1. Laboratoire des sciences de la Terre, Ecole Normale Supérieure de Lyon, Université de Lyon, CNRS UMR 5570, 46 Allée d'Italie, 69364 Lyon Cedex 07, France
2. Équipe de Dynamique des Fluides Géologiques, Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris Cedex 05, France
3. Laboratoire des sciences de la Terres, Université Lyon 1, Université de Lyon, CNRS UMR 5570, 2 rue Raphael Dubois, 69622 Villeurbanne Cedex, France
4. Present address: Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.

Correspondence to: S. Labrosse¹ Correspondence and requests for materials should be addressed to S.L. (Email: stephane.labrosse@ens-lyon.fr).

Abstract

The distribution of geochemical species in the Earth's interior is largely controlled by fractional melting and crystallization processes that are intimately linked to the thermal state and evolution of the mantle. The existence of patches of dense partial melt at the base of the Earth's mantle¹, together with estimates of melting temperatures for deep mantle phases² and the amount of cooling of the underlying core required to maintain a geodynamo throughout much of the Earth's history³, suggest that more extensive deep melting occurred in the past. Here we show that a stable layer of dense melt formed at the base of the mantle early in the Earth's history would have undergone slow fractional crystallization, and would be an ideal candidate for an unsampled geochemical reservoir hosting a variety of incompatible species (most notably the missing budget of heat-producing elements) for an initial basal magma ocean thickness of about 1,000 km. Differences in ¹⁴²Nd/¹⁴⁴Nd ratios between chondrites and terrestrial rocks⁴ can be explained by fractional crystallization with a decay timescale of the order of 1 Gyr. These combined constraints yield thermal evolution models in which radiogenic heat production and latent heat exchange prevent early cooling of the core and possibly delay the onset of the geodynamo to 3.4–4 Gyr ago⁵.

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