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

peridotite (Table 4). Generally, Ca and the forsterite content of olivines in blebs and melt pockets are higher than in th Type I host lherzolite, however in samples Szg-3007 and Szt1069 the small secondary olivines are lower in Mg and higher in Fe compared to the primary phase in the same sample. The latter samples are, however, composite xenoliths composed of Type I as well Type II materials. Type II mantle rocks are considerably iron-rich and less magnesian than Type I peridotites (e.g. MENZIES, 1983, EMBEY-ISZTIN et al. 1990). It may be that the lower forsterite contents in the last two samples are due to the disturbing effect of the Type II component in these samples. Similarly, FRANCIS (1976) and MAALLOE & PRINTZLAU (1979) found that in normal Type I Iherzolites from Nunivak Island, Alaska and Dreiser Weiher, Germany the forsterite content of secondary olivines as well as the Ca content were consistently higher than in the olivines which compose the lherzolite. High levels of CaO in olivine are inidicative of high temperature (ADAMS & BISHOP 1982, KÖHLER & BREY 1990) that must have prevailed in the xenoliths incorporated by the host magma. There may be significant differences in the composition of individual clinopyroxenes in fine-grained zones of the same xenolith (Szt1006, Szt1003). In general, Ca content of clinopyroxenes in blebs and melt pockets is consistently higher than that of the primary phase composing the matrix of the host lherzolite. In contrast, the magnesia content of clinopyroxene in the blebs is consistently smaller than that of the lherzolite. However, clinopyroxenes in blebs of the two composite xenoliths are the richest in iron (FeO 4.00-5.19%) and therefore they show the lowest Mg-values which is a typical Type II feature. The lower crustal granulite xenolith (Bo-3007) has quench olivine with Fo 63.9-65.4 and a very high content of CaO (0.59%) indicative of high temperature that probably prevailed during transport and eruption. GENESIS OF GLASSES IN WPB MANTLE XENOLITHS Mantle xenoliths containing various proportions of glass genetically unrelated to their hosts were reported from a number of occurrences e.g. Hawaii (WHITE 1966), Victoria, Australia (FREY & GREEN 1974, IRVING 1974, MCRAE 1979), Canary Isles (FORBES & STARMER 1974), Alaska (FRANCIS 1976), Eifel, Germany (MAALOE & PRINTZLAU 1979), Olmani, Tanzania (JONES etal. 1 983A), Tahaa, Society archipelago (SCHIANO et al. 1992) and Mongolia (IONOV et al. 1994). These glasses are generally highly aluminous and rich in alkalis. They range from quartz to nepheline normative types and some of them have normative corundum due to the Al-rich nature. The glasses commonly contain quench crystals,