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Article Dans Une Revue Proceedings of the National Academy of Sciences of the United States of America Année : 2020

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

Dongyang Huang
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James Badro
Julien Siebert

Résumé

The 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 (<19.9) (2). This apparent deficit of Nb relative to Ta in the bulk silicate Earth (BSE), known as the "Nb paradox," has been attributed to 1) a hitherto unsampled reservoir with a superchondritic Nb/Ta ratio in the lowermost mantle formed by an ancient subducted crust as early as the Hadean (3-5) and/or 2) the sequestration of Nb into the core during core-mantle differentiation (6-8). The metal-silicate partitioning behavior of Nb, a lithophile element, was first found to mimic that of V, a slightly siderophile element, at elevated pressures (25 GPa) (6).This invariably leads to an increase in Nb solubility in metal relative to Ta in a deep magma ocean and has been proposed as a reason for the sub-chondritic Nb/Ta ratio observed in the BSE. This requires, however, just as in the case of V, the magma ocean to start and remain very reducing (very low FeO concentration) for an extended period during core segregation. Reducing conditions were considered as the only pathway to an increase in the side-rophility of V and Nb. Indeed, all experiments to date (per-formed in piston-cylinder or multianvil press up to ∼25 GPa) show that oxygen fugacity is the dominant parameter (that superimposes pressure [P], temperature [T], and composition) to incorporate Nb in the metal during core-mantle equilibration (7, 8), despite the dependence of partitioning on pressure and temperature (9-12). Further experiments, along with multistage core formation models confirmed that in order to match the Nb abundance and the Nb/Ta in the mantle by core formation alone, a highly reducing magma ocean (very low FeO concentration) was required, with a silicate liquid containing significantly less than 1 wt % FeO (7, 13, 14). The natural consequence of those findings was that the behavior of V and Nb (and to a lesser extent Cr and Ta) during core formation requires Earth to have ac-creted and remained very reduced for a significant amount of time. The initiation of metal-silicate partitioning experiments in the laser-heated diamond anvil cell (LHDAC) extended the P and T conditions of metal-silicate experiments to those directly relevant (43 to 75 GPa and 3,000 to 4,400 K) to core formation on Earth (15). One of the new outcomes of this class of experiments was the significant increase in oxygen concentration in the metal Significance Silicate Earth is widely considered identical to chondrites in its refractory lithophile element ratios. However, its subchondritic Nb/Ta signature deviates from the chondritic paradigm. To resolve this Nb deficit, its sequestration in Earth's core under very reducing core-forming conditions has been proposed based on low-pressure data. Here, we show that under conditions relevant to core formation Nb is siderophile at high pressures under all redox conditions, corroborating Nb inventory in Earth's core. Further core formation modeling shows that Earth's core could have formed under moderately reducing or oxidizing conditions, whereas highly reducing conditions mismatch the geochemical observables; although Earth may have sampled a variety of reservoirs, it is problematic to ac-crete primarily from materials as reduced as enstatite chondrites.
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Dates et versions

hal-03011132 , version 1 (08-12-2020)

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Dongyang Huang, James Badro, Julien Siebert. The niobium and tantalum concentration in the mantle constrains the composition of Earth’s primordial magma ocean. Proceedings of the National Academy of Sciences of the United States of America, 2020, 117 (45), pp.27893-27898. ⟨10.1073/pnas.2007982117⟩. ⟨hal-03011132⟩
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