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Radioactivity in the Oceanic Crust

Radioactivity in the Oceanic Crust. William M. White, Cornell University, USA. Creation of Oceanic Crust. Oceanic crust is produced as magmas rise from the mantle below and ‘freeze’ to fill the gap as lithospheric plates spread apart. Some of this magma erupts on the seafloor as lava flows.

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Radioactivity in the Oceanic Crust

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  1. Radioactivity in the Oceanic Crust William M. White, Cornell University, USA

  2. Creation of Oceanic Crust • Oceanic crust is produced as magmas rise from the mantle below and ‘freeze’ to fill the gap as lithospheric plates spread apart. • Some of this magma erupts on the seafloor as lava flows. • Some freezes in the conduits to the surface (the sheeted dike complex). • Most crystallizes within the crust to form the gabbroic layer.

  3. MORB • Lava flows at mid-ocean ridges are readily sampled (by dredging, among other things; the rest of the crust is less easily sampled). • The lavas that erupt along mid-ocean ridges are basalts with a distinct, and uniform composition (at least by comparison to other environments). • They are given the name “Mid-Ocean Ridge Basalts” or MORB.

  4. Spider Diagrams & Incompatible Elements

  5. Spider Diagrams & Incompatible Elements

  6. MORB are depleted in incompatible elements

  7. Th Distribution in MORB

  8. Mean Concentrations of Th, U, and K in MORB

  9. Back-Arc Basins

  10. Grand Average: MORB + BABB

  11. MORB vs. the Oceanic Crust • Radioactivity in MORB is easy to estimate, but MORB represents only the volcanic layer – ~15% of less of the crust. • Because of igneous differentiation, we expect the gabbroic layer to have different Th, U, and K contents.

  12. Fractional Crystallization • Because minerals crystallizing from basaltic magma have compositions different from the magma, the composition of the magma evolves. • Because most of these minerals exclude K, U, and Th, their concentration increases. • The question is not what composition comes out the top of a mid-ocean ridge volcano, but what goes in the the bottom from the mantle. • We can’t analyze it, we have to model it.

  13. Magma Evolution Model • MORB magma is derived from an olivine-dominated mantle, whose composition (Mg/(Mg+Fe) we think we know (~0.9). • We assume magma entering the crust has this composition. • We use a thermodynamic model of magma evolution to calculate the amount of fractional crystallization that must have occurred, then calculate K, Th, and U in the “parent” magma.

  14. Calculated Parental Magma • ‘MELTS’ model indicates that average erupted MORB has experienced ~39% crystallization, with removal of 5% olivine, 18% plagioclase, 16% clinopyroxene, and <1% spinel-magnetite.

  15. Oceanic Plateaus From Kerr TOG (2013)

  16. Oceanic Plateaus

  17. Basalt-Seawater Interaction • Hydrothermal reactions between oceanic crust and seawater affect U and K concentrations of the oceanic crust. • Staudigel (2013) estimates • 402 mg/kg K uptake • 0.0307 mg/kg U uptake

  18. U, Th, and K in ‘mature’ oceanic crust

  19. Volumes & Masses *Schubert & Sandwell (1980)

  20. Total Radioactivity in Oceanic Crust

  21. Heat Production in the Oceanic Crust Total Estimated Mature (Fresh) Oceanic Crust Heat Production: 0.129 (0.103) TW (0.6 to 0.8% of total terrestrial)

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