The Use of Isotope GeochemistryStan Hart - CIDER 08

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

Tackley, 2000

What’s so hot about mantle plumes?

Workman, 2005

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.

87Rb 87Sr86Sr is not radiogenic

(87Sr/86Sr)now = (87Sr/86Sr)initial + (87Rb/86Sr)now [exp(t) – 1]

Slope ~ Rb/Sr ratio

(87Sr/86Sr)initial

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

Faure 1986

Slope ~ Sm/Nd

~ bulk silicate earth

Anyone see a problem with this plot?

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.

(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)

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!

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).

Faure 1986

207Pb204Pb

⎛⎝⎜

⎞⎠⎟now

−207Pb204Pb

⎛⎝⎜

⎞⎠⎟initial

206Pb204Pb

⎛⎝⎜

⎞⎠⎟now

−206Pb204Pb

⎛⎝⎜

⎞⎠⎟initial

=235U238U

⎛

⎝⎜⎞

⎠⎟now

e235t −1e238t −1

⎛

⎝⎜⎞

⎠⎟

= (238U/204Pb)now

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.

Note that “primitive Earth” samples must lieon the Geochron. A bulletproof test!

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%.

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

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)

More about Helium in a bit -

Let’s look at Sr-Nd-Pb isotopes in 3-D

Workman et al., 2004

Hart et al., 1992

FOZO

high He 3/4

BSE

Workman et al., 2004

The Standard Model

DUPAL Anomaly

Numbers are individual hotspot averages for: (measured 87Sr/86Sr - 0.7000)*10,000

Hart, 1984

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

Hoffman and McKenzie, 1985

-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!

An excellent new textbook thatdoes for Isotope Geochemistrywhat Turcotte and Schubert did for Geodynamics.

(no, I’m not being paid!)

Holden, 196x

Don Anderson