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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)