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

GREEN 1974, FRANCIS 1976). FREY & GREEN ( 1974) also reported glasses associated with phlogopite mica with Na>K despite low Na and high K in the mica. Finally, the high contents of K, Na and P in the glasses would require simultaneous melting of dispersed aggregates of mica, amphibole and apatite, which are extremely rare in peridotites (IONOV etal. 1994). Though melting of amphibole as in our xenolith Szt-1

106 or in those of Nunivak Island (FRANCIS 1976) is unanimously demonstrated, the concept of its decompressional melting can hardly be of universal significance. This is also supported by the fact that most hydrous peridotites described in the literature show no trace of melting of mica, amphibole or apatite, even in small xenoliths. The composition of the glasses is to some extent similar to that of synthetic glasses produced in natural peridotites. This indicates that diopside and spinel must be major components in the melting process. However, there are major deviations between the two types of glasses, especially in the content of K and Ti that are much higher in natural glasses. This fact renders the simple melting model based on the combined effect of heating by the host magma and pressure release during eruption of the xenoliths highly improbable. Partial melting during ascent is also unlikely because of the fact that melting phenomena in peridotite xenoliths are rarely reported. If the partial melting occurred during ascent, all the xenoliths should have been partially molten, as they are all heated approximately to the same temperature and their sizes are also similar (MAALLOE & PRINTZLAU 1979). If we rule out the possibility of near surface decompression melting, then we have to assume that the process had taken place in the mantle itself. High, oceanic-type geotherm observed in the Pannonian Basin (DÖVÉNYI & HORVÁTH 1988) is not inconsistent with partial melting within the spinel peridotite stability field especially if the effect of Cr is taken into account that enlarges the stability field of spinel towards higher depth. Partial melting may have been facilitated by the influx of volatiles such as water and CCX BALI ( 1999) who also found carbonates in association with melt pockets in WPB mantle xenoliths, suggested that these carbonates were formed by carbonatic metasomatism in the mantle. Mantle peridotites contain restricted amounts of alkalis, however, volatiles can carry and add many elements and they are most probably responsible for the enrichment of the glasses in Si, Al and alkali elements. Indeed, the highly variable enrichment of glasses in this elements is best explained by erratic contributions from fluids infiltrating the upper mantle. On the other hand, the SiO^, Ti0

2

, ALO<, Na?0 and K^O content of the glasses cannot be derived from the existing minerals, which are characterised by low concentrations of these oxides. Therefore, it seems that the metasomatically induced in situ mantle melting model is the best explanation for