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
As it can be seen on the back scattered electron images of olivine phenocrysts (Figs 2-6), the interior of these crystals is compositionally homogeneous and rich in magnesium (dark area). Iron enrichment (light coloured area) can only be observed towards the rims of the olivines. This corresponds to the general rule, according to which the firstly precipitated mafic minerals are rich in Mg and poor in Fe. The rate of growth of the Fe/Mg ratio can be evaluated in Fig. 8. Representative olivine core compositions are shown in Table 1. In addition to the major oxides, NiO is present (up to 0.44 wt%), CaO (up to 0.71 wt%) and MnO (0.08—1.39 wt%). As expected, NiO correlates positively and MnO negatively with the Mg/Fe ratio (not shown). Somewhat surprisingly, CaO also shows negative correlation with Mg/Fe. The composition of phenocryst cores in different basalts vary between F073 and F092. (Fo is calculated as the molar 100xMg/(Mg+Fe), whereby all Fe is assumed to be in divalent state). The rate of compositional heterogeneity between cores and rims is highly variable and it depends strongly on the size of the crystals. For example, in an olivine phenocryst from Banská Stiavnica (Selmecbánya) it was as high as Fo86-Fo67- The other exreme case was the basalt of Steinberg (Styria), where virtually no compositional change between cores and rims was observed. The composition of ground mass olivines is similar to that of the phenocryst rims. In Fig. 10 we plotted the frequency distribution of forsterite molecule content of phenocryst cores in different basalts of the study region. The diagram shows a pronounced maximum in the category Fog5_88> many phenocrysts exhibit lower, but some have higher Fo than the peak value. Since olivine is an early crystallising phase, in first approximation we can assume that the interior of the mineral grains were in equilibrium with the melt, and the composition of this melt is now represented by the whole rock composition, provided that the system remained closed all along its evolution. The connection between the Fe/Mg ratio of olivine and that of the melt is governed by the following equation: Fe 2+ M? Kd°J~ l l =—-4-—^=0.3 (ROEDER & EMSLIE 1970) Fe-Mg r, 2+ V ' Fe ! lq M So! The distribution coefficient (KTJ) is independent on the temperature and melt composition (ROEDER & EMSLIE 1970, ULMER 1989). Knowing the value of Ko we can calculate the composition of equilibrium olivine for each rock (melt) composition.
