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Sm-Nd and Lu-Hf geochronology

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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

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Chemical properties

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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 Lu-Hf Lu +3 (REE), Hf +4 (HFSE) Lu =0.93Å, Hf= 0.71Å Larger fractionation High Lu/Hf ratios Better age precision Faster decay constant 1.867E-11/yr Easier to date young rocks

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Decay of 147 Sm Decay of 176 Lu

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Sm-Nd dating

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Lu-Hf dating

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Isochron technique

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

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

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Sample treatment 1.Handpicking 2.Leaching 3.Spiking (mixed 176 Lu/ 180 Hf and 149 Sm/ 150 Nd spikes) 4.Equilibrating spike with a sample 5.Columns chemistry (separation of Yb Lu, Hf, Sm and Nd from matrix) 6.Mass spectrometry (TIMS, MC ICPMS) 7.Data reduction 8.Interpretation

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

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Inclusions affecting Sm-Nd and Lu-Hf garnet 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

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Rock forming mineral inclusions

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

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Influence of inherited inclusions on isochron dates From Prince et al. 2000

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How to deal with inclusions? 1.Handpicking 2.Hot plate digestion (limits refractory minerals dissolution) 3.Handpicking followed by leaching: HNO 3 :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 HClO 4 stepwise dissolution (Baxter et al. 2002) H 2 SO 4 (Anczkiewicz and Thirlwall, 2003)

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

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

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

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H 2 SO 4 leaching

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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 give the isotopic ratio for a single whole garnet; open symbols give the ratios of the outermost 0.05 mm of the respective garnet. The figure illustrates that 176Lu/177Hf ratios will be very low in systems that have high Peclet numbers (slow diffusion relative to growth rate), reflecting a narrow central peak but low overall concentration. If the growth rate is slow compared to diffusion (small Peclet numbers), the 176Lu/177Hf ratio is a function of system size only due to the overall availability of Lu. Rim isotopic compositions are always lower where diffusion is slow or the matrix is depleted. The dependence of 147Sm/144Nd ratios on the Peclet number is quite similar to that calculated for 176Lu/177Hf ratios except that the maximum isotopic ratio that can be obtained is much smaller and the rim isotopic compositions have a much less pronounced effect. Skora et al. 2007

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Possible causes of Sm/Nd Lu/Hf variations on a single isochron Inclusions Growth rates/diffusion rates Zonation of parent/daughter ratio in mineral

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Garnet growth rates Ducea et al. 2003

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Interpretation of garnet dating results Petrology –Major element zonation –Thermodynamic calculations, phase equilibria –Textural relationships Trace elements distribution Closure temperature

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Major element zonation growth diffusion

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Major elements show growth pattern

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Chondrite normalised REE distribution in garnet

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Sm and Nd Rayleigh-like zonation in garnet

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Lu and Hf Rayleigh-like zonation in garnet

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Sm-Nd and Lu-Hf closure temperature in garnet Depends on –Garnet size –Cooling rate –Presence of fluids –Lithology No unique number can universally be assigned to all rocks

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Sm, Nd closure temperature in garnet

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Lu, Hf closure temperature in garnet No experimental data available Tc (Lu-Hf) > Tc (Sm-Nd) Diffusion strongly depends on ionic charge (Van Orman 2002) Hf diffusion slower than Lu

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Age dependence on garnet growth history Fig. 6. Garnet growth models used for age calculations based on Rayleigh fractionation model illustrating the dependence of calculated age with garnet growth histories. The curves with an asterisk match best with measured Lu-Hf and Sm-Nd age data from Lago di Cignana, Italy. Ages are listed as Lu-Hf/Sm-Nd respectively in Ma. From Lapen et al. 2003

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Age dependence on garnet growth history?

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Lu-Hf apatite dating Ap Amph

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Lu-Hf apatite dating

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Dating sedimentation by Lu-Hf

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Good dates, bad dates How many points per isochron? How accurate initial ratio correction should be? Data presentation Which parameters are critical?

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How many points per isochron?

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Initial ratio correction 176 Hf/ 177 Hf WR = Hf/ 177 Hf WR = Change by c. 20ε units

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Data presentation Fraction Samp wt [g] Lu [ppm] Hf [ppm] 176 Lu/ 177 Hf 176 Hf/ 177 HfAge [Ma] εHf(t) apatite ±4416.8± whole rock ±29 grt A ±41 grt b ±28 Age errors at 95% C.L, age calculation by Isoplot (Ludwig, 2003) 176 Hf/ 177 Hf errors are 2SE 176 Lu/ 177 Hf errors are 0.5% 176 Hf/ 177 Hf of JMC475 = ±32 (2SD, n=21) 179 Hf/ 177 Hf= , exponential law 176 Hf/ 177 Hf CHUR(0) = , 176 Lu/ 177 Hf CHUR(0) = (Blichert-Toft and Albarède, 1997) Decay constant λ 176Lu = x yr -1 (Dalmasso et al., 1992; Scherer et al., 2001) Grt A: Total amount of Hf in analyses:2.055 ng Total amount of 176 Hf: ng Amount of radiogenic 176 Hf *: ng = 6.86E-13 g

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