Presentation on theme: "RADIOACTIVITY IN THE OCEANIC CRUST William M. White, Cornell University, USA."— Presentation transcript:
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. Some freezes in the conduits to the surface (the sheeted dike complex). Most crystallizes within the crust to form the gabbroic layer.
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.
Spider Diagrams & Incompatible Elements
MORB are depleted in incompatible elements
Th Distribution in MORB
Mean Concentrations of Th, U, and K in MORB
Grand Average: MORB + BABB
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.
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.
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.
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.
Oceanic Plateaus From Kerr TOG (2013)
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 mg/kg U uptake
U, Th, and K in ‘mature’ oceanic crust
Volumes & Masses Area km 2 Thicknes s km Volume km 3 Density kg/m 3 Mass kg ‘Normal’ Crust 2.8 x x * 5.95 x Plateaus3.79 x 10 6* 2800 * 1.1 x * Schubert & Sandwell (1980)
Total Radioactivity in Oceanic Crust U kg Th kg K kg Mature Normal6.55 x x x Mature Plateaus3.19x10 12 —2.2 x10 16 Total Mature6.58 x x x ν yr x x x 10 7
Heat Production in the Oceanic Crust U µW/kg Th µW/kg K µW/kg Specific heat production x Mass (fresh) 6.59 x x x x x Heat, TW Total Estimated Mature (Fresh) Oceanic Crust Heat Production: (0.103) TW (0.6 to 0.8% of total terrestrial)