年代学-陈福坤-第三节-示踪.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 R a d i o g e n i c i s o t o p e g e o c h e m i s t r y 放射性和放射成因同位素在地质领域的 应用起初主要是同位素年代学，其地球 化学的示踪应用前景较晚才被认识到。 G a s t（19 60 ）首次将铷－锶 R b -Sr 同位素体系应用于有关地幔成份的 地球化学问题。 Gast PW 1960 Limitations on the composition of the upper mantle. Journal of Geophysics Research 65 1287-1297. - Gast 1960 In a given chemical system, the isotopic abundance of 87Sr is determined by four parameters the isotopic abundance at a given initial time, the Rb/Sr ratio of the system, the decay constant of 87Rb, and the time elapsed since the initial time. 87Sr 87Sr0 87Rbeλt– 1 87Sr/86Sr 87Sr/86Sr0 87Rb/86Sreλt– 1 The isotopic composition of a particular sample of strontium, whose history may or may not be known, may be the result of time spent in a number of such systems or environments. In any case, the isotopic composition is the time-intergrated result of the Rb/Sr ratios in all the past environments. - Gast 1960 Local differences in the Rb/Sr ratio will, in time, result in local differences in the abundance of 87Sr. Mixing of material during processes will tend to homogenize these local differences. Once homogenization occurs, the isotopic composition is not further affected by these processes. - Gast 1960 Because of this property and because of the time-intergratingeffect, isotopic compositions lead to useful inferences concerning the Rb/Sr ratio of the crust and the upper mantle. - Gast 1960 Radiogenic isotopes do not fractionate during partial melting of fractional melting processes, so isotopic composition of rocks will reflect characteristics their sources. SiO2wt. 87Sr/86Sr 50607080 .704 .706 .708 Fractionation trend Contamination trend 这一同位素地球化学原理对于其它 同位素体系也成立，如 钐－钕 （Sm-Nd ） 铀－钍－铅 （U -T h -Pb ） 铼－锇 （R e -O s ） 镥－铪 （L u -H f ） 镧－铈 （L a -Ce ） 钾－氩 （K -A r ） 元素地球化学性质与分类元素地球化学性质与分类 根据在星云中凝聚的温度根据在星云中凝聚的温度 难熔元素（难熔元素（Refractory、挥发性元素（、挥发性元素（Volatile 根据在地球中的分配根据在地球中的分配 亲石元素（亲石元素（Lithophile）、亲硫元素（）、亲硫元素（Chalcophile ） 亲铁元素（ ） 亲铁元素（Siderophile）、亲气元素（）、亲气元素（Atmophile 根据在熔融和结晶过程中固根据在熔融和结晶过程中固/液相中的分配液相中的分配 不相容元素（不相容元素（Incompatible） 、相容元素（） 、相容元素（Compatible ）） 根据蚀变过程中的行为根据蚀变过程中的行为 活动性元素（活动性元素（Mobile ）、不活动元素（）、不活动元素（Immobile ）） 根据离子的特征根据离子的特征 高场强元素 （高场强元素 （HFSE）、低场强元素 （）、低场强元素 （LFSE） 或大离子亲石元素 （ ） 或大离子亲石元素 （LILE）） 李献华整理 The Earth’s Interior The inner core is made of solid nickel and iron. The outer core is made of hot liquid nickel and iron. The crust is made of soil, water and rock. The mantle is hot part melted and part solid iron and magnesium. Composition of the Earth’s Layers Bulk Earth CorePrimitive Mantle Modern Mantle Crust Lower Crust Upper Crust Primitive mantle is equivalent to the composition of the bulk Earth less the composition of the core, The subscripts PM, E, and C denote primitive mantle, bulk earth, and core, respectively. mPM[E]PM mE[E]E- mC[E]C The average modern mantle composition is then equivalent to the primitive mantle less the present composition of the crust, The subscripts K denote continental crust. mM[E]M mPM[E]PM- mK[E]K How we know what’s inside Astronomy - Physics Drilling through the crust –deepest is 12 km Earth’s delivery service – Kimberlite Pipes and Xenoliths – Volcanoes Seismic data – Vibrational energy waves – Earthquake data Astronomy - Physics Calculating Earth’s Density - Gravitational influence, mass - Volume, Shape, Diameter - calc. Density of Earth 5.5 g/cm3 - Surface 2.8 g/cm3 Kimberlite pipe Xenolith Earth’s Delivery Service Changes in P-and S- wave velocity reveal Earth’s internal layers Seismology tells us about the density