1. The Use of Isotope Geochemistry Stan Hart - CIDER 08

2. 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

4. Tackley, 2000

6. What’s so hot about mantle plumes?

7. Workman, 2005

8. Basic Isotope Systematics Use 87 Sr/ 86 Sr as an example: 87 Rb decays to 87 Sr with a half-life of 48.8 Gy (decay constant = 1.42e-11 per year) ( 87 Sr) now = ( 87 Sr) initial + ( 87 Rb) now [exp( t) – 1] Divide by a suitable non-radiogenic isotope, i.e. 86 Sr: ( 87 Sr/ 86 Sr) now = ( 87 Sr/ 86 Sr) initial + ( 87 Rb/ 86 Sr) now [exp( t) – 1] Note that the atom ratio 87 Rb/ 86 Sr ~ 2.894 * Rb/Sr (ppm weight ratio) Exactly the same methodology applies to: 147 Sm - 143 Nd, 176 Lu - 176 Hf, 187 Re - 187 Os, 238 U - 206 Pb, 235 U - 207 Pb, 232 Th - 208 Pb Some are more complex: U-Th-He system: 238 U, 235 U and 232 Th all have the same 4 He daughter. Pb-Pb system: the parents 238 U and 235 U are exactly coupled; the parents 238 U and 232 Th are approximately coupled.

9. 87 Rb 87 Sr 86 Sr is not radiogenic ( 87 Sr/ 86 Sr) now = ( 87 Sr/ 86 Sr) initial + ( 87 Rb/ 86 Sr) now [exp( t) – 1] Slope ~ Rb/Sr ratio ( 87 Sr/ 86 Sr) initial

10. Faure 1986 Slope ~ Sm/Nd Here the residue has higher Sm/Nd, compared to previous case where the residue has lower Rb/Sr. ~ bulk silicate earth

11. Faure 1986 Slope ~ Sm/Nd ~ bulk silicate earth Anyone see a problem with this plot?

12. Basic Isotope Systematics Use 87 Sr/ 86 Sr as an example: 87 Rb decays to 87 Sr with a half-life of 48.8 Gy (decay constant = 1.42e-11 per year) ( 87 Sr) now = ( 87 Sr) initial + ( 87 Rb) now [exp( t) – 1] Divide by a suitable non-radiogenic isotope, i.e. 86 Sr: ( 87 Sr/ 86 Sr) now = ( 87 Sr/ 86 Sr) initial + ( 87 Rb/ 86 Sr) now [exp( t) – 1] Note that the atom ratio 87 Rb/ 86 Sr ~ 2.894 * Rb/Sr (ppm weight ratio) Exactly the same methodology applies to: 147 Sm - 143 Nd, 176 Lu - 176 Hf, 187 Re - 187 Os, 238 U - 206 Pb, 235 U - 207 Pb, 232 Th - 208 Pb Some are more complex: U-Th-He system: 238 U, 235 U and 232 Th all have the same 4 He daughter. Pb-Pb system: the parents 238 U and 235 U are exactly coupled; the parents 238 U and 232 Th are approximately coupled.

13. ( 206 Pb) now = ( 206 Pb) initial + ( 238 U) now [exp(t) – 1] Divide by a suitable non-radiogenic isotope, i.e. 204 Pb: ( 206 Pb/ 204 Pb) now = ( 206 Pb/ 204 Pb) initial + ( 238 U/ 204 Pb) now [exp(t) – 1] Initial Pb (FeS in iron meteorites)

14. Because this age depends only on an isotope ratio, and because these can be measured ~ 10 times more precisely than an elemental ratio (such as Sm/Nd, Rb/Sr, etc), Pb-Pb ages can be determined to spectacular precision!

15. 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).

16. Faure 1986 = ( 238 U/ 204 Pb) now

17. Because the solar nebula has a low U/Pb ratio, evolution of Pb on Earth doesn’t really get going until Pb is segregated to the core, thereby raising the U/Pb of the silicate mantle. Here core formation estimated ~ 33 My after Earth accretion.

18. Note that “primitive Earth” samples must lie on the Geochron. A bulletproof test!

19. 4 He/ 3 He Isotope Systematics 238 U – 8 4 He = 206 Pb 235 U – 7 4 He = 207 Pb 232 Th – 6 4 He = 208 Pb ( 4 He/ 3 He) now = ( 4 He/ 3 He) initial + ( 238 U/ 3 He) now [8 (exp( t) – 1) + 7 ( 235 U/ 238 U) now (exp( t) – 1) + 6 ( 232 Th/ 238 U) now (exp( t) – 1)]. Note that ( 235 U/ 238 U) now is a constant = 0.007253. Note that ( 232 Th/ 238 U) now is ~ 3.5 (± 1) in mantle rocks. 4 He production today: 238 U: 235 U: 232 Th = 50%: 2%: 48%. 4 He production at 4.5 Gy: 238 U: 235 U: 232 Th = 31%: 50%: 19%.

20. The “standard model” for He isotope evolution In the standard model, He is more incompatible than U, so that melt removal leaves a residue with higher U/He ratio leading to higher 4 He/ 3 He (or lower 3 He/ 4 He). Thus higher 3 He/ 4 He ratios are deemed more “primitive. In fact no high 3 He/ 4 He mantle 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

21. Bulk silicate Earth Depleted upper mantle Highest 3 He/ 4 He mantle Initial nebula He isotope ratio Now higher 3 He/ 4 He ratios may indicate older mantle, but true primitive mantle will have the LOWEST 3 He/ 4 He ratios! In the inverted model, He is more compatible than U, so that melt removal leaves a residue with lower U/He ratio leading to lower 4 He/ 3 He (or higher 3 He/ 4 He). The “inverted model” for He isotope evolution (Parman et al 2005)

22. More about Helium in a bit - Let’s look at Sr-Nd-Pb isotopes in 3-D

24. Workman et al., 2004

25. Hart et al., 1992 FOZO

26. high He 3/4 BSE

27. Workman et al., 2004 The Standard Model

29. DUPAL Anomaly Numbers are individual hotspot averages for: (measured 87Sr/86Sr - 0.7000)*10,000 Hart, 1984

30. 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

31. Hoffman and McKenzie, 1985

32. -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, then bottom moved right to left, 10 cycles. Ottino, 1989 Geochemists need to know if the mantle looks and acts like this, on < km scales!

33. An excellent new textbook that does for Isotope Geochemistry what Turcotte and Schubert did for Geodynamics. (no, I’m not being paid!)

34. Holden, 196x Don Anderson