The Earth’s missing lead may not be in the core

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Abstract

Relative to the CI chondrite class of meteorites (widely thought to be the ‘building blocks’ of the terrestrial planets), the Earth is depleted in volatile elements. For most elements this depletion is thought to be a solar nebular signature, as chondrites show depletions qualitatively similar to that of the Earth1. On the other hand, as lead is a volatile element, some Pb may also have been lost after accretion. The unique 206Pb/204Pb and 207Pb/204Pb ratios of the Earth’s mantle suggest that some lead was lost about 50 to 130 Myr after Solar System formation2,3,4. This has commonly been explained by lead lost via the segregation of a sulphide melt to the Earth’s core5,6,7, which assumes that lead has an affinity towards sulphide. Some models, however, have reconciled the Earth’s lead deficit with volatilization8. Whichever model is preferred, the broad coincidence of U–Pb model ages with the age of the Moon9,10,11 suggests that lead loss may be related to the Moon-forming impact. Here we report partitioning experiments in metal–sulphide–silicate systems. We show that lead is neither siderophile nor chalcophile enough to explain the high U/Pb ratio of the Earth’s mantle as being a result of lead pumping to the core. The Earth may have accreted from initially volatile-depleted material, some lead may have been lost to degassing following the Moon-forming giant impact, or a hidden reservoir exists in the deep mantle with lead isotope compositions complementary to upper-mantle values; it is unlikely though that the missing lead resides in the core.

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Figure 1: Metal–silicate (black) and sulphide–silicate (grey) partition coefficients for lead.
Figure 2: Summary of the metal–silicate (black) and sulphide–silicate (grey) partition coefficients (1,400 °C, 2 GPa).
Figure 3: Calculation of lead, cadmium, zinc, selenium and tellurium abundances in the Earth’s mantle with partition coefficients summarized in Fig. 2, normalized to Cl and Mg# = 1.
Figure 4: Modelled evolution of the silicate Earth’s 238U/204Pb ratio in time elapsed after Solar System formation, using the partition coefficients summarized in Fig. 2 .
Figure 5: Modelled lead isotope compositions of the present-day upper mantle.

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Acknowledgements

The paper benefited from our discussions with F. Tomaschek, E. E. Scherer, A. Rohrbach, R. Fonseca and K. Mezger, as well as from a review by S. B. Jacobsen. Financial support from the German Research Council through the Priority Programme ‘Mars and the Terrestrial Planets’ to C.B. and C.M. is acknowledged.

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Correspondence to M. Lagos.

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Lagos, M., Ballhaus, C., Münker, C. et al. The Earth’s missing lead may not be in the core. Nature 456, 89–92 (2008) doi:10.1038/nature07375

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