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 (HFSE) INTRODUCTION or Nb and Ta (HFSE) is a feature commonly observed in many Because of their similar crystal-chemical properties (i.e. mantle-derived melts and amphiboles occurring as either disseminated high charge and ionic radius), high field strength elements minerals in mantle xenoliths and peridotite massifs or in vein (HFSE: Ti, Zr, Hf, Nb and Ta) are considered to behave assemblages cutting these rocks. The fractionation of Nb from Zr as a coherent group of substituents in rock-forming silicate seen in natural mantle amphiboles suggests that their incorporation minerals, and thus are not expected to fractionate greatly is governed by different crystal-chemical mechanisms. An extensive during partial melting and crystallization processes (Sun set of new partitioning experiments between pargasite–kaersutite and & McDonough, 1989). Nevertheless, fractionation of melt under upper-mantle conditions shows that HFSE incorporation HFSE from HFSE is a common feature in many and fractionation depends on amphibole major-element composition primitive mantle melts such as island-arc volcanics and and the presence or absence of dehydrogenation. Multiple regression intra-plate alkaline melts, suggesting either the presence analysis shows that DNb/Zr is strongly dependent on the mgof a solid phase with DNb,Ta significantly different from number of the amphibole as a result of a combination of amphibole DZr,Hf, or of two distinct phases controlling these element and melt structure effects, so that the following generalizations pairs. Experimental investigations of HFSE decoupling apply: (1) high-mg-number amphiboles crystallized from unmodified have been so far focused on Ti minerals (rutile and mantle melts more easily incorporate Zr relative to Nb leading to ilmenite), because of their high compatibility for HFSE an increase of the Nb/Zr ratio in the residual melt; (2) low-mgand their consequent capability of altering HFSE signumber amphiboles, such as those found in veins cutting peridotites, natures of equilibrium melts. Rutile has been shown to may strongly deplete the residual melt in Nb and cause very low have partition coefficients for Ta and Nb exceeding that Nb/Zr in residual melts. Implications and applications to mantle for Zr by one to two orders of magnitude, with Zr a environments are discussed. further one to two orders higher than other trace elements ( Jenner et al., 1993; Foley et al., 2000). Rutile could thus cause significant decoupling between Nb and Zr, and has the potential to produce the pattern typical of islandarc basalts (Foley & Wheller, 1990; Foley et al., 2000), in