Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 99. (Budapest 2007)

Embey-Isztin, A. ; Dobosi, G.: Composition of olivines in the young alkali basalts and their peridotite xenoliths from the Pannonian Basin

melting event/s of highly varying degrees. The basaltic liquids generated during these events are not the same as the ones that brought the xenoliths onto the surface. The partial melting events in the mantle lithosphère are very old ( > 1 milliard year; DOWNES et al. 1992), whereas the alkali basalts that originate from the more plastic astenosphere, from below the lithosphère, are young (mostly between three and five million years, EMBEY-ISZTIN et al. 1993). Fig. 12. NiO vs. Mg-value in olivines from mantle-derived peridotite xenoliths The lack of good positive correlation between Ni and Fo in olivines (Fig. 12) from our xenoliths require explanations. Ni is a compatible minor element in the lattice of olivine, and this means that its quantity should continuously increase with increasing degree of the partial melting and consequently with increasing Fo. On the other hand, the abundance of minor elements that are incompatible in the lattice of one of the mineral phases of mantle rocks, decrease with increasing rate of melting. The reason why the Ni-Fo relation is anomalous may reside in the fact that we have analysed an unusually high number of a rare special type of xenolith. In addition to the worldwide known and frequently encountered protogranular, porphyroclastic and equigranular xenoliths, in our region a fairly high number of xenoliths occur with a peculiar poikilitic texture (EMBEY-ISZTIN et al. 1989, DOWNES et al. 1992, EMBEY-ISZTIN et al. 2001). If we take into account the texture types of the xenoliths, then we can say that samples with NiO > 0.5 wt% have poikilitic