Download Isotope Geochemistry: Exploring Mantle Reservoirs with Isotope Ratios and more Slides Geochemistry in PDF only on Docsity! Isotope 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 Docsity.com Isotopes used as tracers in mantle geochemistry 87Rb 87Sr 87Sr/86Sr 147Sm 143Nd 143Nd/144Nd 235U 207Pb 207Pb/204Pb 238U 206Pb 206Pb/204Pb 232Th 208Pb 208Pb/204Pb 40K 40Ar 40Ar/36Ar 176Lu 176Hf 176Hf/177Hf 187Re 187Os 187Os/188Os Parent nuclide Daughter nuclide Tracer ratio (radiogenic/nonradiogenic) Docsity.com Mantle Reservoirs ( ) ( ) 4 144143 144143 101 / / × −= CHUR measured Nd NdNd NdNdε 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 low 143Nd/144Nd Docsity.com Binary mixtures 764.0 )9.01(5009.0100 )9.01(5007.09.01008.0 86 87 = −+× −×+××= M Sr Sr Component A Component B Sr 500 ppm 100 ppm 87Sr/86Sr 0.7 0.8 G. Faure, 1986, 2001 Docsity.com Pb produced by radioactive decay of U & Th 238U → 206Pb 235U → 207Pb 232Th → 208Pb 204Pb is non-radiogenic so, increase of 208Pb/204Pb, 207Pb/204Pb, due to U and Th decay The isotope geology of Pb Docsity.com The isotope geology of Pb Two-stage Pb evolution model of Stacey & Kramers (1975) In this model Pb evolves from primordial 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. Docsity.com Hofmann (2003) Treatise on Geochemistry, Vol. 2: The mantle and core. Hofmann (2003) Treatise on Geochemistry, Vol. 2: The mantle and core. HIMU: requires mantle sources with exceptionally high U/Pb and Th/Pb ratios EM-1: requires mantle sources with high Th/U ratios Docsity.com Mantle isotope tetrahedron Hart et al. (1992) Science 256 Hart et al. (1992) Science 256 FOZO (for focal zone): material from the lower mantle that is present as a mixing component in all deep-mantle plumes Docsity.com Hofmann (2003) : Treatise on Geochemistry Hofmann (2003) : Treatise on Geochemistry = global subducted sediments The 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” Docsity.com = global subducted sediments hidden reservoir with Pb isotopes to the left of the geochron The 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? Docsity.com 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 geochemistry Docsity.com
