Garnet as U-Pb geochronometer

Sergeyeva N. A., Rizvanova N. G., Bogomolov E. S., Lobikov A. F., Levsky L. K.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2016. V. 145. N 5. P. 96-110

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Language: Russian


Currently garnet is used for dating by Sm-Nd and Lu-Hf isotope methods; its use as the U-Pb geochronometer should allow more detailed reconstruction of the changing P—T metamorphic conditions in time. The main problem there is a low concentration of uranium and, subsequently, of radiogenic lead, together with presence of microinclusions of other minerals with a high content of these elements. Such inclusions are most often represented by monazite, zircon, allanite. The paper proposes a new scheme of stepwise leaching of garnet. A criterion allowing to judge about belonging of lead le ached either from microinclusions or the host mineral is the thorium-uranium ratio of each one leach solution. Leaching of two garnets shows the results of applying this technique. The first three stages of Aqua Regia leaching contain common lead and radiogenic lead enriched with the isotope 208Pb. Further leaching with a mixture of hydrofluoric and nitric acids at room temperature resulted in a fraction, depleted the Pb isotope. Thus obtained two-point isochrons record the time closure of the U-Pb garnet system. The result of garnet leaching by sulfuric acid to remove phosphates microinclusions, for the purpose of garnet dating by Pb-Pb method, coincides with the result obtained by application of the new leaching scheme. The result of Sm-Nd garnet dating is in agreement with the result of its Pb-Pb dating.

Key words: garnet, monazite, zircon, radiogenic isotope, leaching, isochron, isotope system, Pb-Pb, Sm-Nd, Th/U.