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Journal of Petrology Volume 42 Number 1 Pages 221-232 2001
© Oxford University Press 2001
Fractionation of Nb and Ta from Zr and Hf at Mantle Depths: the Role of Titanian Pargasite and Kaersutite
1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PAVIA, VIA FERRATA 1, I-27100 PAVIA, ITALY
2 CNR-CENTRO DI STUDIO PER LA CRISTALLOCHIMICA E LA CRISTALLOGRAFIA (CSCC), VIA FERRATA 1, I-27100 PAVIA, ITALY
3MINERALOGISH-PETROLOGISCHES INSTITUT, UNIVERSITÄT GÖTTINGEN, GOLDSCHMIDTSTRASSE 1, 37077 GÖTTINGEN, GERMANY
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-number of the amphibole as a result of a combination of amphibole and melt structure effects, so that the following generalizations apply: (1) high-mg-number amphiboles crystallized from unmodified mantle melts more easily incorporate Zr relative to Nb leading 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.
KEY WORDS: trace elements; high field strength elements; partition coefficients; amphibole; upper mantle
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