of rocks Continental crustContinental crust 2.7 g/cm3 Oceanic crustOceanic crust 3.2 g/cm3 Structure of the Crust and Upper Mantle Upper to 410 km Upper to 410 km olivine olivine spinelspinel 660 km 660 km spinelspinel perovskiteperovskite- -type type SiSiIV IV SiSiVI VI The EarthThe Earth’ ’s Interiors Interior MantleMantle2/3 of the mass of the earth2/3 of the mass of the earth Peridotite ultramaficPeridotite ultramafic UpperUpper to 410 km olivine to 410 km olivine →→ spinel spinel Low Velocity LayerLow Velocity Layer 6060- -220 km220 km Transition ZoneTransition Zone as velocity increases rapidlyas velocity increases rapidly 660 km 660 km γ γ- -OlivinOlivin →→ MgMg- -PerowskitPerowskit Mg Mg- -WW stitstit CpxCpx GntGnt →→ MajoritMajorit Lower MantleLower Mantle has more gradual has more gradual velocity increasevelocity increase 5 to 75 km thick Solid Silicon Si and Oxygen O Continental 5 to 75 km thick Old billions of years in age Deed Density 2.7 g/cm3- Granite Oceanic 5 to 8 km thick Relatively young less than 200 Ma Undeed Density 3.0 g/cm3- Basalt The Crust Isostasy The less dense crust “floats” on the less buoyant, denser mantle Mohorovicic Discontinuity Moho 0.5 of the mass of the earth An overview of the tectonic system Mechanisms of differentiation Mechanisms of differentiation Crystal Settling/Floating REE diagram Plot of concentration as the ordinate y-axis against increasing atomic number. Degree of compatibility increases from left to right across the diagram ConcentrationConcentration La Ce Nd Sm Eu Tb La Ce Nd Sm Eu Tb ErEr DyDy Yb LuYb Lu Rb Ce PrPmEu Gd Tb DyHo Er Tm Yb Lu PaNpPu Am Cm Bk Cf Es Fm Md No Lr NdSm ThU H Li Na Mg BeBCNOF Ne He AlSiPSCl Ar KCa Ga Ge As SeBrKr XeYZr Nb Mo Tc RuPd Ag CdRh TaHf La Ac InSnITeSb RnPt AuTlBi PoAtHgOsWReIrCs Fr Sc TiVCr Mn Fe Co NiCu Zn Ba Ra Sr Pb 57 71 Rare Earth Element Ionic Radii Rare Earth Abundances in Chondrites “Sawtooth”pattern Oddo-Harkins effect of cosmic abundance reflects greater stability of nuclei with even atomic numbers Chondrite Normalized REE patterns By “normalizing” dividing by abundances in chondrites, the “sawtooth” pattern can be removed. Trace element fractionation during partial melting La Nd Rb Melting Residue LaLu LaLu Ni Co Sm Sr Region of Partial Melting Differentiation of the Earth Mantle Continental Crust RbSr NdSm La Lu LaLu La Lu RbLu After partial melt extraction Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elements – Continental crust becomes “incompatible element enriched” – Mantle becomes “incompatible element depleted” Europium anomaly when plagioclase is – a fractionating phenocryst or – a residual solid in source REE diagram for 10 batch REE diagram for 10 batch melting of a hypothetical melting of a hypothetical lherzolite with 20 lherzolite with 20 plagioclase, resulting in a plagioclase, resulting in a pronounced negative Europium pronounced negative Europium anomaly. From Winter 2001 anomaly. From Winter 2001 An Introduction to Igneous and An Introduction to Igneous and Metamorphic Petrology. Metamorphic Petrology. Prentice Hall.Prentice Hall. An overview of the tectonic system An overview of the tectonic system Mid Ocean Ridge Oceanic island Oceanic island arc Continental island arc Magmatism in and around ocean basins The Mid-Ocean Ridge System Minster et al. 1974 Geophys. J. Roy. Astr. Soc., 36, 541-576 Mid-Ocean Ridge Long 70,000 km continuous ridge 1400 km wide and 3000 m high Rift valley along the axis of the ridge Underwater mountain chain Oceanic Crust is generated A divergent boundary