Solubility of manganotantalite, zircon and hafnon in highly fluxed peralkaline to peraluminous pegmatitic melts

authored by: Marieke Van Lichtervelde, Francois Holtz, John M. Hanchar
Abstract: The behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of Ta and Zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous, Li-, F-, P- and B-bearing pegmatitic melts. The pegmatitic melts are synthetic and enriched in flux elements: 0.7-1.3 wt% Li₂O, 2-5.5 wt% F, 2.8-4 wt% P₂O₅ and 0-2.8 wt% B₂O₃, and their aluminum saturation index ranges from peralkaline to peraluminous (ASI_Li = Al/[Na + K + Li] = 0.8 to 1.3) with various K/Na ratios. Dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1,150°C, at 200 MPa and nearly water-saturated conditions. For dissolution experiments, pure synthetic, end member manganotantalite and zircon were used in order to avoid problems with slow solid-state kinetics, but additional experiments using natural manganotantalite and zircon of relatively pure composition (i. e., close to end member composition) displayed similar solubility results. Zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions, and are more sensitive to changes in temperature or ASI of the melt than to flux content. A model relating the enthalpy of dissolution of manganotantalite to the ASI_Li of the melt is proposed: {increment}H_diss (kJ/mol) = 304 × ASI_Li - 176 in the peralkaline field, and {increment}H_diss (kJ/mol) = -111 × ASI_Li + 245 in the peraluminous field. The solubility data reveal a small but detectable competitivity between Zr and Ta in the melt, i. e., lower amounts of Zr are incorporated in a Ta-bearing melt compared to a Ta-free melt under the same conditions. A similar behavior is observed for Hf and Ta. The competitivity between Zr (or Hf) and Ta increases from peraluminous to peralkaline compositions, and suggests that Ta is preferentially bonded to non-bridging oxygens (NBOs) with Al as first-neighbors, whereas Zr is preferentially bonded to NBOs formed by excess alkalies. As a consequence Zr/Ta ratios, when buffered by zircon and manganotantalite simultaneously, are higher in peralkaline melts than in peraluminous melts.
Organisation(s): Institute of Mineralogy
External Organisation(s): Memorial University of Newfoundland
Type: Article
Journal: Contributions to Mineralogy and Petrology
Volume: 160
Pages: 17-32
No. of pages: 16
ISSN: 0010-7999
Publication date: 12.11.2010
Publication status: Published
Peer reviewed: Yes
ASJC Scopus subject areas: Geophysics, Geochemistry and Petrology
Electronic version(s): https://doi.org/10.1007/s00410-009-0462-x (Access: Unknown)

BibTeX

@article{1c0de60450054b6a97040897de24a7a3,
title = "Solubility of manganotantalite, zircon and hafnon in highly fluxed peralkaline to peraluminous pegmatitic melts",
abstract = "The behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of Ta and Zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous, Li-, F-, P- and B-bearing pegmatitic melts. The pegmatitic melts are synthetic and enriched in flux elements: 0.7-1.3 wt% Li2O, 2-5.5 wt% F, 2.8-4 wt% P2O5 and 0-2.8 wt% B2O3, and their aluminum saturation index ranges from peralkaline to peraluminous (ASILi = Al/[Na + K + Li] = 0.8 to 1.3) with various K/Na ratios. Dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1,150°C, at 200 MPa and nearly water-saturated conditions. For dissolution experiments, pure synthetic, end member manganotantalite and zircon were used in order to avoid problems with slow solid-state kinetics, but additional experiments using natural manganotantalite and zircon of relatively pure composition (i. e., close to end member composition) displayed similar solubility results. Zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions, and are more sensitive to changes in temperature or ASI of the melt than to flux content. A model relating the enthalpy of dissolution of manganotantalite to the ASILi of the melt is proposed: {increment}Hdiss (kJ/mol) = 304 × ASILi - 176 in the peralkaline field, and {increment}Hdiss (kJ/mol) = -111 × ASILi + 245 in the peraluminous field. The solubility data reveal a small but detectable competitivity between Zr and Ta in the melt, i. e., lower amounts of Zr are incorporated in a Ta-bearing melt compared to a Ta-free melt under the same conditions. A similar behavior is observed for Hf and Ta. The competitivity between Zr (or Hf) and Ta increases from peraluminous to peralkaline compositions, and suggests that Ta is preferentially bonded to non-bridging oxygens (NBOs) with Al as first-neighbors, whereas Zr is preferentially bonded to NBOs formed by excess alkalies. As a consequence Zr/Ta ratios, when buffered by zircon and manganotantalite simultaneously, are higher in peralkaline melts than in peraluminous melts.",
keywords = "Competition, Dissolution and crystallization experiments, Manganotantalite, Pegmatite, Solubility, Zircon",
author = "{Van Lichtervelde}, Marieke and Francois Holtz and Hanchar, {John M.}",
note = "Funding Information: Acknowledgments This work was supported by the German Science Foundation (DFG project Ho 1337/20). The technical assistance of Renat Almeev, Alexander Bartels, Harald Behrens, Roman Botcharnikov, Sarah Cichy, Otto Diedrich, Wanja Dziony, J{\"u}rgen Koepke and Jan Stelling, as well as constructive reviews by David Dolejs and Fabrice Gaillard, were gratefully appreciated. Copyright: Copyright 2010 Elsevier B.V., All rights reserved.",
year = "2010",
month = nov,
day = "12",
doi = "10.1007/s00410-009-0462-x",
language = "English",
volume = "160",
pages = "17--32",
journal = "Contributions to Mineralogy and Petrology",
issn = "0010-7999",
publisher = "Springer Verlag",
number = "1",
}