Rb-Sr and Sm-Nd Dating8/30/12

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 igneousprocesses

3) The seawater Sr, Nd isotopic curves

Photo of Fe-Ni (left) and chondritic (right) meteorites

87Rb-87Sr decay scheme

87Rb=27.83%85Rb=72.17%

88Sr=82.53%87Sr=7.04%86Sr=9.87%84Sr=0.56%

ALL STABLE

-decays to 87Sr by β-, half-life=48.8 billion years

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)

Rb/Sr ratios for various rocks:Ultrabasic 0.2Basaltic 0.06Granites 0.25-1.7Shale 0.46Sandstone 3

Bottom line:High Rb/Sr rocks contain more 87SrLow Rb/Sr rocks contain less 87Sr

Igneous Processes and 87Sr/86Sr ratios

87Sr/86Sr ratios of igneous rocks:MORB 0.7025Continents 0.7119Ocean Islands >0.704vs.Meteorites 0.699

* Remember that 87Rb likes melt

MORB

87Rb-87Sr decay equation

So how do you determine the initial 87Sr/86Sr ratio?

Because igneous rocks are so heterogeneous,different mineral phases will have different Rb/Srratios, even though they have the same crystallizationage and the same 87Sr/86Sr initial.

87 87 87

86 86 86( 1)t

i

Sr Sr Rbe

Sr Sr Sr

measured measured

when you crystallize a rock,you will always have some Sr present

MANTLE87Sr/86Sr = 0.702

ROCK(87Sr/86Sr) i= 0.702

Rb/Sr=0.6

Rb/Sr=1.2Rb/Sr=0.8

t=T

ime

of

crys

talli

zatio

n

87Rb-87Sr isochrons

87 87 87

86 86 86( 1)t

i

Sr Sr Rbe

Sr Sr Sr

measured measured

when you crystallize a rock,you will always have some Sr present

Sample withlower [Rb]

A schematic Rb-Sr isochron

Sample withhigher [Rb]

If x=(87Rb/86Sr)m

And y=(87Sr/86Sr)m

We have y=b+mx

Where intercept b=(87Sr/86Sr)i

And slope m=(et-1)

Bushveld granite Rb-Sr isochron

More than just an age tool - tracking (87Sr/86Sr)i through time

BABI - Basaltic Achondrite Best Initial = Bulk Earth, undifferentiated

Rb-Sr isochronFrom meteorites

87Sr/86Sr ratios of igneous rocks:MORB 0.7025Continents 0.7119Ocean Islands >0.704T=4.5Ga

Questions:1. Why are all present-day (87Sr/86Sr)

values greater than BABI?2. Why are continental values the highest?

Age in Ga

0 1 2 3 4

(87S

r/86

Sr)

0.695

0.700

0.705

0.710

0.715

0.720

0.725

More than just an age tool - tracking (87Sr/86Sr)i through time

BABI

Average continental crust

MORB

early continental differentiation

continuing continental growth

continuing upper mantle depletion

Ocean islands

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?

enriched

depleted

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 meltshost rock, whichhas high 87Sr/86Sr

Or magma chamberswith different historiesmix prior to eruption

Seawater (87Sr/86Sr) through time

Controls on Seawater SrIsotopic composition

Seawater Sr Isotopic Curve (as measured on old and young carbonates)

mountain-building

hydrothermalactivity

Himalayan uplift

Sr flux rate

Sr isotope ratio

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?

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 decay scheme

147Sm=15%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 0.539MORB 0.32seawater 0.211Shale 0.209Solar 0.31

Nd has a lower ionic potential (charge/radius) than Sm, so the bonds itforms are weaker. Nd is concentrated in melt, while Sm remains in solid.

So… High Sm/Nd rocks contain more 143NdLow 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)

143 143 147

144 144 144( 1)t

i

Nd Nd Sme

Nd Nd Nd

147Sm-143Nd isochrons

If x=(147Sm/144Nd)m

And y=(143Nd/144Nd)m

We have y=b+mx

Where interceptb=(143Nd/144Nd)i

And slope m=(et-1)

- measured by isotope dilution and mass spectrometry

virtually same equationas for Rb/Sr system

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)

“Model Age” calculation:

Present-day ratios: 147Sm/144Nd 143Nd/144NdCHUR 0.1967 0.512638DM 0.222 0.713114

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 / (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 PacificOcean floor

*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