Subcontinental lithospheric mantle origin of high niobium/tantalum ratios in eclogites

摘要

Ratios of non-volatile elements such as zirconium, niobium (Nb) and tantalum (Ta) in the unfractionated Earth are commonly assumed to be similar to those in the class of meteorites known as chondrites(1). However, the Nb/Ta ratio of the Earth's major silicate reservoirs - the crust and mantle - is subchondritic, hinting at the existence of a complementary reservoir with high Nb/Ta (refs 2,3). Eclogites, which are high-pressure metamorphic rocks that often form in subduction zones, can contain the mineral rutile (titanium dioxide) with high Nb/Ta. They have therefore been inferred to constitute this complementary reservoir(2). More recent evidence from natural samples, however, suggests that residual eclogites have low Nb/Ta, which is confirmed by experiments(3-6). Here, we present the Nb and Ta concentrations and hafnium (Hf) isotope compositions of rutiles from eclogite fragments that were entrained from the subcontinental lithospheric mantle in kimberlite melts. Rutiles with high Hf-176/Hf-177 ratios generally have subchondritic Nb/Ta (< 19.9 (refs 3,7)), similar to undisturbed melt residues of broadly basaltic precursors. On the other hand, rutiles with suprachondritic Nb/Ta ratios have low 176Hf/177Hf, suggesting Hf addition from a fluid or melt ultimately derived from ancient (>= 2.9Gyr), chemically modified (metasomatized) subcontinental lithospheric mantle, which has similarly low Hf-176/Hf-177 (ref. 8). Our results suggest that eclogites are unlikely to represent the high Nb/Ta reservoir; instead, the high Nb/Ta signatures of some eclogites are probably generated by metasomatism during long-term residence in the subcontinental lithospheric mantle.