1. Rb-Sr and Sm-Nd Dating 8/30/12
What are the principles behind Rb-Sr and Sm-Nd dating?
What processes can these dating systems address?
What are the main limitations of these methods?
Lecture outline:
dating principles & techniques
Beyond dating - tracking igneous
processes
The seawater Sr, Nd isotopic curves
Photo of Fe-Ni (left) and chondritic (right) meteorites

2. 87Rb-87Sr decay scheme -decays to 87Sr by β-,
half-life=48.8 billion years
Rb/Sr ratios for various rocks:
Ultrabasic 0.2
Basaltic 0.06
Granites
Shale 0.46
Sandstone 3
ALL STABLE
What accounts for huge range in Rb/Sr ratios of rocks?
1. Rb subsitutes for K in K-bearing minerals
while
Sr substitutes for Ca in Ca-bearing minerals
2. Rb and Sr are fractionated by igneous processes:
Rb tends to prefer melt (more “incompatible” than Sr)
Bottom line:
High Rb/Sr rocks contain more 87Sr
Low Rb/Sr rocks contain less 87Sr

3. Igneous Processes and 87Sr/86Sr ratios
MORB
* Remember that 87Rb likes melt
87Sr/86Sr ratios of igneous rocks:
MORB
Continents
Ocean Islands >0.704
vs.
Meteorites 0.699

4. 87Rb-87Sr decay equation
Rb/Sr=
0.8
Rb/Sr=1.2
ROCK
(87Sr/86Sr) i= 0.702
Rb/Sr=0.6
measured
measured
when you crystallize a rock,
you will always have some Sr
present
crystallization
t=Time of
So how do you determine the initial 87Sr/86Sr ratio?
Because igneous rocks are so heterogeneous,
different mineral phases will have different Rb/Sr
ratios, even though they have the same crystallization
age and the same 87Sr/86Sr initial.
MANTLE
87Sr/86Sr = 0.702

5. 87Rb-87Sr isochrons A schematic Rb-Sr isochron measured measured
when you crystallize a rock,
you will always have some Sr
present
Sample with
lower [Rb]
Sample with
higher [Rb]
Bushveld granite Rb-Sr isochron
If x=(87Rb/86Sr)m
And y=(87Sr/86Sr)m
We have y=b+mx
Where intercept b=(87Sr/86Sr)i
And slope m=(eλt-1)

6. More than just an age tool - tracking (87Sr/86Sr)i through time
BABI - Basaltic Achondrite Best Initial = Bulk Earth, undifferentiated
Rb-Sr isochron
From meteorites
87Sr/86Sr ratios of igneous rocks:
MORB
Continents
Ocean Islands >0.704
T=4.5Ga
Questions:
1. Why are all present-day (87Sr/86Sr)
values greater than BABI?
2. Why are continental values the highest?

7. More than just an age tool - tracking (87Sr/86Sr)i through time
Average continental crust
continuing continental growth
enriched
early continental differentiation
BABI
Ocean islands
MORB
depleted
continuing upper mantle depletion
A rock’s (87Sr/86Sr)i value call tell you how enriched or depleted its mantle source was.
i.e. (87Sr/86Sr)i = at 2Ga means a depleted source
How would you explain a (87Sr/86Sr)i value of at 1.4Ga?

8. Rock-forming complexities and (87Sr/86Sr)i
Ex: Mt. Shasta lavas span a wide range of Sr isotopic chemistries
As crystals form,
Rb enriched in melt,
eventually can get
ultra-enriched (87Sr/86Sr)
Crystals form in magma chamber,
Rb stays in melt
Or magma melts
host rock, which
has high 87Sr/86Sr
Or magma chambers
with different histories
mix prior to eruption

9. Seawater (87Sr/86Sr) through time
Controls on
Seawater Sr
Isotopic composition
Questions:
Why is the river Sr isotope
value the highest?
Why is the hydrothermal Sr isotope
value the lowest?
Why is carbonate recrystallization Sr
isotope value equal to that of seawater?
Sr flux rate
Sr isotope ratio
Seawater Sr Isotopic Curve (as measured on old and young carbonates)
mountain-
building
hydrothermal
activity
Himalayan uplift

10. Introduction to Rare Earth Elements
- REE so named because we could not
measure them until high-precision
mass spec techniques developed
- all REE have 3+ charge, ionic radii
decrease with increasing Z
- all REE are “incompatible” (they prefer
the melt), but light REE are more
incompatible (Nd prefers melt more
than Sm)

11. 147Sm-143Nd decay scheme -decays to 143Nd by α,
4 other isotopes
143Nd=12.2%
6 other isotopes
-decays to 143Nd by α,
half-life=106 billion years
Sm/Nd ratios for terrestrial materials:
garnet
MORB 0.32
seawater
Shale
Solar 0.31
Nd has a lower ionic potential (charge/radius) than Sm, so the bonds it
forms are weaker. Nd is concentrated in melt, while Sm remains in solid.
So… High Sm/Nd rocks contain more 143Nd
Low Sm/Nd rocks contain less 143Nd
NOTE: Sm parent will be enriched in “depleted” sources (i.e. MORB)
(opposite to Rb/Sr system, where parent enriched in continents)

12. 147Sm-143Nd isochrons virtually same equation as for Rb/Sr system
- measured by isotope dilution
and mass spectrometry
If x=(147Sm/144Nd)m
And y=(143Nd/144Nd)m
We have y=b+mx
Where intercept b=(143Nd/144Nd)i
And slope m=(eλt-1)

13. Epsilon Nd notation CHUR = “Chonritic Uniform Reservoir”
- typically measured on chondritic meteorites (DePaulo and Wasserburg, 1976a)
- If CHUR and samples are measured in the same lab, then regardless of normalization and corrections, one can compare εNd values
also practical way to report very small Nd isotope changes (~0.0001)
So what’s the ε value for CHUR?

14. CHUR model ages “Model Age” calculation:
- Nd isotope “model ages” can be calculated which represent the time of separation from CHUR evolution
- can also calculate an age from assuming a “depleted mantle” evolution (substitute DM Nd isotope values into equation below)
Present-day ratios:
147Sm/144Nd Nd/144Nd
CHUR DM
“Model Age” calculation:

15. Sm-Nd and Rb-Sr cross-plots

16. Seawater Nd isotopic evolution
- Nd is not well-mixed in the ocean, because it has a short residence time ( yrs*)
Rt= total reservoir / (Σsinks)
- anything with a residence time shorter than the turnover time of the ocean (~1500y) will exhibit concentration and isotopic variability in seawater
- REE are extremely resistant to metamorphism, so can measure Nd isotopes in very old sediments, fish teeth, ferromanganese nodules, etc
- Nd isotope variations reflect large-scale tectonic history
Ferromanganese nodules on Pacific
Ocean floor
*Alibo, D. S. and Nozaki, Y., Rare earth elements in seawater Geochim. Cosmochim. Acta 63:

17. Seawater Nd isotopes and abrupt climate change
Piotrowski et al., EPSL 2004