The continental crust and depleted mantle both have subchondritic Nb/Ta ratios, so mixtures of the two cannot reproduce the chondritic reference composition of the bulk silicate Earth — the long-standing 'missing niobium' (Nb) paradox. Tiepolo and Vannucci (2014) proposed that amphibole-rich mafic cumulates delaminated from deep arc crust into the mantle could serve as the complementary superchondritic Nb/Ta reservoir, but concluded that this can close the budget only if very large volumes of such material are returned to the mantle over Earth history, leaving open whether such volumes are geologically realistic, when the deficit arose, and whether the chondritic-Earth reference assumed in the calculation holds. A decade of follow-up work has produced competing resolution families — rutile-bearing versus amphibole-rich cumulates, metasomatized lithospheric mantle and carbonate-recycling sources, sequestration of Nb in asteroidal and terrestrial cores, and a non-chondritic bulk Earth — none of which is accepted as definitive. The problem of achieving a definitive, mass-balanced resolution of the Nb paradox therefore remains open.

Background

Niobium (Nb) and tantalum (Ta) are refractory lithophile 'geochemical twins' — refractory (condensed at high temperature from the solar nebula, so their bulk planetary abundances are set by the accreting building blocks) and lithophile (partitioned into silicate rock rather than metal) — that occur together in chondritic meteorites (chondrites, the most primitive meteorites, whose refractory-element proportions approximate those of the bulk solar system) at a nearly constant Nb/Ta ratio of about 19.9, established as the reference value for bulk planetary bodies by Münker et al. (2003). The paradox is that every accessible silicate reservoir of the Earth is poorer in Nb relative to Ta than chondrites — the depleted upper mantle (the residue left after extraction of the continental crust) has Nb/Ta near 15–16 and the continental crust near 11–13 — so any mixture of the two is subchondritic and the silicate Earth appears to be 'missing' a substantial fraction of its Nb (Rudnick et al. 2000; Wade & Wood 2001). Because lithophile elements are expected to be stored in silicates, this selective depletion has no easy explanation. Candidate hiding places proposed before 2014 include rutile-bearing refractory eclogites returned to the mantle (eclogite is basaltic rock metamorphosed at high pressure, and rutile is a TiO2 mineral that strongly incorporates Nb and Ta; Rudnick et al. 2000), the Earth's metallic core (Wade & Wood 2001), late-stage residual melts from the magma ocean, the deep layer of silicate melt that covered the early Earth (Nebel et al. 2010), and eclogites in the subcontinental lithospheric mantle, the rigid mantle root beneath old continents (Aulbach et al. 2008). Partition experiments show that calcic amphibole, a hydrous silicate mineral characteristic of deep arc crust, takes in Nb more readily than Ta during arc-magma crystallization — the ratio of its mineral/melt partition coefficients for Nb and Ta, D_Nb/D_Ta, exceeds 1 under these conditions (a partition coefficient is the ratio of an element's concentration in a mineral to that in the coexisting melt) — so amphibole-rich rocks develop superchondritic Nb/Ta; this motivated the 'amphibole sponge' concept for arc differentiation (Davidson et al. 2007). Tiepolo and Vannucci (2014) quantified whether amphibole-rich ultramafic to mafic cumulates (rocks built of crystals accumulated from a magma) delaminated from the deep arc crust — that is, foundered back into the mantle — exemplified by the Cretaceous Kohistan paleo-arc section in Pakistan, could balance the Nb, Nb/Ta and Nb/La budgets by mass-balance modeling among continental crust, depleted mantle and primitive mantle (the composition of the silicate Earth before any crust extraction, used as the bulk reference). They found the paradox solvable only if very large volumes of such material were returned to the mantle over Earth history. Four issues bearing on that conclusion remain unresolved: (1) whether the low Nb/Ta of the depleted mantle was established before and during the Archean or mainly in post-Archean time — some Early Archean basalts and komatiites (ultramafic volcanic rocks erupted almost exclusively in the Archean) already show low Nb/Ta, while Archean hornblendites (rocks composed dominantly of the amphibole hornblende) and Nb-enriched basalts (a rare arc basalt type with anomalously high absolute Nb contents) suggest a later establishment; (2) the total volume of delaminated amphibole-rich cumulates through Earth history is poorly constrained, since the late Phanerozoic Kohistan example may not represent Precambrian arcs that operated under different thermal regimes; (3) whether any realistic integrated volume of delaminated material suffices to close the budget, or whether a residual missing-Nb deficit persists that implies at least one further complementary silicate reservoir; and (4) whether the chondritic bulk-Earth Nb/Ta assumed throughout the calculation holds at all, an assumption that is increasingly debated. Later work sharpened rather than settled these questions: experiments confirmed that amphibole strongly fractionates Nb from Ta during arc magma crystallization (Li et al. 2017; Chen et al. 2021), yet direct in-situ analyses of Kohistan and Gangdese hornblendite cumulates showed their amphibole, though high in Nb/Ta, carries too little absolute Nb and Ta to shift arc-magma budgets significantly, with rutile holding most of the Nb–Ta inventory (Zhang et al. 2021); carbonate-rich sediment subduction has been shown to generate high-Nb/Ta melts and metasomatized (chemically transformed by percolating melts or fluids) lithospheric mantle domains (Bragagni et al. 2024; Yang et al. 2024; Zhao et al. 2026); high-pressure experiments demonstrated that Nb becomes siderophile (preferentially partitioned into metallic iron) under core-formation conditions, reviving storage of the missing Nb in Earth's core (Münker et al. 2017; Huang et al. 2020); and chondrule-scale (chondrules are the millimetre-sized silicate spherules that make up chondrites) fractionation of Nb/Ta in enstatite chondrites undermined the constant-refractory-ratio rule on which the chondritic reference rests (Yoshizaki et al. 2021). Meanwhile (Nb,Ta)/U systematics have shown that the classical three-reservoir silicate-Earth model itself fails mass balance (Hofmann et al. 2022).

