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

During the studies of both the alkali basalts and their xenoliths, the authors completed a large number of electron microprobe analyses of the rock forming minerals including olivines, from which only a smaller representative fraction could be published. The aim of this study was a statistical evaluation of our data set in order to formulate some general statements on the origin and subsequent evolution of the alkali basaltic liquids and the mantle xenoliths therein. ANALYTICAL METHODS The analyses of olivines from basalt and peridotite samples were performed with different types of microprobe (JEOL Superprobe 733, Institute for Geochemical Research, Budapest and Birbeck College, University of London; ARL-SMQ, Petrological Institute, University Vienna; Cambridge Scientific Instruments Microscan 5, Institute of Geology, University of Edinburgh; Cameca SX50, Department of Petrology, University of Heidelberg). The analytical conditions may have varied slightly, but generally the instruments operated under conventional conditions (15 kV, 10—20 nA). The ARL-SMQ and Cameca SX50 microprobes are equipped with wavelength dispersive spectrometers, whereas the JEOL Superprobe 733 in Budapest makes use of both wavelength and energy dispersive spectrometers, but in London only energy dispersive X-ray spectrometers (EDX) were applied. The wavelength dispersive spectrometers are much more sensitive to low elemental concentrations than are EDX detectors usually associated with SEM. Concentrations in the range of 500-1000 ppm can generally be measured, and for some elements within some types of materials, the detection limit can be near 20 ppm. In any case, we estimate that uncertainties for major (>5%) and minor (<5%) oxides analysed by microprobe are better than 2 and 5% of quoted values. RESULTS Olivine phenocrysts in basalts Olivine is the dominant phenocryst phase in the young alkali basaltic lava rocks of the Pannonian Basin. Except for the Nógrád region, where clinopyroxene phenocrysts are also abundant, olivine is frequently the only phenocryst phase and clinopyroxene is restricted to the ground mass. The shape of the olivine phenocrysts is highly variable, ranging from idiomorph (Fig. 2) to xenomorph (Fig. 3) through subidiomorph (Fig. 4). Sometimes they form glomerophiric aggregates (Fig. 5). Early formed large phenocrysts can be resorbed and disintegrate if they are no more in equilibrium with the surrounding liquid phase as illustrated in Fig. 6.