Radioactivity in the Oceanic Crust
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Transcript of Radioactivity in the Oceanic Crust
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RADIOACTIVITY IN THE OCEANIC CRUST
William M. White, Cornell University, USA
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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.
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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.
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Spider Diagrams & Incompatible Elements
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Spider Diagrams & Incompatible Elements
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MORB are depleted in incompatible elements
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Th Distribution in MORB
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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
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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 200132 σmean 0.049 .013 200 0.06 815n 361 362 332 361 242
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Grand Average: MORB + BABB
Uppm
Thppm
Kppm
Gale et al. (2012) Mean
0.123 0.407 1369
2σmean 0.013 .072 125n 2759 2566 2798
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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.
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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.
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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.
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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
ppmMORB 0.123 0.407 1369Bulk Ocean Crust 0.08 0.26 720
Uppm
Thppm
Kppm
MORB 0.123 0.407 1369Bulk Ocean Crust 0.08 0.26 720Implied Source ~0.00
6~0.021 ~60
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Oceanic Plateaus
From Kerr TOG (2013)
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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
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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
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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
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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)
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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
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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)