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The solubility of heat-producing elements in Earth’s core

I. Blanchard1,#,

1Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France

J. Siebert1,2,

1Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France
2Institut Universitaire de France, Paris, France

S. Borensztajn1,

1Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France

J. Badro1,3

1Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France
3Earth and Planetary Science Laboratory, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Blanchard, I., Siebert, J., Borensztajn, S., Badro, J. (2017) The solubility of heat-producing elements in Earth’s core. Geochem. Persp. Let. 5, 1–5.

UnivEarthS Labex at Sorbonne Paris Cité. ANR-10-LABX-0023; ANR-11-IDEX-0005-02 program PARI; PARIS-Idf SESAME #12015908; DFG RU 1323/10-1; PNP Research Program at INSU-CNRS; ERC n°207467 (DECORE); ANR-14-CE33-0017-01 (VolTerre).

Geochemical Perspectives Letters v5  |  doi: 10.7185/geochemlet.1737
Received 01 August 2017  |  Accepted 07 September 2017  |  Published 4 October 2017
Copyright © 2017 European Association of Geochemistry




Figure 1 Thermodynamic models of K and U partitioning between metal and silicate. (a) Equilibrium constant for potassium metal-silicate partitioning (Eq. 1) as a function of P/T. The line corresponds to the least squares fit (R2 = 0.94) of the thermodynamic model (Eq. 3) with a = -2.7 ± 0.03 and c = 65 ± 2.5. (b) Equilibrium constant of uranium metal-silicate partitioning (Eq. 2) as a function of the reciprocal temperature. The line corresponds to the least squares fit (R2 = 0.91) of the thermodynamic model (Eq. 4) with a = -0.55 ± 0.21, b = -10133 ± 642 and εUO = -31.
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Table 1 Coefficients from Eqs. 3, 4 (and S-3, S-4) obtained by least squares linear regression of the experimental data, along with their associated (1 sigma) uncertainties. Details of the calculation are given in the Supplementary Information.
Element iab (K)c (K/GPa)εOi (1873 K)εSi (1873 K)εSii (1873 K)
Potassium–2.7 (0.03)065 (2.5)000
Uranium–0.55 (0.21)–10133 (642)0–3100
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Figure 2 Concentration of K and U in the core from continuous core-formation models, assuming two magma ocean geotherms: cool (A1,2,3) and warm (B1,2,3). The top panel is the abundance of 238U and 235U as a function of time, the middle panel corresponds to that of 40K as a function of time, and the bottom panel is the total power produced in the core by the radioactive decay of these three radionuclides over time.
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Supplementary Figures and Tables


Table S-1 Composition of starting materials. The basalt and Met 1 have been measured by EPMA, whereas the pyrolite has been measured using EDX. Fe is Alfa Aesar spherical iron powder (stock number 00170). N stands for the number of analyses that have been carried.
Starting
material
BasaltPyrolite

Met 1Fe
wt. %N = 17N = 12

N = 15
Na2O2.34 (0.01)-
Fe64.44 (0.81)> 99.9
SiO235.62 (0.05)44.22 (0.1)
S15.63 (0.38)-
MgO10.26 (0.09)35.38 (0.11)
O1.55 (0.19)-
Al2O312.31 (0.03)4.33 (0.02)



P2O50.13 (0.01)-



SO20.69 (0.01)-



K2O5.68 (0.03)4.05 (0.03)



TiO21.32 (0.01)-



FeO17.78 (0.06)6.85 (0.16)



UO22.22 (0.11)0.76 (0.05)



CaO7.44 (0.02)3.14 (0.03)