is a new crust zone where molten magma from the asthenosphere rises, cools, and adds new crust to the edges of the separating plates. The Mid-Ocean Ridge System Most Incompatible Less Incompatible RbCsBa ThUK Nb Ta La Ce Sr NdPHfZr Sm TiYYb 1 10 MORB Rock/Bulk Earth 100 Trace Elements Multi element diagram for MORBs Most Incompatible Less Incompatible RbCsBa Th UK Nb Ta La Ce Sr NdPHfZr Sm TiYYb 0.1 1 Depleted Mantle after a melting event Melt of a Depleted Mantle Original Earth Rock/Bulk Earth 10 Melting of the mantle Incompatible K, Rb, Cs, Ta, Nb, U, Th, Y, Hf, Zr, REEs Compatible Ni, Cr, Co, V, Sc Concentration of incompatible elements decreases with increasing degree of melting Most Incompatible Less Incompatible RbCsBa ThUK Nb Ta La Ce Sr NdPHfZr Sm TiYYb 1 10 MORB Rock/Bulk Earth 100 Trace Elements Multi element diagram for MORBs Most Incompatible Less Incompatible RbCsBa Th UK Nb Ta La Ce Sr NdPHfZr Sm TiYYb 0.1 1 Depleted Mantle after a melting event Melt of a Depleted Mantle Original Earth Rock/Bulk Earth 10 Melting of the mantle Incompatible K, Rb, Cs, Ta, Nb, U, Th, Y, Hf, Zr, REEs Compatible Ni, Cr, Co, V, Sc Concentration of incompatible elements decreases with increasing degree of melting 1 2 3 Hot spot tracks across the North Pacific Ocean Size of volcanic edifices Mauna Loa still active 80.000 km3 average Stratovolcano 500 km3 Size of volcanic edifices Mauna Loa still active 80.000 km3 average Stratovolcano 500 km3 Mauna Loa ation of oceanic island Types of OIB Magmas Two principal magma series Tholeiitic series dominant type – Ocean island tholeiitic basalt, OIT – Similar to MORB, but some distinct chemical and mineralogical differences Alkaline series subordinate – Ocean island alkaline basalt, OIA – Two principal alkaline sub-series silica undersaturated slightly silica oversaturated less common series MORB-normalized Spider Diagrams Data from Sun and McDonough 1989 Wilson 1989 PBS Papuan-Bismarck- Solomon-New Hebrides arc Subduction zones Subduction zones Subduction zones Tatsumi 1989, Tatsumi Nd is more incompatible than Sm; During partial melting of mantle and magma intruding in to crust,Rb-Sr and Sm-Nd will be fractionated. Rb and Nd are easier to go into melt relative Sm and Sr, therefore, mantle will be depleted in Rb and Nd depleted mantle, while the crust will be enriched in Rb and Nd. Sr - Nd Isotopes Ito et al. 1987 Ito et al. 1987 ChemChem Geol 62, 157Geol 62, 157- -176, 176, LeRoexLeRoex et al. 1983 et al. 1983 J Petrol 24, 267J Petrol 24, 267- -318318 Nd and Sr Isotopic systematics of the crust and mantle ** Higher Rb/Sr and 87Sr/86Sr ratio Lower Sm/Nd and 143Nd/144Nd ratio Lower Rb/Sr and 87Sr/86Sr ratio Lower Sm/Nd and 143Nd/144Nd ratio Lower Rb/Sr and 87Sr/86Sr ratio Higher Sm/Nd and 143Nd/144Nd ratio Low Rb/Sr and 87Sr/86Sr ratio High Sm/Nd and 143Nd/144Nd ratio Sm-Nd and Rb-Sr Isotope Systematics ** Upper Crust Lower Crust Upper Mantle Lower Mantle Lithosphere EnrichedDepletedPrimitive Asthenosphere Two-reservoir mantle model An upper depleted mantle A lower mantle ‘primitive’ Nd and Sr isotope ratios of the suboceanic mantle as sampled by oceanic basalts Nd, Hf and Sr isotope ratios of the suboceanic mantle as sampled by oceanic basalts Pb isotope ratios of the suboceanic mantle as sampled by oceanic basalts Nd and Pb isotope ratios of the suboceanic mantle as sampled by oceanic basalts Sr and Pb isotope ratios of the suboceanic mantle as sampled by oceanic basalts Multi-reservoir mantle model Five reservoir types of White 1985 and the components of Zindler and Hart 1986 