MELILITE-MELT EQULIBRIA AND THE ROLE OF M ELILITE IN THE ULTRABASIC MAGMATIC EVOLUTION

The influence of various factors determining the melilite solid soluti on series composition and limiting its crystallisation from the magmat ic melt was experimentally studied on the material of the melilite-mel t equilibrium pairs. The input data include 38 original and 32 publish ed corresponding analyses. It was shown that the melilites could be cr ystallised within very broad compositional range. The content of all e lements comprising melilite series was found not to be critical. Norma tive nepheline in the melt is considered as an essential condition of the melilite crystallisation. The composition of the solid solution is determined primarily by the composition of the coexisting melt. Press ure, temperature and oxygen fugacity are considered as markedly substa ntial factors. It was found that the remaining melt should be always e nriched in Na, Al and Fe whereas Ca is removed. Mg-distribution coeffi cient was estimated as above unit at MgO below 12% in the melt whereas in magnesium enriched melts (greater-than-or-equal-to 12% MgO) this c oefficient is below unit. The Mg-ratio of melilite is always higher in relation to that of the coexisting melt. The agpaitic coefficient of melilite is lower than in corresponding melt if the former is below 0. 65 in the melt and vice versa. It was shown that the melilite composit ion and its variation range could be applied as an indicator of ultrab asic magmatic evolution.