Transcript Document
The Use of Isotope Geochemistry Stan 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 86Sr 87Sr is not radiogenic (87Sr/86Sr)now = (87Sr/86Sr)initial + (87Rb/86Sr)now [exp(t) – 1] Slope ~ Rb/Sr ratio (87Sr/86Sr)initial Here the residue has higher Sm/Nd, compared to previous case where the residue has lower Rb/Sr. Slope ~ Sm/Nd Faure 1986 Anyone see a problem with this plot? Slope ~ Sm/Nd Faure 1986 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) Pb 204 Pb now Pb 204 Pb initial Pb 204 Pb now Pb 204 Pb initial 207 206 207 206 U e235 t 1 238 U now e238 t 1 235 U 1 constant 238 U now 137.88 235 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! 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). = (238U/204Pb)now Faure 1986 Pb 204 Pb now Pb 204 Pb initial Pb 204 Pb now Pb 204 Pb initial 207 206 207 206 U e235 t 1 238 U now e238 t 1 235 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. Note that “primitive Earth” samples must lie on 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: 4He production at 4.5 Gy: 238U: 235U: 232Th = 50%: 2%: 48%. 238U: 235U: 232Th = 31%: 50%: 19%. The “standard model” for He isotope evolution Continental or oceanic crust 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 4He/3He (or lower 3He/4He). Depleted upper mantle Bulk silicate Earth Initial nebula He isotope ratio Bulk silicate Earth Depleted upper mantle Continental or oceanic crust Thus higher 3He/4He ratios are deemed more “primitive. In fact no high 3He/4He mantle samples lie on the Pb-Pb Geochron, so cannot truly be “primitive”. The “inverted model” for He isotope evolution (Parman et al 2005) Initial nebula He isotope ratio Highest 3He/4He mantle 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 4He/3He (or higher 3He/4He). Now higher 3He/4He ratios may indicate older mantle, but true primitive mantle will have the LOWEST 3He/4He ratios! More about Helium in a bit - Let’s look at Sr-Nd-Pb isotopes in 3-D Workman et al., 2004 FOZO Hart et al., 1992 BSE high He 3/4 The Standard Model Workman et al., 2004 DUPAL Anomaly Hart, 1984 Numbers are individual hotspot averages for: (measured 87Sr/86Sr - 0.7000)*10,000 Nd isotope variations along the East Pacific Rise spreading center Global average N-MORB (mid-ocean ridge basalt) Global average OIB (ocean island basalt ~ plumes) ±1 CIDER 2004 Working Group 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, 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! An excellent new textbook that does for Isotope Geochemistry what Turcotte and Schubert did for Geodynamics. (no, I’m not being paid!) Holden, 196x