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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:
1)
dating principles & techniques
2)
Beyond dating - tracking igneous
processes
3)
The seawater Sr, Nd isotopic curves
Photo of Fe-Ni (left) and chondritic (right) meteorites
87Rb-87Sr
decay scheme
-decays to 87Sr by β-,
85Rb=72.17%
half-life=48.8 billion years
87Rb=27.83%
88Sr=82.53%
87Sr=7.04%
86Sr=9.87%
84Sr=0.56%
ALL STABLE
Rb/Sr ratios for various rocks:
Ultrabasic
0.2
Basaltic
0.06
Granites
0.25-1.7
Shale
0.46
Sandstone
3
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
Igneous Processes and 87Sr/86Sr ratios
MORB
* Remember that 87Rb likes melt
87Sr/86Sr
ratios of igneous rocks:
MORB
0.7025
Continents
0.7119
Ocean Islands
>0.704
vs.
Meteorites
0.699
decay equation
Sr  87 Sr  87 Rb t
  86   86 (e  1)
86
Sr  Sr i
Sr
87
measured
measured
when you crystallize a rock,
you will always have some Sr
present
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.
Rb/Sr=
0.8
Rb/Sr=1.2
ROCK
i= 0.702
(87Sr/86Sr)
Rb/Sr=0.6
t=Time of
crystallization
87Rb-87Sr
MANTLE
= 0.702
87Sr/86Sr
87Rb-87Sr
isochrons
Sr  87 Sr  87 Rb t
  86   86 (e  1)
86
Sr  Sr i
Sr
87
measured
A schematic Rb-Sr isochron
measured
when you crystallize a rock,
you will always have some Sr
present
Sample with
lower [Rb]
Bushveld granite Rb-Sr isochron
If
And
We have
Sample with
higher [Rb]
x=(87Rb/86Sr)m
y=(87Sr/86Sr)m
y=b+mx
Where intercept
And slope
b=(87Sr/86Sr)i
m=(e t-1)
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
T=4.5Ga
ratios of igneous rocks:
MORB
0.7025
Continents
0.7119
Ocean Islands
>0.704
Questions:
1. Why are all present-day (87Sr/86Sr)
values greater than BABI?
2. Why are continental values the highest?
More than just an age tool - tracking (87Sr/86Sr)i through time
0.725
(87Sr/86Sr)
0.720
Average continental crust
continuing continental growth
0.715
0.710
early continental differentiation
BABI
0.705 Ocean islands
0.700 MORB
continuing upper mantle depletion
0.695
0
1
2
3
4
Age in Ga
A rock’s (87Sr/86Sr)i value call tell you how enriched or depleted its mantle source was.
i.e. (87Sr/86Sr)i = 0.7020 at 2Ga means a depleted source
How would you explain a (87Sr/86Sr)i value of 0.728 at 1.4Ga?
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
Seawater (87Sr/86Sr) through time
Controls on
Seawater Sr
Isotopic composition
Sr flux rate
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 isotope ratio
Seawater Sr Isotopic Curve (as measured on old and young carbonates)
mountainbuilding
hydrothermal
activity
Himalayan uplift
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)
147Sm-143Nd
147Sm=15%
4 other isotopes
143Nd=12.2%
6 other isotopes
decay scheme
-decays to 143Nd by α,
half-life=106 billion years
Sm/Nd ratios for terrestrial materials:
garnet
0.539
MORB
0.32
seawater
0.211
Shale
0.209
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)
147Sm-143Nd
isochrons
Nd  143 Nd  147 Sm t
  144   144
(e  1)
144
Nd  Nd i
Nd
143
virtually same equation
as for Rb/Sr system
- measured by isotope dilution
and mass spectrometry
If
And
We have
x=(147Sm/144Nd)m
y=(143Nd/144Nd)m
y=b+mx
Where intercept
And slope
b=(143Nd/144Nd)i
m=(e t-1)
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?
CHUR model ages
- 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
CHUR
0.1967
DM
0.222
143Nd/144Nd
0.512638
0.713114
“Model Age” calculation:
Sm-Nd and Rb-Sr cross-plots
Seawater Nd isotopic evolution
- Nd is not well-mixed in the ocean,
because it has a short residence time
(400-900yrs*)
Rt= total reservoir / (sinks)
Ferromanganese nodules on Pacific
Ocean floor
- 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 largescale tectonic history
*Alibo, D. S. and Nozaki, Y., 1999. Rare earth elements in seawater . . . Geochim. Cosmochim. Acta 63: 363-372.
Seawater Nd isotopes and
abrupt climate change
Piotrowski et al., EPSL 2004