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

Spider Diagrams & Incompatible Elements

MORB are depleted in incompatible elements

Th Distribution in MORB

Mean Concentrations of Th, U, and K in MORB

Uppm

Thppm

Kppm

Th/U K/U

Mean 0.119 0.404

1328 3.1 14836

2σmean 0.026 .077 71 0.029 227

n 2205 2367 2466 2167 1743

Back-Arc Basins

Uppm

Thppm

Kppm

Th/U K/U

(MORB) (0.119)

(0.404)

(1328) 3.1 14836

Mean 0.137 0.399 2258 2.69 20013

2 σmean 0.049 .013 200 0.06 815

n 361 362 332 361 242

Grand Average: MORB + BABB

Uppm

Thppm

Kppm

Gale et al. (2012) Mean

0.123 0.407 1369

2σmean 0.013 .072 125

n 2759 2566 2798

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. U

ppmTh

ppmK

ppm

MORB 0.123 0.407 1369

Bulk Ocean Crust 0.08 0.26 720

Uppm

Thppm

Kppm

MORB 0.123 0.407 1369

Bulk Ocean Crust 0.08 0.26 720

Implied Source ~0.006

~0.021 ~60

Oceanic Plateaus

From Kerr TOG (2013)

Oceanic Plateaus

Uppm

Thppm

Kppm

MORB 0.123 0.407 1369

Plateau Basalts 0.4 1.3 3039

Bulk ‘Normal’ Crust

0.08 0.26 720

Bulk ‘Plateau’ Crust

0.25 0.82 1600

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

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

Uppm

Thppm

Kppm

Bulk ‘Normal’ Mature Crust

0.08 0.26 1120

Bulk ‘Plateau’ Mature Crust

0.29 0.82 2000

Continental Crust 1.31 5.61 15200

Volumes & Masses

Areakm2

Thicknesskm

Volume

km3

Density

kg/m3

Masskg

‘Normal’ Crust

2.8 x 108

7 2.06 x 109

2800* 5.95 x 1021

Plateaus

3.79 x 106*

2800* 1.1 x 1019

*Schubert & Sandwell (1980)

Total Radioactivity in Oceanic Crust

Ukg

Thkg

Kkg

Mature Normal

6.55 x 1014 1.55 x 1015 6.67 x 1018

Mature Plateaus

3.19x1012 — 2.2 x1016

Total Mature

6.58 x 1014 1.56 x 1015 6.69 x 1018

ν yr-1 2.57 x 106 1.33 x 106 1.33 x 107

Heat Production in the Oceanic Crust

UµW/kg

ThµW/kg

KµW/kg

Specific heat production

98.14 26.36 3.45 x 10-3

Mass(fresh)

6.59 x 1014

4.79 x 1014 1.56 x 1015 6.69 x 1018

4.30 x 1018

Heat, TW 0.0650.047

0.0410.041

0.0210.015

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