Sm-Nd and Lu- Hf geochrono logy

1. Sm-Ndand Lu-Hfgeochronology

2. Content • Background • Sample treatment and analytical methods • Interpretation of garnet dating • Major elements • Trace elements • Closure temperature • Diffusion rates vs. growth rates • Lu-Hf apatite dating • Good dates, bad dates • Data presentation and evaluation

3. Chemical properties

4. Chemical properties Lu-Hf Lu+3 (REE), Hf+4 (HFSE) Lu =0.93Å, Hf= 0.71Å Largerfractionation High Lu/Hf ratios Betterage precision Fasterdecayconstant 1.867E-11/yr Easier to dateyoungrocks Sm-Nd REE +3 Nd=1.08, Sm=1.04Å Limited fractionation Low Sm/Nd ratios Limits age precision Slow decay constant 6.54E-12 /yr Difficult to date young rocks

5. Decay of 147Sm Decay of 176Lu

6. Sm-Nd dating

7. Lu-Hf dating

8. Isochron technique

9. Datable minerals Sm-Nd Garnet Staurolite Lu-Hf Garnet Apatite Xenotime Gadolinite Duchene et al. 1997

10. PG 14 garnet amphibolite PG 73 blueschist 0.2 mm 0.1 mm PG 31 eclogite PG 5 eclogite 2 mm 2 mm

11. Sample treatment • Handpicking • Leaching • Spiking (mixed 176Lu/180Hf and 149Sm/150Nd spikes) • Equilibrating spike with a sample • Columnschemistry (separation of Yb Lu, Hf, Sm and Nd from matrix) • Mass spectrometry (TIMS, MC ICPMS) • Data reduction • Interpretation

12. Disadvantages of garnet geochronology • Common inclusions • Prolonged growth • Retrograde reactions Advantages of garnet geochronology • Rock forming mineral • Commonly used for PT estimates • High resolution dating (core and rim dating) • Prograde growth

13. Inclusions affecting Sm-Nd and Lu-Hfgarnet dating Sm-Nd • Monazite, Xenotime, Apatite • Epidote • Sphene Lu-Hf • Zircon (metamict) • ± Rutile • Problems: • Lower parent/daughter ratio and hence reduce age precision • Cause wrong age estimate (inheritance) • Make dating impossible

14. Rock forming mineral inclusions

15. Accessory mineral inclusions Very similar influence of zircon on Lu-Hf

16. Influence of inheritedinclusions on isochrondates From Prince et al. 2000

17. How to deal with inclusions? • Handpicking • Hot plate digestion (limits refractory minerals dissolution) • Handpicking followed by leaching: HNO3:HCl leaching (Zhou and Hensen 1994) HCl stepwise dissolution (De Wolf et al. 1996) HF and HCl stepwise dissolution (Amato et al. 1999) HF and HClO4 stepwise dissolution(Baxter et al. 2002) H2SO4 (Anczkiewicz and Thirlwall, 2003)

18. HF+HClleaching: Sm-Nd • Grt A - not leached • Grt B L- leaching in 2 steps: • HF • HCL • Leachates 1 and 2 are joined and analysed together. • Grt B R- residue

19. HF+HClleaching: Lu-Hf • Grt A - not leached • Grt B L- leaching in 2 steps: • HF • HCL • Leachates 1 and 2 are joined and analysed together. • Grt B R- residue

20. HF+HCl leaching:Sm-Nd vs. Lu-Hf

21. HF+HCl leaching:Sm-Nd vs. Lu-Hf

22. H2SO4 leaching

23. Diffusion limited REE uptake Fig. 10 Plot of modeled 176Lu/177Hf (a) and 147Sm/144Nd (b)ratios against log Peclet numbers for different system sizes(modeled garnet is 1 mm, grown in 10 m.y.). Filled symbols givethe isotopic ratio for a single whole garnet; open symbols give theratios of the outermost 0.05 mm of the respective garnet. Thefigure illustrates that 176Lu/177Hf ratios will be very low insystems that have high Peclet numbers (slow diffusion relative togrowth rate), reflecting a narrow central peak but low overallconcentration. If the growth rate is slow compared to diffusion(small Peclet numbers), the 176Lu/177Hf ratio is a function ofsystem size only due to the overall availability of Lu. Rimisotopic compositions are always lower where diffusion is slow orthe matrix is depleted. The dependence of 147Sm/144Nd ratios onthe Peclet number is quite similar to that calculated for176Lu/177Hf ratios except that the maximum isotopic ratio thatcan be obtained is much smaller and the rim isotopic compositionshave a much less pronounced effect. Skora et al. 2007

24. Possiblecauses of Sm/Nd Lu/Hf variations on a single isochron • Inclusions • Growth rates/diffusion rates • Zonation of parent/daughter ratio in mineral

25. Garnet growth rates Ducea et al. 2003

26. Interpretation of garnetdatingresults • Petrology • Major element zonation • Thermodynamic calculations, phase equilibria • Textural relationships • Trace elements distribution • Closure temperature

27. Major element zonation diffusion growth

28. Major elements show growth pattern

29. Chondrite normalised REE distribution in garnet

30. Sm and Nd Rayleigh-likezonationingarnet

31. Lu and Hf Rayleigh-likezonationingarnet

32. Sm-Nd and Lu-Hf closuretemperatureingarnet • Depends on • Garnet size • Cooling rate • Presence of fluids • Lithology No unique number can universally be assigned to all rocks

33. Sm, Nd closuretemperature in garnet

34. Lu, Hf closuretemperatureingarnet • No experimental data available • Tc(Lu-Hf) > Tc(Sm-Nd) • Diffusion strongly depends on ionic charge(Van Orman 2002) • Hf diffusion slower than Lu

35. Agedependence on garnet growth history Fig. 6. Garnet growth models used for age calculations basedon Rayleigh fractionation modelillustrating the dependenceof calculated age with garnet growthhistories. The curveswith an asterisk match best with measured Lu-Hf and Sm-Nd age data from Lago di Cignana, Italy. Ages are listed asLu-Hf/Sm-Nd respectively in Ma. From Lapen et al. 2003

36. Agedependence on garnet growth history?

37. Lu-Hf apatitedating Ap Amph

38. Lu-Hf apatitedating

39. Datingsedimentation by Lu-Hf

40. Gooddates, baddates • How many points per isochron? • How accurate initial ratio correction should be? • Data presentation • Which parameters are critical?

41. How many points per isochron?

42. Initialratiocorrection 176Hf/177Hf WR= 0.282606 176Hf/177Hf WR= 0.282000 Change by c. 20ε units

43. REPRODUCIBILITY Data presentation Grt A: Total amount of Hf inanalyses: 2.055 ng Total amount of 176Hf: 0.108666 ng Amount of radiogenic176Hf *: 0.000686 ng = 6.86E-13 g Age errors at 95% C.L, age calculation by Isoplot (Ludwig, 2003) 176Hf/177Hf errors are 2SE 176Lu/177Hf errors are 0.5% 176Hf/177Hf of JMC475 = 0.282186±32 (2SD, n=21) 179Hf/177Hf= 0.7325, exponential law 176Hf/177HfCHUR(0) = 0.282772 , 176Lu/177HfCHUR(0) = 0.0332 (Blichert-Toft and Albarède, 1997) Decay constant λ176Lu= 1.865 x 10-11 yr-1 (Dalmassoet al., 1992; Scherer et al., 2001)