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

Back-Arc Basins

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)

Oceanic Plateaus

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)