Problem Statement

Achieve a definitive resolution of the silicate Earth's niobium paradox. Tiepolo & Vannucci (2014) posed it as a mass-balance question: because the mixture of continental crust and upper mantle does not generate the Nb/Ta and Nb/La ratios of chondrites, a reservoir with superchondritic Nb/Ta and Nb/La is required in the Earth's system, and the question is whether amphibole-rich mafic lower arc crust returned to the mantle is that reservoir — their modeling shows it can solve the paradox only if large volumes of material are supposed to be returned into the mantle during Earth history. The following aspects of that question are unresolved: (i) whether the subchondritic Nb/Ta of the depleted mantle was established before and during the Archean or mainly in post-Archean time, as recorded in Archean mafic rocks, komatiites, hornblendites and Nb-enriched basalts; (ii) the total volume (mass) of amphibole-rich deep-arc cumulates delaminated into the mantle over Earth history, including whether the Phanerozoic Kohistan example is representative of Precambrian arcs; (iii) the identity of the complementary reservoir or combination of reservoirs — among candidates such as delaminated amphibolite, rutile-bearing eclogite, metasomatized lithospheric mantle, recycled carbonate-rich sediments or altered oceanic crust, late magma-ocean residues, and the Earth's core — together with the masses and Nb concentrations that account for the Nb missing from continental crust plus depleted mantle; and (iv) whether the bulk Earth's Nb/Ta is chondritic at all, or whether the missing-Nb problem dissolves into a non-chondritic bulk composition established during accretion and core formation. Resolving the paradox requires a quantitatively closed, physically consistent Nb (and Nb/Ta) mass balance for the Earth: within the chondritic-reference frame, the complementary reservoir or reservoirs that balance the subchondritic continental crust and depleted mantle, together with the timing of the deficit and the integrated delamination flux that the balance implies; or, if the deficit is primordial — for example hosted in the metallic core — or the bulk Earth's Nb/Ta is non-chondritic, the account that closes the same mass balance quantitatively in its own terms. The balance is constrained by the geological record of arc magmatism and delamination and by the cosmochemical constraints on Earth's building blocks.

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