the use of isotope geochemistry stan hart - cider 08
TRANSCRIPT
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The Use of Isotope GeochemistryStan Hart - CIDER 08
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The Use of Isotope Geochemistry (only one?).
The Uses of Isotope Geochemistry (well, let me count the ways!!).
What am I really going to talk about? How Isotope Geochemistry can inform us about:
The presence and time evolution of chemical heterogeneities in the mantle.
• where are they?• how big are they?• how old are they?• what’s their pedigree?
(a.k.a. - animals run amok in the zoo)
CIDER 2008
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Tackley, 2000
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What’s so hot about mantle plumes?
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Workman, 2005
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Basic Isotope Systematics
Use 87Sr/86Sr as an example: 87Rb decays to 87Sr with a half-life of 48.8 Gy (decay constant = 1.42e-11 per year)
(87Sr)now = (87Sr)initial + (87Rb)now [exp(t) – 1]
Divide by a suitable non-radiogenic isotope, i.e. 86Sr:
(87Sr/86Sr)now = (87Sr/86Sr)initial + (87Rb/86Sr)now [exp(t) – 1]
Note that the atom ratio 87Rb/86Sr ~ 2.894 * Rb/Sr (ppm weight ratio)
Exactly the same methodology applies to:147Sm -143Nd, 176Lu -176Hf, 187Re -187Os, 238U -206Pb, 235U -207Pb, 232Th -208Pb
Some are more complex:U-Th-He system: 238U, 235U and 232Th all have the same 4He daughter.Pb-Pb system: the parents 238U and 235U are exactly coupled;
the parents 238U and 232Th are approximately coupled.
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87Rb 87Sr86Sr is not radiogenic
(87Sr/86Sr)now = (87Sr/86Sr)initial + (87Rb/86Sr)now [exp(t) – 1]
Slope ~ Rb/Sr ratio
(87Sr/86Sr)initial
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Faure 1986
Slope ~ Sm/Nd
Here the residue has higher Sm/Nd,compared to previous case where theresidue has lower Rb/Sr.
~ bulk silicate earth
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Faure 1986
Slope ~ Sm/Nd
~ bulk silicate earth
Anyone see a problem with this plot?
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Basic Isotope Systematics
Use 87Sr/86Sr as an example: 87Rb decays to 87Sr with a half-life of 48.8 Gy (decay constant = 1.42e-11 per year)
(87Sr)now = (87Sr)initial + (87Rb)now [exp(t) – 1]
Divide by a suitable non-radiogenic isotope, i.e. 86Sr:
(87Sr/86Sr)now = (87Sr/86Sr)initial + (87Rb/86Sr)now [exp(t) – 1]
Note that the atom ratio 87Rb/86Sr ~ 2.894 * Rb/Sr (ppm weight ratio)
Exactly the same methodology applies to:147Sm -143Nd, 176Lu -176Hf, 187Re -187Os, 238U -206Pb, 235U -207Pb, 232Th -208Pb
Some are more complex:U-Th-He system: 238U, 235U and 232Th all have the same 4He daughter.Pb-Pb system: the parents 238U and 235U are exactly coupled;
the parents 238U and 232Th are approximately coupled.
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(206Pb)now = (206Pb)initial + (238U)now [exp(t) – 1]
Divide by a suitable non-radiogenic isotope, i.e. 204Pb:
(206Pb/204Pb)now = (206Pb/204Pb)initial + (238U/204Pb)now [exp(t) – 1]
Initial Pb (FeS in iron meteorites)
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207Pb204Pb
⎛⎝⎜
⎞⎠⎟now
−207Pb204Pb
⎛⎝⎜
⎞⎠⎟initial
206Pb204Pb
⎛⎝⎜
⎞⎠⎟now
−206Pb204Pb
⎛⎝⎜
⎞⎠⎟initial
=235U238U
⎛
⎝⎜⎞
⎠⎟now
e235t −1e238t −1
⎛
⎝⎜⎞
⎠⎟
235U238U
⎛
⎝⎜⎞
⎠⎟now=constant=
1137.88⎛⎝⎜
⎞⎠⎟
Because this age depends only on an isotope ratio, and because these can bemeasured ~ 10 times more precisely than an elemental ratio (such as Sm/Nd, Rb/Sr, etc), Pb-Pb ages can be determined to spectacular precision!
