Uran-Serien-Methode(n)

Transkript

1 Uran-Serien-Methode(n)

2 Uran-Serien-Methode(n) Besonderheiten einsetzbar auf Zeitskalen (Tage!!! bis 10er kilo- Jahre), mit denen (physikalische) Prozesse quantifiziert werden können Es wird nicht die kkumulation eines stabilen Tochternuklids gemessen Das lter ist vielmehr eine Funktion des existierenden Ungleichgewichtes

3 Uran-Serien-Methode(n)

4 Decay of radionuclide (N) to stable radiogenic daughter (D*) N = N 0 e -λt D* = N 0 - N D* = N 0 - N 0 e -λt D* = N 0 (1 - e -λt ) t = ln(1 + D*/N 0 )/λ

5 Decay series Decay of a long-lived parent N 1 to a short lived daughter N 2 (similar to U-Pb decay-chain) Radiochemical equilibrium is established after several half-lifes of N 2

6 Visualisierung

7 Visualisierung Spout size varies

8 U Zerfallsreihe (nur langlebige Nuklide gezeigt)

9 Uran-Serien-Methode(n) Disturbing secular equilibrium: Removal of immediate daughter atoms Production of new daughter atoms will continue, so the number of daughter atoms will gradually increase at rate determined by its half life, until secular equilibrium is restored (after about 5 half-lives). Removal of immediate parent atoms The amount of daughter atoms will decrease at rate determined by its half life until secular equilibrium is reestablished with the lower abundance levels of parent atoms. The daughter atoms are said to be unsupported. i.e. there are not enough of the parent atoms left to supply daughter atoms by decay

10 Uran-Serien-Methode(n) (1) (2) If we solve Equations (1) and (2) for the first two nuclides in a decay chain, we obtain:

11 General principles Bateman equation (1910) Evolution of the daughter activity (N 2 ) as a function of time: λ2t λ2 ( N ) = ( N ) e + ( N )(1 e t ) (1 e λ t λ t unsupported For the U- Th method Bateman s equation can be written as: = e + )

12 ctivity ratios for intermediate daughter and parent pairs ( U decay chain) as a function of time after disturbance

13 Uran-Serien-Methode(n) State of secular equilibrium : N U.λ = N 234U.λ 234 = N Th.λ = N 226Ra.λ 226 =. BUT all of these nuclides are of different elements and will have different chemical properties, so it is possible to disturb this secular equilibrium between nuclides

14 Relation between activity and isotope ratio für λ N 1 1 = = 1 gilt : λ N 2 2 = λ λ N N 1 2 Th U = a a 1 1 =

15 U-series disequilibrium in young volcanic rocks U 234 U Th 206 Pb (excluding some shorties ) T 1/2 = 75 ka T 1/2 = 1.6 ka Equilibrium: λ 1 N 1 = λ 2 N 2 1 = 2 ( Th/ U) = 1 llègre & Condomines (1976) EPSL 28, (1982) Nature 299

16 Uranium series dating U 234 U Th 206 Pb (excluding some shorties ) T 1/2 = 75 ka For equilibrium: λ 1 N 1 = λ 2 N 2 1 = 2 ( Th/ U) = 1

17 Uranium series dating (1 e λ t λ t = Bateman s equation: e + ) unsupported llègre & Condomines (1976) EPSL 28,

18 Übung Zeichnen Sie ein Ungleichgewichstdiagramm mit den Datenpunkten und berechnen Sie das lter der Lava (1 e λ t λ t = e Die Steigung entspricht dem usdruck (1 - e -λ t ) + )

19 China hat (Blackfood lava field)

20 Belfond dome (Saint Lucia): sphenochron Sphenochron Y. Chen et al.1996 Science 274

21 cigöl, Cappadocia

22 U-Th Isotope in jungen Vulkaniten = = = N N λ λ N N = λ λ Condomines et al. (1981) EPSL 55, N N Thorium-Uranium ratio: = =

23 U-Th isotopes in arc magmas Th U = λ λ U Th = 2.57 Newman et al. (1983) EPSL 65, 17-33

24 U Th isotopes Hawkesworth et al. (1997) Science 276

25 U Th isotopes

26 Melting of a subducting oceanic slab? Sigmarsson et al. (1998) Nature 394

27 Timescales of melt transport U 234 U Th 226 Ra 206 Pb (excluding some shorties ) T 1/2 = 75 ka T 1/2 = 1.6 ka Lesser ntilles Chabaux et al. (1999) Chem Geol 153 Ultrafast source to surface movement: m/year

28 Oldoinyo Lengai, Tanzania

29 U-series dating

30 Timescales of exolution and fractionation Oldoinyo Lengai (Tanzania) exolution occurred only 7 years before eruption! Williams & Gill (1986) GC Ra- Th equilibrium reched after 8000 a 228 Ra- Th equilibrium reached after 30 a

31 U-Serien Elemente U 234 U Th 206 Pb 222 Rn 210 Pb 222 Rn U 234 U Th 226 Ra Gelöst in Meerwasser U: ~ 3 ppb Th: < ppb Residenzzeit: U: Ma Th: ~300 a In oxidized waters, U is soluble as U VI, Uranyl ions = UO 2 +2, strong complexes with carbonates (UO 2 )(CO 3 ) 3 4 Th is present as Th IV ; highly insoluble; adsorbs on surfaces RT = d/ dt 226 Ra ufgrund der schnellen Sedimentation von Th aus dem Meerwasser weisen junge pelagische Gesteine einen Th-Überschuß auf! Th ( 231 Pa, 210 Pb, etc.)

32 Die Th (Ionium) Methode U 234 U Th 206 Pb Th-Überschuß wird in der Folgezeit durch Zerfall von Th abgebaut. us der ktivitätsabnahme mit der Tiefe im Sediment lassen sich Sedimentationsraten bestimmen R = R o e -λt T = 1/λ ln(r o /R) Bestimmung von Sedimentationsraten: a = h/t (h = Tiefe) R = R o e -λh/a lnr = lnr o λh/a 1 normales Muster 2 Änderung in Sedimentationsrate 3 Bioturbation 4 Th Nachbildung durch U-Zerfall Stosch:

33 Die 234 U- U Methode Stosch: The activity of 234 U in seawater not in secular equilibrium with that of U 234 U is preferentially leached out of radiation damaged crystal lattice sites during weathering. The 234 U / U ratio of sea water is about 1.15.

34 Die 234 U- U Methode G. Faure: Principles of Isotope Geology Muscheln, Korallen und Foraminiferen weisen eine gegenüber dem Wasser tausendfach höhere U-Konzentration auf und enthalten fast kein Th. Sobald es vom Meerwasser isoliert ist, zerfällt das überschüssige 234 U bis sich ein sekundärer Gleichgewichtszustand zwischen den Uranisotopen 234 U und U eingestellt hat.

35 Die Th- U und Th- 234 U Methoden U 234 U Th 206 Pb Th/U isotope ratio can be used to date carbonates provided we know: (1) the initial 234 U/ U ratio Exponential decay towards equilibrium (2) that the initial Th/ U ratio was close to zero and that the sample has remained closed with respect to U, Th and their intermediate isotopes