Abstract
Except for the first 50–100 million years or so of the Earth’s history, when most of the mantle may have been subjected to melting, the differentiation of Earth’s silicate mantle has been controlled by solid-state convection1. As the mantle upwells and decompresses across its solidus, it partially melts. These low-density melts rise to the surface and form the continental and oceanic crusts, driving the differentiation of the silicate part of the Earth. Because many trace elements, such as heat-producing U, Th and K, as well as the noble gases, preferentially partition into melts (here referred to as incompatible elements), melt extraction concentrates these elements into the crust (or atmosphere in the case of noble gases), where nearly half of the Earth’s budget of these elements now resides2. In contrast, the upper mantle, as sampled by mid-ocean ridge basalts, is highly depleted in incompatible elements, suggesting a complementary relationship with the crust. Mass balance arguments require that the other half of these incompatible elements be hidden in the Earth’s interior. Hypotheses abound for the origin of this hidden reservoir3,4,5,6. The most widely held view has been that this hidden reservoir represents primordial material never processed by melting or degassing. Here, we suggest that a necessary by-product of whole-mantle convection during the Earth’s first billion years is deep and hot melting, resulting in the generation of dense liquids that crystallized and sank into the lower mantle. These sunken lithologies would have ‘primordial’ chemical signatures despite a non-primordial origin.
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Acknowledgements
The ideas for this paper were conceived at Rice University and fine-tuned at the 2008 Cooperative Institute for Deep Earth Research workshop at the Kavli Institute. We thank G. Masters, M. Ishii, M. Manga, M. Jellinek, B. Romanowicz, T. Plank, H. Gonnermann, L. Stixrude and C. Lithgow-Bertelloni for discussions and S. Parman for reviews. G. Masters also helped with velocity calculations. We thank the Packard Foundation and the NSF for support. J. Li also acknowledges support from the University of Illinois.
Author Contributions C.-T.A.L. planned the paper, performed the modelling and wrote the paper; P.L. helped with the modelling and interpretation and figures; T.H. helped with the geodynamic interpretation; J.L. helped with the density modelling and interpretation; R.D. helped with the petrologic and geochemical interpretations; and J.H. helped with the geodynamic and mineral physics interpretations.
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Lee, CT., Luffi, P., Höink, T. et al. Upside-down differentiation and generation of a ‘primordial’ lower mantle. Nature 463, 930–933 (2010). https://doi.org/10.1038/nature08824
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DOI: https://doi.org/10.1038/nature08824
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