Rhenium-osmium isotope systematics and platinum group element concentrations in oceanic crust from DSDP/ODP Sites 504 and 417/418

摘要: [1] Rhenium, Os, 187Os/188Os, Ir, Ru, Pt, and Pd analyses of 43 samples from DSDP/ODP Hole 504B in the eastern equatorial Pacific crust with an age ∼6.7 Myr are used to calculate composition a composite section through upper oceanic crust. Weighted average Re (1526 pg/g), Os (21 187Os/188Os (0.213), 187Re/188Os (353), initial (0.173), Ir (9 Pt (304 (265 pg/g) similar sample DSDP Sites 417/418 western Atlantic ∼120 (2153 pg Re/g, 33 Os/g, = 1.042 ± 0.016, 349; 187Os/188Osi 0.344, 25 Ir/g, 698 Ru/g, 804 Pt/g, 2001 Pd/g). These sections complement existing data for ODP Site 735B (11–12 Myr) and, combined, provide best available information on Re-PGE-Os isotope systematics middle concentrations volcanic zone, transition zone stockwork mineralization, sheeted dike complex decrease 30 pg/g 18 pg/g. Rhenium highest (3867 leading 885, compared values 178 393 complex. The present-day (0.329 0.230, respectively). This also has (899 (523 concentrations. Of all lithologic units, breccias most enriched (4874 depleted (8 (3 (131 (173 have radiogenic (0.710 0.324, We contend that this is indicative intense alteration, loss PGE, early addition hydrogenous Os. Pillow lavas massive flows dikes affected by additions causing superchonditic 187Os/188Os. weighted Os/Ir 2.4 entire core significantly higher than reported mantle peridotites estimates likely indicates fractionation during melting melt extraction or intracrustal differentiation rather differential mobility alteration. Addition altered crust, although locally important, does not increase concentration more few percent. magnitude mantle-derived seawater cannot be reliably estimated our data, but steady state mass balance exceed 10% crustal inventory. Similarly, primary This, however, leads significant ∼0.17. characteristic fractionated PGE pattern fresh MORB fundamentally changed hydrothermal ingrowth storage subducted prolonged periods primarily determined magmatic processes at ocean ridges possibly, additional processing subduction zones.