The accessible silicate Earth — continental crust (~12–13) and mantle (~14–15) — has a niobium/tantalum ratio well below the chondritic reference (~19.9), so geochemical mass balance requires either a complementary reservoir with superchondritic Nb/Ta or a revision of the chondritic-composition premise itself. Aulbach et al. (2008) showed that the high Nb/Ta of kimberlite-borne eclogite rutiles is not primary but metasomatic, acquired during residence in the subcontinental lithospheric mantle (SCLM), removing the then-leading candidate (rutile-bearing subducted eclogite) and leaving the reservoir's identity, mass, and location unresolved. The problem remains open: at least five incompatible solutions are actively pursued — Earth's core (high-pressure diamond-anvil-cell experiments, Huang et al. 2020), sequestration into asteroidal cores before accretion (Münker et al. 2017), metasomatized/enriched lithospheric mantle (Pfänder et al. 2012; Yang et al. 2024), deep magma-ocean residual melts (Nebel et al. 2010), and subducted-crust reservoirs — none accepted as definitive.

Background

Niobium (Nb) and tantalum (Ta) are refractory high-field-strength elements (HFSE) — metals that form small, highly charged ions and behave coherently during most geological processing because their charges (+5) and ionic radii are nearly identical. Both are lithophile (silicate-preferring) and refractory (they condense at high nebular temperatures and are not volatilized), so the bulk silicate Earth (BSE: the mantle plus crust, i.e., the Earth outside the metallic core) is expected to have the same Nb/Ta ratio as chondritic meteorites, the unfractionated building blocks of the solar system. High-precision isotope-dilution measurements (mass spectrometry calibrated by adding a known isotope spike to the sample, yielding precise trace-element abundances) broke this expectation: chondrites cluster at Nb/Ta ≈ 19.9 ± 0.6 (Münker et al. 2003; cf. the lower ~17.6 estimate of Jochum et al. 2000), whereas the BSE is ~14–15, the continental crust ~12–13 (Barth et al. 2000), and the depleted mantle (the upper-mantle source of mid-ocean-ridge basalts, depleted in incompatible elements by ancient melt extraction) similarly subchondritic. Because a mixture of crust and depleted mantle — the two sampleable silicate reservoirs — cannot reproduce a chondritic ratio, either some hidden reservoir with superchondritic Nb/Ta holds the complement, or the premise of a chondritic starting composition fails. This is the 'missing Nb' (or Nb/Ta) paradox. Named candidate reservoirs include: (1) rutile-bearing refractory eclogites — high-pressure metamorphic basalts whose accessory oxide rutile (TiO2_2) strongly concentrates Nb and Ta and fractionates them (rutile–melt partitioning gives melts high Nb/Ta but low absolute Nb; Schmidt et al. 2004), proposed by Rudnick et al. (2000) and Kamber & Collerson (2000) as deeply subducted slabs storing the missing Nb; (2) the metallic core, into which Nb becomes siderophile (metal-preferring) relative to Ta under sufficiently reducing conditions during core formation (Wade & Wood 2001; Münker et al. 2003). Aulbach, O'Reilly, Griffin & Pearson (2008) tested the eclogite hypothesis on rutiles from kimberlite-borne eclogite xenoliths (fragments of mantle rock carried to the surface by volcanic eruptions) using hafnium isotopes (176^{176}Hf/177^{177}Hf, a radiogenic tracer of ancient chemical separation of Hf from Lu). They found that rutiles with high, presumably primary 176^{176}Hf/177^{177}Hf have subchondritic Nb/Ta, while those with suprachondritic Nb/Ta carry low 176^{176}Hf/177^{177}Hf fingerprinting interaction with ancient (≥2.9 Gyr) metasomatized subcontinental lithospheric mantle (SCLM: the stiff mantle keel beneath continents, chemically modified by percolating fluids/melts — 'metasomatism'). They concluded that eclogites are unlikely to be the missing reservoir and that their high Nb/Ta signatures are secondary. Whether metasomatized SCLM itself, or any other candidate, has the necessary mass fraction and Nb–Ta concentrations to close the global budget was left open.

Problem Statement

Close Earth's niobium budget. Determine what balances the subchondritic Nb/Ta (~14–15) of the bulk silicate Earth against the chondritic reference (~19.9 ± 0.6): either (a) identify the reservoir or combination of reservoirs — its location, mass fraction, Nb and Ta concentrations, and Nb/Ta — whose addition to the crust plus depleted mantle restores a chondritic BSE Nb/Ta within measurement uncertainties, together with the process that fractionated Nb from Ta and sequestered it there; or (b) demonstrate quantitatively that no hidden reservoir residual to the crust plus depleted mantle is required because the premise fails — e.g., because Earth accreted from material whose Nb/Ta was already non-chondritic (such as silicate portions of reduced asteroids whose Nb was sequestered in asteroidal cores), or because the chondritic reference value adopted for the comparison is incorrect. Candidate reservoirs, any one or a combination of which may provide the missing complement, include the metallic core, deep magma-ocean residual cumulates/melts, subducted oceanic crust and sediments, metasomatized or enriched subcontinental lithospheric mantle, and mica- or amphibole-bearing restites (solid melting residues) of the deep crust.

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