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
ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Volume 99 Budapest, 2007 pp. 5-22. Composition of olivines in the young alkali basalts and their peridotite xenoliths from the Pannonian Basin A. EMBEY-ISZTIN 1 & G. DOBOSI 2 1 Department of Mineralogy and Petrology, Hungarian Natural History Museum H-1083, Budapest, Ludovika tér 2-6, Hungary. E-mail: embey@ludens.elte.hu Institute for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaörsi út 45. Hungary. E-mail: dobosi@geochem.hu Abstract - A statistical evaluation of a large number of microprobe analyses of olivine phenocrysts and olivines from mantle-derived peridotite xenoliths originating from the young alkali basaltic lavas of the Pannonian Basin is presented. It has been argued that the composition of olivines in peridotites attests to a melt depletion process of varying degrees in the mantle lithosphère. The variability of Ca abundance in olivines indicates that probably the whole mantle lithosphère section has been sampled by the xenoliths from about a depth of 25-30 to 70 km. The range of core composition of olivine phenocrysts in different lava rocks is considerable. Three cases can be distinguished: (1) phenocryst cores are in equilibrium with the host basalt, (2) they are more magnesian than the equilibrium value, (3) less magnesian olivine cores than the calculated equilibrium value. Within (l) we can separate a subgroup in which the equilibrium conditions are close to that of mantle-derived primitive basalts, whereas in other basalts the phenocrysts re-equilibrated with more evolved melts. With 13 figures. Keywords - Olivine composition, alkali basalt, peridotite xenolith, Pannonian Basin. INTRODUCTION Magnesian olivine (Mg, Fe)2 SÍO4, is an important rock forming mineral phase in most mafic and ultramafic rocks. At moderate and low pressures it forms a liquidus phase in basic magmas and therefore it is present as large phenocrysts in most terrestrial, lunar and martian basalts. Olivine was also found in many meteorites including the so-called HED group that is widely believed to come from the asteroid 4 Vesta (e.g. McSWEEN 1989). However, olivine is a dominant mineral phase not only in basalts that cover the surface of
