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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2017: 2.31

SCImago Journal Rank (SJR) 2017: 1.440
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1945-3027
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Volume 88, Issue 10

Issues

Experimental hydrothermal alteration of partially metamict zircon

Thorsten Geisler
  • Corresponding author
  • Mineralogisch-Petrographisches Institut, Universität Hamburg, Grindelallee 48, 20146 Hamburg, Germany
  • School of Applied Geology, Curtin University of Technology, PO Box U 1987, Perth 6845, WA, Australia 2School of Applied Geology, Curtin University of Technology, PO Box U 1987, Perth 6845, WA, Australia
  • Institut für Mineralogie, Universität Münster, Corrensstr. 24, D-48149 Münster, Germany
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/ Robert T. Pidgeon
  • School of Applied Geology, Curtin University of Technology, PO Box U 1987, Perth 6845, WA, Australia
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/ Reinhardt Kurtz
  • Mineralogisch-Petrographisches Institut, Universität Hamburg, Grindelallee 48, 20146 Hamburg, Germany
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/ Wilhelm Van Bronswijk
  • School of Applied Chemistry, Curtin University of Technology, PO Box U 1987, Perth 6845, WA, Australia
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/ Helmut Schleicher
  • Mineralogisch-Petrographisches Institut, Universität Hamburg, Grindelallee 48, 20146 Hamburg, Germany
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Published Online: 2015-03-31 | DOI: https://doi.org/10.2138/am-2003-1013

Abstract

We present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from Sri Lanka in the temperature range 350 to 650 °C and with different solutions (2 M AlCl3, 2 M CaCl2, pure H2O, and a multi-cation solution). Under these conditions, sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems. The reaction zones are characterized by a marked increase in the cathodoluminescence intensity, a decrease of the back-scattered electron emission, and an increased degree of structural order, as revealed by micro-Raman and infrared spectroscopy. Sensitive high-resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations (e.g., Ca2+, Ba2+, Mg2+, Al3+) from the solution and lost variable amounts of Zr, Si, Hf, the REE, U, Th as well as radiogenic Pb. A comparison between “dry” and “hydrothermal” annealing trends shows that the kinetics of structural recovery, including recrystallization of the amorphous phase in metamict zircon, is strongly enhanced under hydrothermal conditions. This finding suggests that water “catalyzes” structural recovery processes in metamict zircon. We found that the structure of the reacted areas does not resemble that of crystalline zircon, i.e., is still characterized by a temperature-dependent degree of disorder, which would not be expected if the reaction is controlled by a coupled dissolution and re-precipitation process. Instead, the alteration process can be described best by a diffusion-reaction-recrystallization model. In this model, it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts. We found that the rate and the extent of solid-state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements. This interpretation is indicated by the observation that trace elements, including U and Th, were preferentially lost during the reaction with a fluid at low temperatures, where recrystallization of the amorphous material was slow or not activated at all. The observed chemical alteration patterns are believed to reflect a competition between the kinetics of long-range diffusion and ion exchange and the kinetics of the short-range diffusion necessary for the recrystallization process.

About the article

Received: 2002-09-26

Accepted: 2003-05-09

Published Online: 2015-03-31

Published in Print: 2003-10-01


Citation Information: American Mineralogist, Volume 88, Issue 10, Pages 1496–1513, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2003-1013.

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© 2015 by Walter de Gruyter Berlin/Boston.

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