The continental crust has a lower Nb/Ta ratio (11–13) than the basalts from which it is inferred to have differentiated (15–16) and than chondritic meteorites (~19.9), so the crust must be balanced by a complementary high-Nb/Ta reservoir. Tang et al. (2019) proposed that this reservoir is rutile-bearing garnet pyroxenite cumulates ('arclogites') formed during deep differentiation of continental-arc magmas beneath crust thicker than ~50 km and subsequently foundered into the mantle, with pressure-dependent rutile saturation linking crustal thickness to Nb/Ta fractionation. The proposal rests on quantities that remain unverified: the partitioning of Nb versus Ta between rutile and hydrous felsic melt at the 800–1000 °C conditions of arc-cumulate formation, the pressure of rutile saturation as a function of melt composition and water content, the mass and timing of foundering required by the continental Nb–Ta budget, and the fate of rutile and its high-Nb/Ta signature during mantle recycling. Subsequent research shows that the experimental partitioning gap was filled by Chen et al. (2021), whose results challenged rather than confirmed the model — equilibrium rutile crystallization does not lower melt Nb/Ta, kinetic diffusive fractionation during rutile growth may, and amphibole fractionation may explain the same global systematics — leaving the mechanism, the quantitative arclogite budget, and the recycling signature contested and open.

Background

Niobium (Nb) and tantalum (Ta) are refractory high-field-strength elements (HFSEs — ions of high charge and small radius) whose near-identical geochemical behavior makes the Nb/Ta ratio a robust tracer of processes that fractionate them, chiefly the crystallization of Ti-bearing accessory minerals. Because both are refractory, the bulk Earth is expected to have the chondritic ratio (~19.9) measured in meteorites, and basaltic melts derived from the mantle have Nb/Ta of 15–16. The andesitic continental crust, however, has Nb/Ta of only 11–13, a mismatch recognized since the 1980s–2000s: if the crust was made from basaltic parents by differentiation, some complementary reservoir with high Nb/Ta must exist (Rudnick et al. 2000; Münker et al. 2003). Whether the bulk silicate Earth (BSE, the silicate portion of the planet excluding the core) is itself subchondritic in Nb/Ta — the 'missing niobium' question, attributed by some to Nb sequestration into the core during accretion (Wade & Wood 2001) — is a related but separate long-standing problem.

Tang, Lee, Chen, Erdman, Costin & Jiang (2019, Nature Communications 10:235) proposed a specific answer for the crustal imbalance. From a global compilation of arc magmas they showed that Nb/Ta decreases with differentiation (increasing SiO2) only where arc crust is thicker than ~50 km, and that deep-seated garnet-rich pyroxenite cumulates from continental-arc roots ('arclogites', sampled as xenoliths beneath, for example, the Sierra Nevada and central Arizona) carry high Nb/Ta (average ~19) because they contain magmatic rutile (TiO2, the dominant host mineral of Nb and Ta in mafic-to-felsic systems) with high Nb/Ta. The link to crustal thickness comes from rutile saturation thermodynamics: in the Ryerson–Watson solubility model, rutile saturates in hydrous differentiated magmas only at high pressure, controlled by a melt-composition parameter FMFM and water content, so deep-crustal (garnet-stability-field) differentiation drives Nb and Ta into rutile-bearing cumulates while evolved melts — and ultimately the continental crust — evolve to low Nb/Ta. Because arclogites are denser than mantle peridotite, they eventually founder (delaminate) into the mantle; Nb–Ta mass balance then requires the foundered mass to be ~0.3–2.5 times the present continental crust (a minimum bound).

