Significance of silicate melt pockets in upper mantle xenoliths from the Bakony-Balaton Highland Volcanic Field, Western Hungary

Silicate melt pockets with or without carbonate occur in 10% of upper mantle xenoliths from the alkali basalts of the Bakony-Balaton Highland Volcanic Field (BBHVF). Western Hungary. Based on the estimated bulk composition of the melt pockets, both the carbonate-free and the carbonate-bearing ones are considered to be the result of the reaction between primary mantle clinopyroxene and/or amphibole and external CaO, Al2O3, alkali-rich and MgO-poor fluids/melts, as metasomatic agents, migrating in the upper mantle. The metasomatic melt that produced the carbonate-bearing melt pockets was extremely rich in volatiles, whereas metasomatic melt that contributed to the formation of the carbonate-free melt pockets was particularly rich in silica and relatively poor in volatiles. These metasomatizing melts could have originated from the melting of the previously metasomatized upper mantle due to Middle Miocene mantle diapirism. (C) 2002 Elsevier Science B.V. All rights reserved.