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 a mphiboles occurring as either disseminated minerals in mantle xenoliths and peridotite massifs or in vein assemblages cutting these rocks. The fractio nation of Nb from Zr seen in natural mantle amphiboles suggests that their incorporation is governed by different crystal-chemical mechanisms. An exte nsive set of new partitioning experiments between pargasite-kaersutite and melt under upper-mantle conditions shows that HFSE incorporation and fracti onation depends on amphibole major-element composition and the presence or absence of dehydrogenation. Multiple regression analysis shows that D-Amph/ I(Nb/Zr) is strongly dependent on the mg-number of the amphibole as a resul t of a combination of amphibole and melt structure effects, so that the fol lowing generalizations apply: (1) high-mg-number amphiboles crystallized fr om unmodified mantle melts more easily incorporate Zr relative to Nb leadin g to an increase of the Nb/Zr ratio in the residual melt; (2) low-mg-number 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.