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

Olivines in mantle-derived peridotite xenoliths The composition of olivines from peridotite xenoliths exhibits a highly restricted variability (Table 2). The Fo-molecule content is invariably high and range from 88 to 92 with a pronounced frequency peak at F090-91 (Fig. 11). Only one xenolith (G1005) from the locality Gérce has a Fo-content as high as 93. Olivine is the dominant phase in mantle peridotites; therefore its Fo-content can be safely used to characterise the whole rock composition. The variability of Fo-content in mantle peridotites is governed by a partial fusion and melt extraction process (e.g. KUNO & AOKI 1970). Primitive, fertile mantle has higher Fe/Mg and hence lower Fo, whereas barren, depleted mantle, from which basaltic melt was extracted, has low Fe/Mg and high Fo. In this context the depleted mantle is the solid restite, that has been left after the formation of a basaltic magma. Thus the Fo, alias Mg-value functions as an indicator for the degree of depletion, as not only Fe, but also other "basaltophile" elements, such as AI, Ca and Na exhibit negative correlation with it (not shown). The frequency distribution of Mg-values of Pannonian Basin peridotite xenoliths suggests that most of the samples are moderately depleted in basaltophile elements (Fig. 11). Contrary to expectations, there is only a fairly weak positive correlation between NiO- and Fo-content in the Pannonian Basin mantle xenoliths (Fig. 12). The CaO-content of olivines varies between 0.04 and 0.18 wt%. The abundance of CaO in olivines from mantle peridotites is a function of the temperature of equilibration (KÖHLER & BREY 1990). Fig. 13 demonstrates this relationship for the Pannonian Basin mantle-derived peridotite xenoliths, showing that the temperature of equilibration of the peridotite samples encompasses a fairly high range from about 800 to 1200 °C. DISCUSSION In the time of N. L. BOWEN, the key problem of petrology was how basalts differentiate, nowadays the question is much more, how they form. Our subject is related to both these topics. Mantle xenoliths are samples originating from the mantle lithosphère, i.e. the part of the mantle that coupled with the overlaying crust, forms the rigid plates of our planet. The composition of olivines in the mantle xenoliths of the Pannonian Basin clearly demonstrates that most of the mantle samples underwent at least a single or multiple partial