Total98.84 (0.08)100

95.83 (0.19)100
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Table S-2 Experimental conditions. Pressure has been corrected for thermal pressure following Siebert et al. (2012). Oxygen fugacity of each run has been calculated relative to the IW buffer using an ideal model assuming the activity of iron in metal and silicate is 1. The estimated uncertainties for temperatures and pressures are ±150 K and ±5 GPa respectively (e.g., Fiquet et al., 2010; Siebert et al., 2012).
Run #T (K)P (GPa)ΔIWStarting composition
BAS K 52360062-0.586Basalt + Met 1
BAS K Fe 40350049-0.861Basalt + Fe
BAS K Fe 60370070-0.633Basalt + Fe
BAS K Fe 70400081-0.711Basalt + Fe
PYR K FeS 40370049-0.82Pyrolite + Met 1
PYR K FeS 60400071-0.95Pyrolite + Met 1
PYR K Fe 40400054-1.01Pyrolite + Fe
PYR K Fe 50380060-0.923Pyrolite + Fe
PYR K Fe 60410071-0.75Pyrolite + Fe
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Table S-3 Composition of the metallic phases. *Totals include Cu, Zn, Mn and Pb which are not reported here. N stands for the number of analyses that have been performed.
Run #BAS K 52BAS K Fe 40BAS K Fe 60BAS K Fe 70PYR K Fe 40PYR K FeS 60PYR K FeS 40PYR K Fe 50PYR K Fe 60
EPMA (wt. %)N = 3N = 8N = 3N = 2N = 5N = 3N = 4N = 2N = 4
O10.17 (0.11)6.79 (0.55)10.41 (0.06)12.11 (0.02)9.14 (0.51)9.20 (0.56)7.73 (0.57)10.54 (0.39)12.38 (0.25)
Mg0.34 (0.01)0.24 (0.01)0.29 (0.02)0.82 (0.06)1.18 (0.06)0.78 (0.43)0.56 (0.08)1.13 (0.42)1.03 (0.03)
Al0.26 (0.01)0.15 (0.02)0.28 (0.01)1.16 (0.17)0.16 (0.02)0.08 (0.03)0.07 (0.02)0.17 (0.04)0.13 (0.01)
Si1.43 (0.02)1.10 (0.16)1.23 (0.001)2.71 (0.23)5.78 (0.54)1.60 (0.09)1.74 (0.35)3.12 (0.14)3.45 (0.06)
P0.43 (0.01)0.60 (0.03)0.19 (0.02)0.17 (0.02)-----
S8.76 (0.09)4.13 (0.13)8.08 (0.04)10.39 (0.03)-4.12 (0.05)5.19 (0.07)--
K0.211 (0.001)0.10 (0.003)0.24 (0.01)0.67 (0.04)0.21 (0.11)0.47 (0.23)0.14 (0.01)0.24 (0.01)0.31 (0.01)
Ca0.107 (0.001)0.06 (0.01)0.07 (0.003)0.26 (0.01)-----
Ti0.34 (0.03)0.14 (0.03)0.24 (0.02)0.27 (0.07)-----
Fe71.09 (0.82)77.19 (0.89)70.11 (0.65)66.87 (0.46)77.76 (1.99)73.87 (0.40)73.61 (1.10)75.74 (0.01)76.29 (0.26)
U0.05 (0.01)0.03 (0.01)0.05 (0.004)0.07 (0.01)0.20 (0.04)0.31 (0.03)0.13 (0.04)0.41 (0.01)0.57 (0.01)
Total*96.61 (1.03)93.37 (0.35)94.07 (0.44)98.09 (0.39)97.80 (1.32)98.45 (1.57)96.54 (0.58)97.53 (0.93)99.39 (0.35)
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Table S-4 Composition of the metallic phase given as mol. %. N stands for the number of analyses that have been performed.
Run #BAS K 52BAS K Fe 40BAS K Fe 60BAS K Fe 70PYR K Fe 40PYR K FeS 60PYR K FeS 40PYR K Fe 50PYR K Fe 60
Mol. %N = 3N = 8N = 3N=2N = 5N = 3N = 4N = 2N = 4
K0.230.120.270.680.230.530.170.260.33
Fe54.7367.4255.0347.5360.9958.961.1759.3956.85
U0.0090.0070.0080.0120.0370.0590.0250.0750.099
O27.3120.7128.5230.0325.0225.5922.4228.8532.19
S11.756.2911.0412.8605.727.5100
Si2.181.911.913.839.012.542.884.875.11
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Table S-5 Compositions of silicate phases. N stands for the number of analyses that have been performed.
Run #BAS K 52BAS K Fe 40BAS K Fe 60BAS K Fe 70PYR K Fe 40PYR K FeS 60PYR K FeS 40PYR K Fe 50PYR K Fe 60
EPMA (wt. %)N = 3N = 3N = 3N = 3N = 6N = 6N = 5N = 4N = 5
