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
Th
ppm
0.404
K
ppm
1328
Th/U
K/U
Mean
U
ppm
0.119
3.1
14836
2σmean
0.026
.077
71
0.029
227
n
2205
2367
2466
2167
1743
Back-Arc Basins
U
ppm
Th
ppm
K
ppm
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
361
362
332
361
242
n
Grand Average: MORB + BABB
Gale et al. (2012) Mean
2σmean
n
U
ppm
Th
ppm
K
ppm
0.123
0.013
2759
0.407
.072
2566
1369
125
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
ppm
Th
ppm
K
ppm
MORB
0.123
0.407
1369
Bulk Ocean Crust
0.08
0.26
720
~0.006
~0.021
~60
Implied Source
Oceanic Plateaus
From Kerr TOG (2013)
Oceanic Plateaus
U
ppm
Th
ppm
K
ppm
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
MORB
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
U
ppm
Th
ppm
K
ppm
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
‘Normal’
Crust
Area
km2
Thicknes
s
km
Volume
km3
Density
kg/m3
Mass
kg
2.8 x 108
7
2.06 x 109
2800*
5.95 x 1021
3.79 x 106*
2800*
1.1 x 1019
Plateaus
*Schubert
& Sandwell (1980)
Total Radioactivity in Oceanic Crust
U
kg
Th
kg
K
kg
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
Specific heat
production
Mass
(fresh)
Heat, TW
U
µW/kg
Th
µW/kg
K
µW/kg
98.14
26.36
3.45 x 10-3
6.59 x 1014
4.79 x 1014
0.065
0.047
1.56 x
1015
0.041
0.041
6.69 x 1018
4.30 x 1018
0.021
0.015
Total Estimated Mature (Fresh) Oceanic Crust Heat Production:
0.129 (0.103) TW (0.6 to 0.8% of total terrestrial)