin section, and then counting the number of tracks per unit area. 2. Determination of the U concentration of the sample. This is usually done by neutron irradiation and counting of the tracks resulting from neutron-induced fission. As fission is a rare event in any case, fission track dating generally uses uranium rich minerals. Minerals commonly used are apatite, sphene titanite, and zircon. Fission tracks are subject to annealing at geologically low to moderate temperatures. Relationship between the percentage of tracks annealed, temperature, and time for apatite and sphere. In general, closure temperatures for fission tracks are below those of conventional isotope geochronometers, so they are particularly useful in analysis of low temperature events and in determining cooling histories. When combined with estimates of geothermal gradients, fission track ages, particularly if ages for a variety of minerals are determined, are a useful tool in studying uplift and erosion rates. Apparent closure annealing temperatures of fission tracks as a function of cooling rate for a variety of minerals. U-decay series dating （铀系不平衡） U and Th do not decay directly to Pb, rather the transition from U and Th to Pb passes through many intermediate radioactive daughters. 238U235U 232Th 206, 207, 208 Pb Half-lives and constants of long- lived U and Th daughters Disequilibrium Dating Can be applied when a radioactive daughter product becomes separated from its parent isotope 234U 230Th in sea water 222Rn 210Pb in the atmosphere 234U and 222Rn are decay products of 238U series 230Th –238U dating on sediments 230Thu 230Thue-λ230t 0 230Th/232Thu 230Th/232Thue-λ230t 0 230Th –238U isochron diagram Faure, 1986 Slope of the isochron changes as a function of time Faure, 1986 230Th/232Th ratio as a function in depth Huh T1/2 12.3 years Low activity 1 part in 1018varies by region T r i t i u m d a t i n g Used to trace water sources age of „recent“ materials Sources directly fed by rainwater will contain the same tritium levels as rainwater Trapped aquifers will have no tritium Slow traveling aquifers will have a reduced amount Tritium dating 10Be dating 1 0 Be d a t i n g 10Be is produced by reactions of high energy cosmic ray protons with N2and O2 in the atmosphere and at the surface of minerals exposed to atmosphere 10Be then undergoes decay to 10B with a half-life of about 1.5 Ma 1 0 Be d a t i n g Growth of Mn nodule 锰结核） Manganese nodules are dark, potato-shaped little balls where metals and other minerals have accumulated around a core over a few million years. They contain a relatively high percentage of metals, i.e. Nickel, Copper, Cobalt, Manganese and Iron and are mostly found in water depths of 4000-6000 meters a few thousand km from the closest continent shores. Their growth rate can be dated with the help of cosmogenic nuclides. Depth mm Ln26Al/10BeLnd.p.m/kg Mn nodule Magma chamber 10Be 10Be sediments Volcano accreted prism material return to mantle Incorporation of cosmogenic 10Be in island arcs 10Be in the sediments may be transferred to the overlying mantle wedge and incorporated in magmas, and eventually lavas that are erupted by volcanoes. This provides unambiguous evidence that sediments are recycled through subduction zones, as the half life of 10Be is too short for it to be still present in the mantle. Late Proterozoic basement in the northwestern Turkey Chen et al. 2002 Intern. J. Earth Sciences East European plat Black Sea Turkey Study area Medirerranean Sea African plat Haydoutov, 1995 Atlantic Ocean V A R I S C A N B E L T A L P I N E B E L T Caledonian cores in the Alpine belt Pre-Caledonian massif in the Alpine belt Pre-Variscan massif in the Variscan belt 50 km North Anatolian fault zone Cankiri Kastamonu Bolu zone Black Sea Study area After Yilmaz Sr 0.7217 WR-Mu 610 /- 8 Ma; Sr 0.7200 Ap-Bi 574 /- 6 Ma; Sr 0.7191 Mu-Bi 572 /- 6 Ma; Sr 0.7282 i i i i 0.6 1.6 2.6 3.6 0100200300400 Paragneiss 8786 Sr / Sr 8786 Rb / Sr Biotite Bi Whole rock WR Apatite Ap WR-Bi 548 /- 5 Ma; Sri 0.7105 Ap-Bi 547 /- 5 Ma; Sri 0.7136 0.7 0.8 0.9 1.0 1.1 01020304050 8786 Rb / Sr 8786 Sr / Sr Orthogneiss Biotite Bi Whole rock WR Apatite Ap WR-Bi-Ap 545 /- 5 Ma; Sri 0.7048 MSWD 2.6 0.511 0.512 0.513 0.514 0.515 0.516 0.00.20.40.60.81.0 147144 Sm / Nd 143144 Nd / Nd Orthogneiss Whole rock WR Garnet Gt 559 /- 8 Ma 207206 Pb/Pb age 590 /- 5 Ma 200 300 400 500 600 700 800 900 530540550560570580590600 Age Ma Temperature C o Zircon age Garnet Sm-Nd age Biotite Rb-Sr ages B L A C K S E A Tauride - Anatolide plats T U R K E Y Sakarya zone East European plat Moesian plat Istanbul zone Rhodope Istanbul Study area Okay et al. 1994 Gondwana Africa Arabia Southern America India Australia China Late Proterozoic India Australia Antarctica Africa Study area S. European suture Haydoutov, 1995 Avalonian Cadomian belt Russia South America North America Arabia Source Late Precambrian reconstruction Murphy Nance, 1991 30 30 0 60 谢谢
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