High
Field Strength Element ratios, in particular Nb/Ta and Zr/Hf (~17,5 and 36,
respectively; Sun & McDonough 1989), in the unfractionated Earth are
assumed to follow those of the chondrites. However, the Nb/Ta ratio of the
Earth’s crust and mantle show subchondritic values (~ 12-15) which have led to
so-called “Nb/Ta paradox” and searching for a suprachondritic Nb/Ta reservoir and
“missing Nb”. Recent studies have shown that Nb/Ta and Zr/Hf can fractionate
during magmatic processes, despite their similar geochemical characteristics. However,
the fractionation usually lowers the ratios and suprachondritic values are rare
being restricted to rutile-bearing eclogites (Rudnick et al. 2000) and subcontinental
lithospheric mantle (Aulbach et al. 2008).

            We present here Nb, Ta, Zr and Hf
concentrations of 1.86-1.85 Ga monzogabbros from southwestern Finland. The
analysed rocks show OIB-like enriched geochemical features and elevated Nb/Ta
(~ 16-38) and Zr/Hf (~ 41-48) ratios with high initial ε_Hf values. This
is in contrast to the older 1.90-1.88 Ga magmatism within the same area (Kara
et al. 2018). Our results suggest a residence of a high Nb/Ta and Zr/Hf
reservoir under the central Fennoscandian shield during the Paleoproterozoic
but the nature of the reservoir, however, remains debatable.