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A Mercury-like component of early Earth yields uranium in the core and high mantle 142Nd

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Abstract

Recent 142Nd isotope data indicate that the silicate Earth (its crust plus the mantle) has a samarium to neodymium elemental ratio (Sm/Nd) that is greater than that of the supposed chondritic building blocks of the planet. This elevated Sm/Nd has been ascribed either to a ‘hidden’ reservoir in the Earth1,2 or to loss of an early-formed terrestrial crust by impact ablation3. Since removal of crust by ablation would also remove the heat-producing elements—potassium, uranium and thorium—such removal would make it extremely difficult to balance terrestrial heat production with the observed heat flow3. In the ‘hidden’ reservoir alternative, a complementary low-Sm/Nd layer is usually considered to reside unobserved in the silicate lower mantle. We have previously shown, however, that the core is a likely reservoir for some lithophile elements such as niobium4. We therefore address the question of whether core formation could have fractionated Nd from Sm and also acted as a sink for heat-producing elements. We show here that addition of a reduced Mercury-like body (or, alternatively, an enstatite-chondrite-like body) rich in sulfur to the early Earth would generate a superchondritic Sm/Nd in the mantle and an 142Nd/144Nd anomaly of approximately +14 parts per million relative to chondrite. In addition, the sulfur-rich core would partition uranium strongly and thorium slightly, supplying a substantial part of the ‘missing’ heat source for the geodynamo.

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Figure 1: Sulphide–silicate partitioning data.
Figure 2: Core content of U (p.p.b.) and mantle 142Nd anomaly (p.p.m.).

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Acknowledgements

We acknowledge support from the European Research Council grant number 267764. We thank J. Wade for his advice and comments. A. Hofmann and T. Elliott provided advice and suggestions about Th/U of silicate Earth.

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Authors

Contributions

Both authors performed experiments and microanalysis, with about two-thirds done by A.W. Both authors contributed to writing the manuscript, with about two-thirds done by B.J.W.

Corresponding authors

Correspondence to Anke Wohlers or Bernard J. Wood.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Partition coefficients for U, Nd and Sm with changing log[FeO] content in silicate melt (wt%).

a, Results for D values of experiments performed at 1.5 GPa and 1,500 °C. b, D value results at 1.5 GPa and 1,650 °C.

Extended Data Figure 2 Core content of U (p.p.b.) and mantle 142Nd anomaly (p.p.m.) at DU/DSm=3.

The effect on the Nd and U content using the same parameters (DNd/DSm at 1.4 and DTh/DU at 0.1) as in Fig. 2 but with a higher DU/DSm ratio. a and b show the calculated effect of adding to Earth a reduced body of 20% of the Earth's mass or 45% of the Earth's mass, containing 0.15 mass fraction sulfide. c and d illustrate the same scenario except that the reduced body contains 0.22 mass fraction sulfide.

Extended Data Figure 3 REE fractionation at 3.2% and 8.1% S in the core.

The calculated REE pattern in the bulk silicate Earth (BSE) for the two extreme cases of Fig. 1a (3.2% S) and Extended Data Fig. 2d (8.1% S). Black diamonds represent REE concentrations relative to chondritic abundances and normalized to Yb = 1, at 3.2% S in the core (20% reduced mass impactor containing 0.15 mass fraction sulfide). White diamonds illustrate the REE fractionation at elevated S content (8.1% S in the core, 35% reduced mass impactor containing 0.22 mass fraction sulfide). We assumed DSm = [(Sm in sulfide)/(Sm in silicate)]=1 and Di/DSm ratios for other elements from experiment 464. Both scenarios result in very small depletions of light REE relative to heavy REE in the BSE. The trend is broadly consistent with that seen in the depleted mid-ocean-ridge basalt (MORB)–mantle composition (blue diamonds) but much smaller. The effect on the REE pattern of the BSE would, as can be seen, be undetectable. Blue diamonds illustrate the measured ratio of depleted MORB mantle (from Salters and Stracke31) to the BSE (Palme and O'Neill8, assuming chondritic abundances of refractory lithophile elements in the latter. Error bars are from propagated error calculation and correspond to 1 s.d.

Extended Data Table 1 Major-element composition of silicate glass.
Extended Data Table 2 Major-element composition of sulfides.
Extended Data Table 3 Trace-element concentration in silicates.
Extended Data Table 4 Trace-element concentration in sulfides.

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Wohlers, A., Wood, B. A Mercury-like component of early Earth yields uranium in the core and high mantle 142Nd. Nature 520, 337–340 (2015). https://doi.org/10.1038/nature14350

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