Sm-Nd and Lu-Hf Sm-Nd and Lu-Hf geochrono geochrono logy logy
Mar 30, 2015
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