The niobium and tantalum concentration in the mantle constrains the composition of Earth’s primordial magma ocean

International audienceThe bulk silicate Earth (BSE), and all its sampleable reservoirs, have a subchondritic niobium-to-tantalum ratio (Nb/Ta). Because both elements are refractory, and Nb/Ta is fairly constant across chon-drite groups, this can only be explained by a preferential seques-tration of Nb relative to Ta in a hidden (unsampled) reservoir. Experiments have shown that Nb becomes more siderophile than Ta under very reducing conditions, leading the way for the accepted hypothesis that Earth's core could have stripped sufficient amounts of Nb during its formation to account for the subchon-dritic signature of the BSE. Consequently, this suggestion has been used as an argument that Earth accreted and differentiated, for most of its history, under very reducing conditions. Here, we present a series of metal-silicate partitioning experiments of Nb and Ta in a laser-heated diamond anvil cell, at pressure and temperature conditions directly comparable to those of core formation; we find that Nb is more siderophile than Ta under any conditions relevant to a deep magma ocean, confirming that BSE's missing Nb is in the core. However, multistage core formation modeling only allows for moderately reducing or oxidizing accretionary conditions , ruling out the need for very reducing conditions, which lead to an overdepletion of Nb from the mantle (and a low Nb/Ta ratio) that is incompatible with geochemical observations. Earth's pri-mordial magma ocean cannot have contained less than 2% or more than 18% FeO since the onset of core formation. core formation | Nb/Ta ratio | metal-silicate partitioning | high pressure N iobium and tantalum are among those "geochemical twins" (e.g., Zr and Hf, Mo and W) located vertically adjacent to each other in the periodic table that share similar chemical properties during planetary differentiation. As both refractory and lithophile elements, Nb and Ta may be assumed to have condensed into planetary bodies in chondritic ratio (1) yet solely remained in the silicate portion of planets after core-mantle (metal-silicate) differentiation. However, in contrast to other trace element ratios (e.g., Zr/Hf and Ce/Pb), which indicate the complementarity between Earth's mantle and (continental or oceanic) crust with respect to chondritic meteorites, Nb/Ta ratios in all major reservoirs of silicate Earth (the crust, depleted mid-ocean ridge basalt mantle, and ocean island basalts) are invariably subchondritic (