Fractionation of Nb and Ta from Zr and Hf at mantle depths: the role of titanian pargasite and kaersutite

Selective enrichment or depletion in either Zr and Hf (HFSE4+) or Nb and Ta (HFSE5+) is a feature commonly observed in many mantle-derived melts and amphiboles occurring as either disseminated minerals in mantle xenoliths and peridotite massifs or in vein assemblages cutting these rocks. The fractionation of Nb from Zr seen in natural mantle amphiboles suggests that their incorporation is governed by different crystal-chemical mechanisms. An extensive set of new partitioning experiments between pargasite/kaersutite and melt under upper-mantle conditions shows that HFSE incorporation and fractionation depends on amphibole major-element composition and the presence or absence of dehydrogenation. Multiple regression analysis shows that Amph/LDNb/Zr is strongly dependent on the mg# of the amphibole due to a combination of amphibole- and melt structure effects, so that the following generalisations apply: i) high-mg# amphiboles crystallised from unmodified mantle melts more easily incorporate Zr relative to Nb leading to an increase of the Nb/Zr ratio in the residual melt; ii) low-mg# amphiboles, such as those found in veins cutting peridotites, may strongly deplete the residual melt in Nb and cause very low Nb/Zr in residual melts. Implications and applications to mantle environments are discussed.