The proposal depends on quantities that were not pinned down in 2019. (1) The partition coefficients DNbD_{\mathrm{Nb}} and DTaD_{\mathrm{Ta}} (mineral/melt concentration ratios) and their ratio DNb/DTaD_{\mathrm{Nb}}/D_{\mathrm{Ta}} govern whether crystallizing rutile raises or lowers the Nb/Ta of the residual melt. Most pre-2019 experiments (e.g., Schmidt et al. 2004; Xiong et al. 2011) were run at temperatures above 1000 °C and returned DNb/DTa<1D_{\mathrm{Nb}}/D_{\mathrm{Ta}}<1 — under which rutile crystallization would raise, not lower, melt Nb/Ta — while Xiong et al. (2011) found the ratio rises with melt evolution and eventually exceeds 1 below ~1000 °C (at water contents <10 wt.%, per the summary in Tang et al. 2019); natural arclogite rutiles formed at roughly 800–1000 °C, so the applicable values at those conditions were unknown. (2) The calculated pressure of rutile saturation — the quantitative link between crustal thickness and Nb/Ta fractionation — depends sensitively on FMFM and water content: switching FMFM from 3 to 4 moves the saturation pressure at 900–950 °C from ~1–1.5 GPa to above 3 GPa, and real arc-magma water contents are variable. (3) The foundered mass estimate is a minimum bound, sensitive to assumed arclogite and crust compositions, and the timing of foundering is unconstrained. (4) The fate of foundered rutile-bearing arclogites in the mantle is unknown: rutile may be exhausted during low-degree melting, so recycled arclogites may or may not impart the high-Nb/Ta, high-Dy/Yb signatures (Dy and Yb are heavy rare-earth elements whose ratio diagnoses residual garnet) observed in some small-volume intracontinental basalts, for which carbonatite metasomatism (chemical modification of the lithospheric mantle by carbonate-rich melts) is an alternative source of high Nb/Ta (Pfänder et al. 2012). Tang et al. also note that mineral-scale Nb and Ta measurements by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) carry oxide-versus-glass matrix-effect uncertainties. Reviews such as Ballouard et al. (2020) situate these Nb–Ta fractionation questions within the broader evolution of felsic magmas and rare-metal mineralization.

Problem Statement

Determine mechanistically and quantitatively how the continental crust acquired its low Nb/Ta (11–13) relative to its basaltic parents (15–16) and chondrites (~19.9), by resolving, as one coupled problem:

(1) The partitioning and saturation behavior of rutile at the conditions under which arc cumulates actually form — approximately 800–1000 °C, hydrous (a few to ~20 wt.% H2O), 1–3 GPa, andesitic-to-rhyolitic melt compositions: specifically DNb/DTaD_{\mathrm{Nb}}/D_{\mathrm{Ta}} and its dependence on temperature, water content, and melt composition, including whether natural rutile Nb/Ta reflects equilibrium partitioning or kinetic diffusive fractionation during crystal growth, and the pressure at which rutile saturates as a function of these variables, so that the crustal-thickness threshold for Nb/Ta fractionation is pinned rather than adjustable through FMFM and H2O choices.

(2) Whether the complementary high-Nb/Ta reservoir required by the continental Nb–Ta mass balance is foundered rutile-bearing arclogite, amphibole-rich cumulates, or another reservoir — and, if arclogite, with what total mass and timing of foundering, given that the foundered material is by construction unsampled and current estimates are minimum bounds (0.3–2.5 times the continental crust).

(3) The fate of the foundered material in the mantle: whether recycled rutile-bearing arclogites survive storage and melt to produce the high-Nb/Ta (and high-Dy/Yb) signatures observed in some intracontinental basalts, or whether rutile is exhausted during low-degree melting and those signatures instead reflect carbonatite metasomatism or other sources.

A resolution of this problem must be simultaneously consistent with the experimental partitioning data, the global arc-magma compilations (Nb/Ta–SiO2 by crustal thickness; Nb/Ta–Dy/Yb in arcs, intracontinental and oceanic basalts), and the natural cumulate/xenolith record, and must determine which observations discriminate among the candidate mechanisms. The separate question of why the bulk silicate Earth itself is subchondritic in Nb/Ta is not part of this problem.

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