Experimental investigations of the behaviour of metamict and crystalline zircons during hydrothermal treatment in 2M Na2CO3 solution at T=400-800oC, P=1 and 5 kbar were performed. The obtained data show the existance of three main processes in the treated zircons:
1) The zircon matrix recrystallization is accompanied by reducing number of imperfections, homogenization of main elements and impurity contents and loss of impurities whih become slightly soluble.
2) Initially, Na saturation of zircon matrix causes isomorphous substitutions and then formation and growth of Na2ZrSi2O7 crystals (above 12 wt.% of Na); this process is more intensive in metamict zircon at 600-700oC, 1 kbar and in crystalline zircon at 700oC, 1 kbar. It was noted that Na appeared only in the parts of zircon grains with defect structure. The part of zircon grains with thermally stabile structure contain less than 0.5 wt.% of Na.
3) The phase transformation of zircon into baddeleyite and siliceous remainder begins in metamict zircon at 400oC, 1 kbar, in crystalline zircon at 600oC, 1 kbar. The intensity of this reaction depends on the initial material crystallinity degree. It dominates at 700-800oC, 5 kbar.
This experimental study shows appreciable U and Pb losses for metamict zircon. It could take place at lower P-T than greenschist grade facies metamorphism. The uranium and lead loss from crystalline zircon took place under conditions corresponding with granulite facies of metamorphism.