Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 92. (Budapest 2000)

Embey-Isztin, A. ; Scharbert, H. G.: Glasses in peridotite xenoliths from the western Pannonian Basin

the formation of glass patches in the xenoliths of the WPB. The viability of this model was established by the study of IONOV etal. (1994) who used trace element as well as Sr, Nd isotope evidence to show that the formation of glass in Mongolian peridotite xenoliths had been initiated by a metasomatic enrichment in incompatible elements due to reactions with a migrating fluid phase in the upper mantle. In contrast to peridotite xenoliths in which the formation of glass - as we have seen- is rather exceptional, melting phenomena and glass are fairly often observed in lower crustal granulite xenoliths. This is understandable, since granulites have mafic compositions that melts at considerably lower temperatures than do ultramafic peridotites. The size of granulite xenoliths is comparable to that of the peridotites and they are heated to the same temperature. All these suggest that in the case of granulite xenoliths, we are really dealing with decompressional melting in rapidly ascending xenoliths. The high abundance of small quench crystals, especially plagioclase in the vitreous matrix of xenolith Bo-3007 and elsewhere also support this view. The lack of enrichment in potash and highly incompatible elements in these particular glasses also indicates that a fluid-driven metasomatic event can not be responsible for the melting in the lower crustal granulite xenoliths. * * * Acknowledgements - The first author kindly acknowledges the financial support by Hungarian National Scientific Research Fund (OTKA grant No. T-025978) as well as that of the OMFB-British Council (project GB-65/98). We are indebted to H. DIETRICH for performing the microprobe analyses. REFERENCES ADAMS, G. E. & BISHOP F. C. (1982): Experimental investigation of Ca-Mg exchange between olivine, orthopyroxene, and clinopyroxene: potential for geobarometry. - Earth Planet. Sei. Letters SI: 241-250. BALI, E. (1999): Geochemical research of melt pockets from upper mantle xenoliths (BakonyBalaton Highlands and Little Hungarian Plain, Western Hungary. - In: Berichte der Deutschen Mineralogischen Gesellschaft. Beihefte zum Eur. J. Mineral. 11: 24. BRUNET, F. & CHAZOT, G. (1999): Significance of phosphorus content of olivine, pyroxene, amphibole and glass in spinel lherzolites from Yemen. - Ofioliti 24: 13-1 A. DOWNES, H., EMBEY-ISZTIN, A. & THIRLWALL, M. F. ( 1992): Petrology and geochemistry of spinel peridotite xenoliths from the western Pannonian Basin (Hungary): evidence for an association between enrichment and texture in the upper mantle. - Contrib. Min. Petr. 109: 340-354.