Multi-reservoir mantle model Five reservoir types of White 1985 and the components of Zindler and Hart 1986 Five mantle reservoir types ** PREMA prent mantle DMM depleted MORB mantle EMI enriched mantle I EMII enriched mantle II HIMU high- mantle 238U/204Pb Mantle reservoirs BSE Bulk Sillicate Earth or Primary Uni Reservoir Mantle reservoirs Mantle Reservoirs 1.DM Depleted Mantle N-MORB source Zindler and Hart 1986, Staudigel et al. 1984, Hamelin et al. 1986 and Wilson 1989. Zindler and Hart 1986, Staudigel et al. 1984, Hamelin et al. 1986 and Wilson 1989. Subduction process and recycled crustal material into mantle Nd and Sr isotope ratios in modern marine sediments Ben Othman et al., 1989 Lower crust Upper crust Pb isotope ratios in modern marine sediments Asmeron and Jacobsen 1993 Pb isotope ratios in modern marine sediments Asmeron and Jacobsen 1993 Nd and Sr isotopic composition of granulite and xenoliths in volcanic rocks Rudnick, 1992 Pb isotope ratios in lower crustal xenoliths Rudnick and Goldstein, 1990 Isotopically enriched reservoirs EMI, EMII, and HIMU are too enriched for any known mantle process, and they correspond to crustal rocks and/or sediments HIMU – enriched in 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, depleted in 87Sr/86Sr Origin Recycled oceanic crust, which has lost alkalis Rb and Pb during alteration and subduction Metasomatically enriched oceanic lithosphere EMI – slightly enriched in 87Sr/86Sr, but not in Pb isotopic composition, very low 143Nd/144Nd Origin Recycling of delaminated subcontinental lithosphere Recycling of subducted ancient pelagic sediment high Th/U and low U/Pb and Th/Pb ratios EMII – more enriched, especially in 87Sr/86Sr and radiogenic Pb isotopes Origin Recycled oceanic crust and minor subducted sediment Recycling of melt-impregnated oceanic lithosphere Hart et al. 1992, Science 256 Mantle Isotope Tetrahedron Global Material Recycling Global material cycling through Earth’s history results in present mantle heterogeneities. What does an isotope ratio reflect time-integrated parent-daughter ratio Re-melting of ancient ocean floor Models of Mantle Circulation Hofmann, 1997 Slab-recycling model whole mantle convection Mantle tomography - the fate of the slab Plum pudding model The heterogeneities are remnants of recycled oceanic and continental crust Helffrich Wood, 2001 Compositional stratification in the deep mantle Kellogg et al., 1999 Transition-zone water filter model Bercovici Karato, 2003 Tr a c i n g p e t r o l o g i c p r o c e s s Magma differentiationMagma differentiation Fractional CrystallizationFractional Crystallization Assimilation, fractional crystallizationAssimilation, fractional crystallization SiO2wt. 87Sr/86Sr 50607080 .704 .706 .708 Fractionation trend Contamination trend Magma mixing 87Sr/86Sr 0.7040.7060.7080.710 -4 0 4 Mixing trend εNd End member 1 End member 2 e.g. depleted mantle e.g. crustal material Pl o t o f r a t i o s o f e l e me n t s o r i s o t o p e s f o r mi x i n g o f e n d me mb e r s r a1*b2/a2*b1 for Sr and Nd isotopes r 144Nd1*86Sr2/ 144Nd2*86Sr1 M i x i n g h y p e r b o l a f o r me d b y c o mp o n e n t s A a n d B Faure 1986 Isotopic compositions of subduction-related magmas Ocean-continent plate convergence - Subduction of oceanic crust and sediments I s l a n d a r c v o l c a n i c s -s u b d u c t e do c e a n i c c r u s t a n d s e d i me n t s Pb Isotope Geochemistry 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. 204Pb is non-radiogenic 206Pb 238U 207Pb 235U 208Pb 232Th Evolution o