Zircon reaction and stability of the U-Pb isotope system during interaction with carbonate fluid: experimental hydrothermal study |
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Authors: | N G Rizvanova O A Levchenkov A E Belous N I Bezmen A V Maslenikov A N Komarov A F Makeev L K Levskiy |
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Institution: | (1) Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Makarova Emb., 2, St-Petersburg, 199034, Russia, E-mail: lev@ad.iggp.ras.spb.rv, RU;(2) AO “Mekhanobr-Analit”, 8a, 21 liniya, 199026, St-Petersburg, Russia, RU;(3) Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow Distr., 142432, Russia, RU |
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Abstract: | The interpretation of metamorphically induced U-Pb isotopic discordance requires a thorough understanding of zircon-fluid
interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems
by cathodoluminescence after treatment by 2M Na2CO3 solution at T = 200–800 °C and P = 1–5 kbar for 3–14 days, X-ray diffraction, microprobe and isotope dilution analysis. The data indicate that zircon transformation
under hydrothermal conditions depends on the experimental conditions and the degree of structural damage. Reconstitution of
defective and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of
crystallinity) was observed at 400 °C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed
baddeleyite accommodating U from 2M Na2CO3 solution and Zr-Na-silicate were recognized. This process intensified with increasing pressure. Study of crystalline zircon
indicates that migration of U and Pb took place only during dissolution of zircon at T above 650 °C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T than is typical for greenschist facies metamorphism.
Received: 4 October 1997 / Accepted: 6 December 1999 |
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