Trusov S.V., Pletchov P.Yu. and Kotelnikov A.R.

The aim of this work was a melt inclusion synthesis during the process of new phase growth on natural crystals of similar composition. Inclusions were trapped with subhedral growing of crystal single blocks or within-growing crystal rims. The synthesis of melt inclusions was carried out with the alkaline feldspar growth in the subliquidus area of the granite system under gradual temperature decrease. Mixture composition corresponded to feldspar liquid-solid equilibrium under a temperature of 780^oC (Q - 20%, Ab 56%, Or - 24%).

Grains of natural Or(fraction 0,5-1 mm) were used as nuclear centers for crystal growth. Mixture with grains of natural Or was heated to 810^oC (approximately by 50^oC higher than the liquidus temperature) and exposed under these conditions for 4 hours (up to complete melting and homogenization of mix). Then the temperature was reduced to subliquidus during next 6 days with decreasing the rate to 10^o per day.

There are particularly melting and strongly rezorbed grains of Or, which are overgrown by newly-formed alkaline feldspar (fig.1). New formations of feldspar are presented by block crystal with radial direction to large Or crystals. Via step-by-step temperature decreasing, crystals have got clearly expressed composition zones. Later (outer) zones are enriched in Na and depleted in K as compared with earlier (inner) zones of newly-formed alkaline feldspar.

Fig. 2 shows the results of chemical analyses of experimental products. Compositions of artificial melt inclusions are situated on the cotectic line or above it (in the Q field). Trapping of inclusions took place in the feldspar crystallization field. The composition of captured melt is on the line, which connected feldspar composition and initial mixture composition. Such effect is observed at feldspar crystallization on the inclusion walls. That is why some compositions of glass inclusions, which were captured in the feldspar field, are on the cotectic line.

The other inclusions compositions are located in the crystallization field of Q. Such effect can be described at further feldspar growth, after the system comes to the cotectic. Q can not crystallize because one it has not nucleation centers.

Thus, we showed the principal possibility of the melt inclusions synthesis in the process of new formation growth on nuclear centers. This method is easier than crack-forming synthesis methods [2]. It allows one to observe the evolution of melt composition by the study of melt inclusion compositions in the different newly- formed zones.

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