Chapter 6 . Age and Growth of Continental Crust from Radiogenic Isotopes

1. Chapter 6. Age and Growth of Continental Crust from Radiogenic Isotopes pp. 321-348

2. What is the crust? Continental crust?

3. 1. Total mass = 0.6% silicate earth 2. Main reservoir for K-U-Th (heat) and mineral resources 3. Primary archive of the earth history study of the continental crust is critical to our understanding of the origin & differentiation of the Earth.

4. When did continental crust form?How to get the age? • How did it form? What mechanism of its formation? • What is its geodynamics?

5. Three major topics • Age of the continental crust • 2. Mechanism of continental growth and formation • 3. Geodynamics for continental growth: supercontinent and continental growth

6. 1. Age of the continental crust The age of the continental crust is the amount of time the crustal rocks has been isolated from the mantle sources (DePaolo et al., 1991).

7. 1.1 How to determine age of crust ------ Isotopic dating • 40Ar/39Ar • Rb–Sr Dating • Sm–Nd Dating • U–Pb Dating TIMS(Thermal Ionization Mass Spectrometry)—The IsotopeDilution Method SHRIMP (Sensitive High Mass Resolution Ion MicroProbe)

8. Rb–Sr 40Ar/39Ar SHRIMP U–Pb TIMs AGESOF METAMORPHISM

10. 40Ar/39Ar • Rb–Sr Dating • Sm–Nd Dating • U–Pb Dating Metamorphic events ?Age of crust Igneous events Inherited or xenocryst zircon 200 Ma Upper crust How to determine initial age of crust? How to constrain on age of lower crust? Rock probe MC Lower crust 300 Ma 1000 Ma

11. Neodymium Model AGE 143Nd is produced by α-decay of 147Sm. Sm/Nd ratio of the mantle> the crust and hence143Nd/144Nd ratio >crust. Sm and Nd are not mobile, so ages and initial ratios are relatively insensitive to weathering and metamorphism. εNd is simply the relative deviation of the 143Nd/144Nd ratio from the chondritic ratio: where all 143Nd/144Nd values are specified at the age of interest (t). Preset-day (146Nd/144Nd)CHUR=0.512638.

12. Nd--variations in 143Nd/144Nd relative to the primitive mantle (CHUR). Nd<0----crust Nd>0-----mantle

13. Nd同位素模式年齡TDM TDM定義爲由樣品現在的147Sm/144Nd和143Nd/144Nd值反演到其143Nd/144Nd值與虧損地幔源區（DM）的該比值一致的時間: =0.00654/Ga，（O）代表現在值。下標SA和DM分別表示樣品和虧損地幔(DePaolo, 1988), (143Nd/144Nd)DM(O) = 0.51325, (147Sm/144Nd)DM(O) = 0.2168。 地殼樣品從虧損地幔中分離後，无Sm/Nd變化，TDM值爲殼幔分異年齡或地殼形成年齡。若有Sm/Nd值變化，它代表在不同的化學體系中演化時間的加權平均年齡。

14. 10 DM 8 T CF 6 Continental crust 4 E Nd 2 CHUR 0 Nd=DM 1000 Ma -2 -4 T = T C DM T C -6 0 200 400 600 800 1000 1200 Age, Ma Nd = -4 0 Ma 200 Ma 1000 Ma Nd = -2 200 Ma

15. Model age can not be used if parental felsic melts was generated by partial melting of mixed sources of various ages and compositions. 10 DM 8 T CF 6 Continental crust 4 E Nd 2 CHUR 0 -2 Mixing of various sources ? -4 -6 0 200 400 600 800 1000 1200 Age, Ma

16. V.I. Kovalenko et al. (2004)

17. Condie 1998

18. Tectonic ages

19. Patchett (2005)

20. Patchett (2005)

21. 1.2 Growth events of continental crust

22. 大陆生长量 大陆的幕式生长 时间

23. Condie 1998

24. K.C. Condie Tectonophysics 322 (2000)

25. 大陆模式生长 Models for continental growth 大陆 体积 大陆生长模式 时间

26. 1.3 Phanerozoic continental growth It is widely agreed that the production of the continental crust was essentially completed in the Precambrian, and was minor in the Phanerozoic. However, recent isotope investigations in the western North America (Sierra Nevada, Peninsular Range, and Canadian Cordillera) and eastern Australia (Lachlan and New England Foldbelts) have revealed that a substantial proportion of the Phanerozoic crust is juvenile.

27. IGCP420 Phanerozoic continental growth: evidence from central Asia (1998-2003, Bor-ming Jahn) Two issues:

28. Central Asian Orogenic Belt is the largest and typical Phanerozoic juvenile crust

29. 1.3 Phanerozoic continental growth Jahn (2004)

33. CAOB NCC COBC Pacific Qinling belt (diamond) SCB Dabei belt

34. εNd(t) Intermediate-acid rocks NCB and Dabei Blet Dabei belt (87Sr / 86 Sr)i

35. Production of the continental crust not only mainly occurred in the Precambrian, but also in the Phanerozoic. • Central Asian Orogenic Belt is the largest Phanerozoic juvenile crust

36. 2 How did continental crust form? Formation mechanism and processes

37. Juvenile continental crust is produced at two tectonic settings and ways: Subduction upper crust mantle plumes lower crust

38. Two major mechanism: Vertical growth Horizontal growth

39. 2.1 Horizontal growth Mantel oceanic crust continental crust

40. Chemical process Mantel oceanic crust continental crust Melting in subduction zone

41. Horizontal growth Physical process Chemical process Barr et al., 1999, Lithos

42. Arc magmatism and lateral accretion of arcs

43. 2.2 Vertical growth from below The plumesgive rise to juvenile crust:by partialmelting as they arrive at the base of the lithosphere,or by heating the upper mantle. Oceanic ridge subduction

44. Identify new juvenile compositions from below • Probable sources for post-orogenic granites (a) Recycle from young crustal (including ocean crust) by horizontal accretion----- horizontal growth (b) New underplating mantle-derived magma---- post-orogenic vertical growth (c) pre-orogenic underplating mantle-derived magma----- pre-orogenic vertical growth

45. How to identify new juvenile compositions from below εNd(t)>0 ? ? Jahn (2004)

46. Vertical evidence from Eastern Tianshan, NW China

47. Vertical growth

48. Vertical evidence from Eastern Tianshan, NW China Vertical growth

49. What is the geodynamics for continental growth? Why did continental grow?

50. Continental crustal growth Supercontinental cycle andare two of the most important subjects in the earth sciences. Thesetwo problems have long been studied separately. Crustal growth: 3.6–3.5, 2.7–2.6, 2.0–1.8, 1.2–1.0, and 0.5–0.3 Ga. The supercontinental accumulation: 2.6, 1.8, 1.0, 0.65, and 0.25 Ga. Continental crustal growth Supercontinental cycle Mantle convection