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High field strength element systematics and Lu-Hf & Sm-Nd garnet geochronology of orogenic eclogites
(2008)
Concerning the Bulk Silicate Earth (BSE), the depleted mantle and the continental crust are thought to balance the budget of refractory and lithophile elements, resulting in complementary trace element patterns. However, the two high field strength elements (HFSE) Niob and Tantal appear to contradict this mass balance. All reservoirs of the silicate Earth exhibit subchondritic Nb/Ta ratios, possibly as a result of Nb depletion. The two HFSE Zr and Hf on the other hand seem not to be fractionated between the silicate reservoirs. They show more or less chondritic Zr/Hf ratios. In this study a series of orogenic eclogites from different localities was analyzed to determine their HFSE concentrations and to contribute to the question if eclogites could form a hidden reservoir to account for the mass imbalance of the BSE. The results show that the orogenic eclogites have subchondritic Nb/Ta ratios and near chondritic Zr/Hf ratios. The investigated eclogites show no fractionation of Nb/Ta ratios and no enrichment of Nb compared to e.g. MOR-basalts, the likely precursor of these rocks. With an average Nb/Ta ratio of 14.9 these eclogites could not balance the differences between BSE and chondrite. Additionally, with an average Nb/Ta ≈ MORB they also cannot balance the small differences in the Nb/Ta of the crust and the mantle. LA-ICPMS analyses of rutiles in these eclogites reveal a zonation of Nb/Ta ratios in this mineral, with rutile cores having higher Nb/Ta than rutile rims. As a consequence, Laser Ablation data of rutiles have to be evaluated carefully and cannot necessarily reflect a bulk rock Nb and Ta composition, although over 90% of these elements reside in rutile.