**5. Conclusions**

**Figure 7.**

**Figure 8.**

**34**

*A) the majority of the olivine crystals fall into the range of Fo80–87, the cores of the normal zoned crystals are Mg rich (Fo75–90). Their Ni-concentration correlates positively with the Fo content, Mg-rich crystal cores could reach a Ni content of 2500 ppm, while in crystal rims only 100–700 ppm of Ni can be analyzed. B) the clinopyroxene phenocrysts of the SAN trachybasalt and SOR basanite are dominantly ferroan-diopsides, rarely augites. They are dominantly sector zoned with a primitive core and a more evolved rim, MgO-content varies between 9.2 and 15.9 wt. %. CaO varies between 21.1–23.2 wt. %, higher values can be measured in crystal rims. Especially in the SOR basanite clinopyroxenes are extremely rich in Ca, several crystals exceed Wo50.*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

*Results of olivine and clinopyroxene fractionation. The composition of olivine crystals suggest only olivine fractionation in the case of the BRE basanite (A) and the RAC2 trachybasalt (B), while a significant degree of clinopyroxene fractionation played role in the formation of the SAN trachybasalt (C) and SOR basanite (D).*

*increasing degree of olivine fractionation along the curves. Blue, underlined numbers show the degree of pure olivine fractionation, blue and green italic numbers indicate total degree of olivine and clinopyroxene fractionation (wt. %), respectively. The most primitive olivine crystal in the RAC trachybasalt (B) is in equilibrium with the bulk rock composition and it is the starting point of the olivine fractionation curve. In the other samples the starting point is a hypothetical olivine which is in equilibrium with the bulk rock composition.*

*Ni values, black numbers indicate*

*Calculated olivine fractionation curves are based on the indicated Dolliq*

The results of fractionation modeling are consistent with the petrographic observations, i.e. for the olivine phyric basalts (BRE, RAC2) only olivine fractionation have been calculated, while for those which have a considerable amount of phenocrystic clinopyroxene a significant clinopyroxene fractionation have been revealed by the model. The olivine/pyroxene ratio in the rock samples is 4/1 for SAN and 5/5 for SOR, this is fairly the same as the results of modeling (5.5/16.5 for SAN and 7.4/7.6 for SOR).

The calculated parental melts composition may resemble a mafic melt from a fertile peridotite. Parental melts composition and Mg# have been calculated with the Primelt2 software [14] too. For the olivine phyric rocks it gave a similar Mg# (BRE – 75.13, RAC2–73.27).

In the case of the SAN trachybasalt the Mg# calculated by the Primelt2 software is obviously too high (Mg# = 76.31), which is consistent with the high degree of clinopyroxene fractionation that was indicated by the fractionation modeling. This coincides with the fact that the Primelt2 program does not count on the possibility of clinopyroxene fractionation during primary melt calculations except that it gives a warning about the possible pyroxene fractionation.

The SOR basanite is little bit more interesting as the fractionation modeling have resulted the same amount of olivine and clinopyroxene fractionation, and the Mg# of the parental melts based on the modeling is somewhat higher compared to the result given by the Primelt2 [14]. In this case two important facts have to be considered. First, the modeled mineral fractionation fits very well to the petrographic observations in all case. It suggests that the pyroxene fractionation predicted by the modeling should also be realistic in the case of SOR basanite. Secondly, the Primelt2 program gives us warnings, if the calculation may be inaccurate because of pyroxenite source rocks or because of clinopyroxene fractionation. In the case of SOR basanite a pyroxene fractionation warning was given by the software, although the suggested olivine addition to reach primary melt (6.6 wt. %) is almost the same as it was given by fractionation modeling (7.39 wt. %).

The uncertainty, whether the result of the fractionation modeling or the Primelt2 is the more realistic may have arisen from two factors. Neither olivine, nor clinopyroxene is a dominant phase and probably this amount (<10 wt. %) of pyroxene is close to the limit what can be detected by the difference in primary melts' Mg#.

Despite its limitations, the presented olivine and clinopyroxene fractionation modeling based on olivine and bulk rock compositional data draw attention that it is not only olivine fractionated basalts that could be useful for primary melt calculations.

It is more important, that all possible tools have to be applied from petrographic observations to quite simple or more complicated geochemical modeling methods. We hope that the modeling presented in this chapter will inspire the reader to develop further ideas and methods to give a more realistic and better description of the fractionation process of alkaline basaltic melts.
