Nb-Ta fractionation in hydrothermal magnetite: implications for the “missing Nb” paradox

Abstract:

The subchondritic Nb/Ta ratios in continental crust and depleted mantle require complementary reservoirs to balance the deficit Nb observed in most terrestrial silicate reservoirs. In this work, we provide textural and in-situ trace elemental data of magnetite from Baishiya iron deposit, East Kunlun Orogenic Belt, Northern Tibet Plateau to constrain the Nb-Ta fractionation in hydrothermal magnetite, a potential mineral to explain the missing Nb paradox. Four stages of mineralization and alteration have been identified based on field work and petrography study: iron skarnization, hydrothermal alteration, metasomatic carbonatation and quartz-sulfide mineralization. Back scattering images reveal that magnetite in iron skarnization stage develops obvious oscillatory zonation patterns, whereas those formed in the hydrothermal alteration and metasomatic carbonatation stages display metasomatic texture, and those in sulfide mineralization stage develops euhedral zoned texture. Corresponding to the textural signatures, systematic variation of trace elements is also observed in their trace element concentration which is related to the efficient factors during fluid-rock interaction. Therefore, some factors on Nb-Ta fractionation could be excluded through their variation with the Nb/Ta ratios. Above it all, a negative exclusively correlation of Nb/Ta (0.55-27) and the content of V and Ni, are considered to be key elements due to their siderophile and chalcophile affinity respectively. It confirms that increased oxygen and sulfur fugacity are primary factors that controlled Nb/Ta fractionation in hydrothermal magnetite. Similarly, the band iron formations with huge amounts of magnetites in subduction process presumably carried off mass Nb from the mantle due to the dehydration of the oceanic crust and fluid-rock interaction subsequently. We contend that the BIFs in deep mantle may be a potential complementary reservoir that has been overlooked before.