Fractionation of Nb and Ta from Zr and Hf at mantle depths

The role of Titanian pargasite and kaersutite

M. Tiepolo*, P. Bottazzi, S. F. Foley, R. Oberti, R. Vannucci, A. Zanetti

*Corresponding author for this work

Research output: Contribution to journalArticle

122 Citations (Scopus)

Abstract

Selective enrichment or depletion in either Zr and Hf (HFSE4+) or Nb and Ta (HFSE-5+) 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 mechanism. 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.

Original languageEnglish
Pages (from-to)221-232
Number of pages12
JournalJournal of Petrology
Volume42
Issue number1
Publication statusPublished - 2001
Externally publishedYes

Keywords

  • Amphibole
  • High field strength elements
  • Partition coefficients
  • Trace elements
  • Upper mantle

Fingerprint Dive into the research topics of 'Fractionation of Nb and Ta from Zr and Hf at mantle depths: The role of Titanian pargasite and kaersutite'. Together they form a unique fingerprint.

Cite this