isotope geochemistry - uni- · pdf filein this model pb evolves from ... reference line (nhrl)...

Isotope GeochemistryIsotope Geochemistry

• Isotopes do not fractionate during partial melting or fractional crystallization processes. So they will reflect the characteristics of the mantle source

• OIBs, which sample a great expanse of oceanic mantle in places where crustal contamination is minimal, provide incomparable evidence for the nature of the mantle

Isotopes used as tracers in mantle geochemistry

87Rb 87Sr 87Sr/86Sr147Sm 143Nd 143Nd/144Nd235U 207Pb 207Pb/204Pb 238U 206Pb 206Pb/204Pb232Th 208Pb 208Pb/204Pb40K 40Ar 40Ar/36Ar176Lu 176Hf 176Hf/177Hf 187Re 187Os 187Os/188Os

Parent nuclide Daughter nuclideTracer ratio

(radiogenic/nonradiogenic)

Mantle ReservoirsMantle Reservoirs

Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

µµµµ

Mantle ReservoirsMantle Reservoirs

DM (Depleted Mantle) = N-MORB source

BSE (Bulk Silicate Earth) or the Primary Uniform Reservoir

PREMA (PREvalent MAntle)

Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

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measuredNd NdNd

NdNdε

BSE

HIMU (high-µ, µ = 238U/204Pb)

Mantle ReservoirsMantle Reservoirs

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The high Sr ratios in EM I and EM II also require a high Rb content and a similarly long time to produce the excess 87Sr. This signature correlates well with continental crust (or sediments derived from it). Oceanic crust and sediment are other likely candidates for these reservoirs

The high Sr ratios in EM I and EM II also require a high Rb content and a similarly long time to produce the excess 87Sr. This signature correlates well with continental crust (or sediments derived from it). Oceanic crust and sediment are other likely candidates for these reservoirs

EM-II = enriched mantle-2 87Sr/86Sr well above any reasonable mantle sources

EM I = enriched mantle-1 has low 87Sr/86Sr (near primordial) and very low143Nd/144Nd

Binary mixturesBinary mixtures

764.0)9.01(5009.0100

)9.01(5007.09.01008.086

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MSrSr

Component A Component B

Sr 500 ppm 100 ppm87Sr/86Sr 0.7 0.8

G. Faure, 1986, 2001

Pb produced by radioactive decay of U & Th238U → 206Pb235U → 207Pb232Th → 208Pb

204Pb is non-radiogenic

so, increase of 208Pb/204Pb, 207Pb/204Pb, due to U and Th decay

The isotope geology of PbThe isotope geology of Pb

The isotope geology of PbThe isotope geology of Pb

µ = 238U/204Pb

Primeval lead (Isotope ratios of Pb in troilite of the iron meteorite Canyon Diablo)

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The isotope geology of PbThe isotope geology of Pb

The straight lines are isochronsfor selected values of t. Point P:207Pb/204Pb and 206Pb/204Pbratios of a lead mineral (e.g. galena that was withdrawn 3x109

years ago from a source regionwith a present µ-value of 9.

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The isotope geology of PbThe isotope geology of Pb

Two-stage Pb evolution model of Stacey & Kramers(1975)In this model Pb evolves fromprimordial isotope ratios between 4.6 and 3.7 Ga in a reservoir with a µ-(238U/204Pb) value of 7.2. At 3.7 Ga the µ-value of the reservoir was changed by geochemical differentiation to 9.7.

Hofmann (2003) Treatise on Geochemistry, Vol. 2: The mantle and core.

Hofmann (2003) Treatise on Geochemistry, Vol. 2: The mantle and core.

HIMU: requiresmantle sourceswith exceptionallyhigh U/Pb and Th/Pb ratios

EM-1: requiresmantle sourceswith high Th/U ratios

Mantle isotope tetrahedronMantle isotope tetrahedronHart et al. (1992) Science 256

Hart et al. (1992) Science 256

FOZO (for focal zone): material from the lowermantle that is present as a mixing component in all deep-mantle plumes

• Isotopically enriched reservoirs (EM-1, EM-2, and HIMU) are too enriched for any known mantle process, and they correspond to crustal rocks and/or sediments

• HIMU – (enriched in 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, depleted in 87Sr/86Sr) Origin: a) recycled oceanic crust, which has lost alkalis (Rb) and Pb during alteration and subduction b) metasomatically enriched oceanic lithosphere

• EM-1 (slightly enriched in 87Sr/86Sr, but not in Pb, very low 143Nd/143Nd) Origin: a) recycling of delaminated subcontinental lithosphereb) recycling of subducted ancient pelagic sediment (because of their high Th/U and low (U,Th)/Pb ratios)

• EM-2 (more enriched, especially in 87Sr/86Sr and radiogenic PbOrigin: a) recycled ocean crust and small amount of subducted sedimentb) recycling of melt-impregnated oceanic lithosphere

Mantle reservoirs/flavorsMantle reservoirs/flavors

– U, Pb, and Th are concentrated in continental crust (high radiogenic daughter Pb isotopes)

– Oceanic crust has elevated U and Th content (compared to the mantle) as well as sediments derived from oceanic and continental crust

– Pb isotopes are a sensitive measure of crustal (including sediment) components in mantle isotopic systems

The isotope geology of PbThe isotope geology of Pb

Hofmann (2003) : Treatise on Geochemistry

Hofmann (2003) : Treatise on Geochemistry

= global subducted sediments

The lead paradoxThe lead paradox

Kruste und Mantel: komplementär bzgl. Pb-

Konzentration – nicht aber

bzgl. Isotopie!

The fact that MORBs do not plot to the left of the geochron is called the “First Lead Paradox”

= global subducted sediments

hidden reservoir with Pb isotopes to the left of the geochron

The lead paradoxThe lead paradox

Ave. oceanic and cont. crust close to geochron� little net fractionation of U/Pb during crust-mantle differentiation

• uptake of lead by the core (“core pumping”)?• storage of unradiogenic lead in the lower cont. crust or subcont. lithosphere?

Wilson (1989)

The 207Pb/204Pb vs 206Pb/204Pb data, especially from the northern hemisphere form a linear mixing line between DM and HIMU, a line called the Northern Hemisphere Reference Line (NHRL)

Pb isotope geochemistryPb isotope geochemistry

Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984)

Northern Hemisphere Reference Lines (NHRL)

207Pb/204Pb = 0.1084 x (206Pb/204Pb) + 13.491

208Pb/204Pb = 1.209 x (206Pb/204Pb) + 15.627

∆7/4=[(207Pb/204Pb)gem – (207Pb/204Pb)NHRL] x 100

∆8/4=[(208Pb/204Pb)gem – (208Pb/204Pb)NHRL] x 100

Pb isotope geochemistryPb isotope geochemistry

Mapping the geographic distribution of isotopic data

Pb isotope geochemistry

Hart (1984)

DUPAL = DUPre & ALlegre

SOPITA = SOuth Pacific Isotopic and Thermal Anomaly