SmNd and LuHf geochronology

1
Sm-Nd and Lu-Hf geochronology
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

• Sm-Nd
• REE 3
• Nd1.08, Sm1.04Å
• Limited fractionation
• Low Sm/Nd ratios
• Limits age precision
• Slow decay constant
• 6.54E-12 /yr
• Difficult to date young rocks

Lu-Hf Lu3 (REE), Hf4 (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
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
• Columns chemistry (separation of Yb Lu, Hf, Sm
  and Nd from matrix)
• Mass spectrometry (TIMS, MC ICPMS)
• Data reduction
• Interpretation

12
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-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

14
Rock forming mineral inclusions
15
Accessory mineral inclusions
Very similar influence of zircon on Lu-Hf
16
Influence of inherited inclusions on isochron
dates
From Prince et al. 2000
17
How to deal with inclusions?

• Handpicking
• Hot plate digestion (limits refractory minerals
  dissolution)
• Handpicking followed by leaching
• HNO3HCl 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
HFHCl leaching 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
HFHCl leaching 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
HFHCl leachingSm-Nd vs. Lu-Hf
21
HFHCl leachingSm-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 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
24
Possible causes 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 garnet dating results

• 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-like zonation in garnet
31
Lu and Hf Rayleigh-like zonation in garnet
32
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

33
Sm, Nd closure temperature in garnet
34
Lu, Hf closure temperature in garnet

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

35
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
36
Age dependence on garnet growth history?
37
Lu-Hf apatite dating
38
Lu-Hf apatite dating
39
Dating sedimentation by Lu-Hf
40
Good dates, bad dates

• 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
Initial ratio correction
176Hf/177Hf WR 0.282606 176Hf/177Hf WR
0.282000 Change by c. 20e units
43
Data presentation
REPRODUCIBILITY
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
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 0.28218632 (2SD, n21) 179Hf/177Hf
0.7325, exponential law 176Hf/177HfCHUR(0)
0.282772 , 176Lu/177HfCHUR(0) 0.0332
(Blichert-Toft and Albarède, 1997) Decay constant
?176Lu 1.865 x 10-11 yr-1 (Dalmasso et al.,
1992 Scherer et al., 2001)