Na2O2.03 (0.03)3.06 (0.13)2.18 (0.04)2.25 (0.14)- ----
SiO229.65 (0.28)33.26 (1.09)30.14 (0.28)33.08 (0.70)40.81 (2.78)31.64 (0.71)34.13 (0.74)32.78 (0.55)33.67 (2.38)
MgO7.03 (0.08)8.59 (0.37)6.53 (0.10)7.06 (0.13)27.78 (2.36)21.02 (0.46)20.75 (0.39)21.57 (0.67)21.71 (2.31)
Al2O39.24 (0.15)10.07 (0.42)10.75 (0.16)12.31 (0.28)4.45 (0.46)3.43 (0.13)3.29 (0.04)4.01 (0.14)3.31 (0.42)
P2O50.21 (0.05)0.21 (0.11)0.19 (0.07)0.19 (0.04)-----
SO21.44 (0.12)0.26 (0.16)1.10 (0.09)1.13 (0.16)-0.24 (0.04)0.60 (0.27)--
K2O5.57 (0.14)6.17 (0.16)6.35 (0.06)6.32 (0.12)6.54 (0.86)10.66 (1.66)6.52 (0.12)8.51 (0.26)8.00 (1.64)
TiO21.58 (0.04)1.40 (0.05)1.39 (0.05)1.44 (0.01)-----
FeO33.12 (0.54)29.29 (1.23)32.94 (0.59)28.59 (1.13)25.79 (7.05)24 78 (1.09)28.17 (1.72)25.88 (1.32)28.65 (5.27)
CuO0.70 (0.11)0.56 (0.13)1.14 (0.26)0.81 (0.19)1.42 (0.21)3.19 (0.22)2.53 (0.28)2.53 (0.13)2.50 (0.43)
ZnO2.85 (0.07)3.04 (0.10)2.78 (0.05)2.85 (0.05)0.36 (0.06)0.67 (0.08)0.48 (0.04)0.46 (0.06)0.55 (0.06)
UO20.25 (0.02)0.31 (0.04)0.19 (0.02)0.25 (0.05)0.97 (0.06)1.10 (0.10)1.10 (0.07)0.96 (0.09)1.99 (0.17)
PbO0.57 (0.04)0.40 (0.05)0.49 (0.03)0.40 (0.04)-----
CaO2.67 (0.06)3.13 (0.07)1.90 (0.01)2.52 (0.07)2.59 (0.28)2.64 (0.05)3.03 (0.10)2.87 (0.10)3.16 (0.28)
MnO0.23 (0.06)0.17 (0.03)0.21 (0.04)0.19 (0.03)-----
Total97.92 (0.96)99.94 (0.58)98.29 (0.25)99.40 (0.24)110.75 (1.17)99.66 (1.62)101.10 (0.99)99.62 (1.04)103.62 (0.28)
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Table S-6 Literature data that have been used along with our data to perform thermodynamic regression.
ReferenceRun#
ReferenceRun#
Potassium
Uranium
Corgne et al., 2007PC509
Wheeler et al., 2006PC408
Corgne et al., 2007PC504
Malavergne et al., 2007#2309
Corgne et al., 2007PC508
Malavergne et al., 2007#414
Corgne et al., 2007PC514
Bouhifd et al., 2013969
Corgne et al., 2007PC515
Bouhifd et al., 2013975
Corgne et al., 2007PR380
Bouhifd et al., 20131027
Corgne et al., 2007PR381
Bouhifd et al., 20131022
Corgne et al., 2007PR382
Bouhifd et al., 2013986
Corgne et al., 2007PR390
Chidester et al., 2017B22
Corgne et al., 2007PR394
Chidester et al., 2017B23
Corgne et al., 2007PR395
Chidester et al., 2017B42



Chidester et al., 2017B56



Chidester et al., 2017B50



Chidester et al., 2017B66



Chidester et al., 2017B49
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Figure S-1 Backscattered scanning electron microscopy (FEG-SEM) image of a typical recovered sample. Samples are excavated from the melted region, thinned down to a thickness of ~3 microns, and welded to a TEM grid using a FIB instrument. The metal and silicate phases are analysed by electron microprobe analysis.
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Figure S-2 Comparison of equilibrium constant (log K) and the exchange coefficient (log KD) for potassium (a) and uranium (b) metal-silicate exchange reactions. The difference between log(K) and log(KD) is the activity coefficient ratio EQUATION . While activities are limited for potassium (a), because all interaction parameters are 0, they are obvious in uranium (b) because of a large interaction parameter between U and O in the metal.
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