Sm-Nd and Lu-Hf geochronology. Content Background Sample treatment and analytical methods...

Sm-Nd and Lu-Hf Sm-Nd and Lu-Hf geochronogeochronologylogy

ContentContent•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

Chemical Chemical prpropertiesoperties

Chemical Chemical prpropertiesoperties

Sm-NdREE +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-HfLu+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

Decay of Decay of 147147SmSm

QNdSm 14360

14762

QvHfLu 17672

17671

Decay of Decay of 176176LuLu

Sm-Nd datingSm-Nd dating

)1(144

147

144

143

144

143

t

i

eNd

Sm

Nd

Nd

Nd

Nd

*i NdNdNd 143143143

)1(147143143 ti eSmNdNd

Lu-Hf datingLu-Hf dating

)1(176176176 ti eLuHfLu

)1(177

176

177

176

177

176

t

i

eHf

Lu

Hf

Hf

Hf

Hf

*i HfHfLu 176176176

IIssochron techniqueochron technique

Datable mineralsDatable minerals

Sm-Nd

Garnet

Staurolite

Lu-HfGarnetApatiteXenotimeGadolinite

Duchene et al. 1997

0.1 mm0.1 mm 0.2 mm0.2 mm

2 mm2 mm

PG 31 eclogitePG 31 eclogite

PG 14 garnet amphibolitePG 14 garnet amphibolite PG 73 blueschistPG 73 blueschist

PG 5 eclogitePG 5 eclogite

2 mm2 mm

Sample treatmentSample treatment1. Handpicking

2. Leaching

3. Spiking (mixed 176Lu/180Hf and 149Sm/150Nd 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

Advantages of garnet Advantages of garnet geochronologygeochronology

• Rock forming mineral• Commonly used for PT estimates• High resolution dating (core and rim dating)• Prograde growth

Disadvantages of garnet Disadvantages of garnet geochronologygeochronology

• Common inclusions• Prolonged growth• Retrograde reactions

Inclusions affecting Sm-Nd Inclusions affecting Sm-Nd and Lu-Hfand Lu-Hf garnet datinggarnet dating

Sm-Nd• Monazite, Xenotime, Apatite• Epidote• Sphene

Lu-Hf• Zircon (metamict)• ± Rutile

Problems:Problems:• Lower parent/daughter ratio and hence reduce

age precision• Cause wrong age estimate (inheritance)• Make dating impossible

Rock forming mineral Rock forming mineral inclusionsinclusions

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 0.02 0.04 0.06 0.08 0.1

weight fraction of amphibole

Sm

/Nd

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1

weight fraction of amphibole

Lu

/Hf

Accessory mineral Accessory mineral inclusionsinclusions

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 0.00005 0.0001 0.00015 0.0002

weight fraction of spheneS

m/N

d

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5E-07 0.000001 1.5E-06 0.000002

weight fraction of monazite

Sm

/Nd

Very similar influence of zircon on Lu-Hf

Influence of Influence of inheritedinherited inclusionsinclusions on isochron on isochron

datesdates

From Prince et al. 2000

How to deal with How to deal with inclusions?inclusions?

1. Handpicking

2. Hot plate digestion (limits refractory minerals dissolution)

3. 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)

HF+HCl leachingHF+HCl leaching: Sm-: Sm-NdNd

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

HF+HCl leachingHF+HCl leaching: Lu-Hf: 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

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

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

HH22SOSO44 leaching leaching

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

Possible causes of Sm/Nd Possible causes of Sm/Nd Lu/Hf variations on a Lu/Hf variations on a

single isochronsingle isochron

• Inclusions

• Growth rates/diffusion rates

• Zonation of parent/daughter ratio in mineral

Garnet growth ratesGarnet growth rates

Ducea et al. 2003

Interpretation of garnet Interpretation of garnet dating resultsdating results

• Petrology– Major element zonation– Thermodynamic calculations, phase equilibria– Textural relationships

• Trace elements distribution

• Closure temperature

Major element zonationMajor element zonation

0.0

0.2

0.4

0.6

0.8

1.0

0 51

Mo

le fr

acti

on Almandine

Pyrope

Grossular

Spessartine

Fe/(Fe+Mg)

Rim Rim

growth diffusion

Major elements show Major elements show growth patterngrowth pattern

0.5130

0.5131

0.5132

0.5133

0.5134

0.5135

0 0.2 0.4 0.6

147Sm/144Nd

14

3N

d/1

44N

d

PG 10 eclogiteSanta Catalina

Franciscan Complex

Age = 115.6 ± 7.5MSWD = 1.9

143Nd/144Ndi = 0.513037 ± 18

Cpx

Grt B

Grt A

0.281

0.283

0.285

0.287

0.289

0 1 2 3

176Lu/177Hf

17

6H

f/1

77H

f

Age = 116.3 ± 0.6MSWD = 1.13

176Hf/177Hfi = 0.283110 ± 8

Cpx

Grt B

Grt A

PG 10 eclogiteSanta Catalina

Franciscan Complex

0.0

0.2

0.4

0.6

0.8

1.0

0 51

Mo

le fr

acti

on Almandine

Pyrope

Grossular

Spessartine

Fe/(Fe+Mg)

Rim Rim

Chondrite normalised REE Chondrite normalised REE distribution in garnetdistribution in garnet

Sm and Nd Rayleigh-Sm and Nd Rayleigh-like zonation in garnetlike zonation in garnet

Lu and Hf Rayleigh-like Lu and Hf Rayleigh-like zonation in garnetzonation in garnet

Sm-Nd and Lu-Hf Sm-Nd and Lu-Hf closure temperature in closure temperature in

garnetgarnet• Depends on

– Garnet size– Cooling rate– Presence of fluids– Lithology

No unique number can universally No unique number can universally be assignbe assigneded to all to all rocksrocks

Sm, Nd closure temperature in garnet

Sm, Nd closure temperature in garnet

Lu, Hf closure Lu, Hf closure temperature in garnettemperature 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

Age dependence on Age dependence on garnet growth historygarnet 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

Age dependence on Age dependence on garnet growth history?garnet growth history?

Lu-Hf apatite datingLu-Hf apatite dating

Ap

Amph

Lu-Hf apatite datingLu-Hf apatite dating

DatingDating sedimentationsedimentation by by Lu-HfLu-Hf

Good dates, bad datesGood dates, bad dates

• How many points per isochron?

• How accurate initial ratio correction should be?

• Data presentation

• Which parameters are critical?

How many How many pointspoints per per isochron?isochron?

Initial ratio correctionInitial ratio correction

176Hf/177Hf WR= 0.282606

176Hf/177Hf WR= 0.282000

Change by c. 20ε units

Data presentationData presentationFraction

Samp wt [g]

Lu[ppm]

Hf [ppm]

176Lu/177Hf 176Hf/177Hf Age[Ma]

εHf(t)

apatite 0.09988 0.248 0.032 1.1080 0.282840±44 16.8±0.3 -10.4

whole rock 0.05323 0.491 0.178 0.3909 0.282582±29

grt A 0.03372 2.506 0.061 5.8206 0.284303±41

grt b 0.02649 3.119 0.073 6.0255 0.284356±28

Age errors at 95% C.L, age calculation by Isoplot (Ludwig, 2003)176Hf/177Hf errors are 2SE176Lu/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 (Dalmasso et al., 1992; Scherer et al., 2001)

Grt A:

Total amount of Hf in analyses: 2.055 ng

Total amount of 176Hf: 0.108666 ng

Amount of radiogenic 176Hf *: 0.000686 ng= 6.86E-13 g