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Amelin et al 2002
Pb-Pb ages on Ca-Al rich inclusions from a CV3 carbonaceous chondrite (Efremovka) and on individual chondrules from Acfer (a weird Fe-metal rich CH3 chondrite).
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Faure 1986
207Pb204Pb
⎛⎝⎜
⎞⎠⎟now
−207Pb204Pb
⎛⎝⎜
⎞⎠⎟initial
206Pb204Pb
⎛⎝⎜
⎞⎠⎟now
−206Pb204Pb
⎛⎝⎜
⎞⎠⎟initial
=235U238U
⎛
⎝⎜⎞
⎠⎟now
e235t −1e238t −1
⎛
⎝⎜⎞
⎠⎟
= (238U/204Pb)now
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Because the solar nebula has a low U/Pb ratio, evolution of Pb on Earthdoesn’t really get going until Pb is segregated to the core, thereby raising the U/Pbof the silicate mantle. Here core formation estimated ~ 33 My after Earth accretion.
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Note that “primitive Earth” samples must lieon the Geochron. A bulletproof test!
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4He/3He Isotope Systematics
238U – 8 4He = 206Pb 235U – 7 4He = 207Pb 232Th – 6 4He = 208Pb
(4He/3He)now = (4He/3He)initial + (238U/3He)now [8 (exp(t) – 1) + 7 (235U/238U)now(exp(t) – 1) + 6 (232Th/238U)now(exp(t) – 1)].
Note that (235U/238U)now is a constant = 0.007253. Note that (232Th/238U)now is ~ 3.5 (± 1) in mantle rocks.
4He production today: 238U: 235U: 232Th = 50%: 2%: 48%. 4He production at 4.5 Gy: 238U: 235U: 232Th = 31%: 50%: 19%.
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The “standard model” for He isotope evolution
In the standard model, He is moreincompatible than U, so that melt removalleaves a residue with higher U/He ratioleading to higher 4He/3He (or lower 3He/4He).
Thus higher 3He/4He ratios are deemedmore “primitive. In fact no high 3He/4Hemantle samples lie on the Pb-Pb Geochron,so cannot truly be “primitive”.
Bulk silicate Earth
Depleted upper mantle
Continental or oceanic crust
Bulk silicate Earth
Continental or oceanic crust
Depleted upper mantle
Initial nebula He isotope ratio
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Bulk silicate EarthDepleted upper mantle
Highest 3He/4He mantle
Initial nebula He isotope ratio
Now higher 3He/4He ratios may indicate older mantle, but true primitive mantle will have theLOWEST 3He/4He ratios!
In the inverted model, He is morecompatible than U, so that melt removalleaves a residue with lower U/He ratioleading to lower 4He/3He (or higher 3He/4He).
The “inverted model” for He isotope evolution (Parman et al 2005)
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More about Helium in a bit -
Let’s look at Sr-Nd-Pb isotopes in 3-D
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Workman et al., 2004
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Hart et al., 1992
FOZO
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high He 3/4
BSE
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Workman et al., 2004
The Standard Model
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DUPAL Anomaly
Numbers are individual hotspot averages for: (measured 87Sr/86Sr - 0.7000)*10,000
Hart, 1984
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CIDER 2004 Working Group
Global average OIB (ocean island basalt ~ plumes)
± 1
Global average N-MORB (mid-ocean ridge basalt)
Nd isotope variations along the East Pacific Rise spreading center
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Hoffman and McKenzie, 1985
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-2 blobs of dye in glycerine.
-red dye placed in a region of chaotic mixing.
- green dye placed in an island of non-chaotic mixing.
- Top moved left to right, thenbottom moved right to left, 10 cycles.
Ottino, 1989
Geochemists need to knowif the mantle looks and actslike this, on < km scales!
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An excellent new textbook thatdoes for Isotope Geochemistrywhat Turcotte and Schubert did for Geodynamics.
(no, I’m not being paid!)
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Holden, 196x
Don Anderson
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