**1. Introduction**

Intracontinental monogenetic alkaline basaltic volcanic fields consist of various types of small-scale volcanoes (e.g. scoria cones, tuff rings, maars, diatremes and related lava flows) [1]. However, the size of the volcanic edifices is usually smaller than 1 km<sup>3</sup> and they are short-lived volcanoes, the total magma output rate of a monogenetic volcanic field can be compared with a polygenetic volcano [1]. The lifespan of the volcanic field can last for several millions of years (e.g. [2, 3]). Based on the geochemical and petrographic characteristics of the individual volcanoes, monogenetic basaltic volcanic fields could be very diverse both spatially and temporally (e.g. [4]). This diversity depends on several factors. Basaltic melt could be generated in the asthenosphere or in the lithosphere. Partial melting of the asthenosphere could be caused by a mantle plume (e.g. [5–7]), or mantle flows related to active rifting, collision or lithospheric delamination (e.g. [8–10]). Melt generation in the lithospheric mantle is often related to metasomatized peridotites

and pyroxenites which are rich in volatiles and alkalis (e.g. [3, 11–13]). Primitive alkaline basalts are excellent tools to study the petrological and geochemical heterogeneity and thermal characteristics of the upper mantle and the type of melt generation (e.g. [5, 14–17]), however a seemingly primitive basaltic magma could have undergone fractionation [18]. Only with fairly accurate knowledge on the primary melt composition it is possible to reveal the whole story and characterize the source region of the basalts (e.g. [14, 19, 20]).

(Fo and Ni-rich) population of olivine crystals will cluster dominantly around the upper part of the predicted exponential curve on the Fo-Ni plot and with decreasing Ni and Fo content at some point – when the clinopyroxene also begins to crystallize the curve will change to diagonal [27]. Accordingly if other significant process that

*Modeling of Olivine and Clinopyroxene Fractionation in Intracontinental Alkaline Basalts…*

*Ni* higher than 1. This value is the highest for magnetite (up to 30) and olivine (up to 25), and almost a magnitude lower for clinopyroxene (1–3) and amphibole (0.6–3) [28–31]. To avoid any processes than olivine and clinopyroxene fractionation during magma ascent the following filters have been used during sample selection. 1: only fresh, unaltered olivine and olivine-clinopyroxene phyric basalts without amphibole content were investigated. 2: samples contain only minor magnetite content (<1%) with very low Ni concentration (200–700 ppm). 3: there is no sign of olivine or clinopyroxene incorporation (mega- or antecrysts with signifi-

1.Creating a corrected major elements composition for each rock sample from

assuming a Fe3+/Fe2+ ratio of 0.15; recalculating Ni and Cr concentrations from ppm into NiO and Cr2O3 wt. % values. The Fe3+/Fe2+ will be reflected in the *Mg*# of the rock (*Mg*#*rock* <sup>¼</sup> *Mg<sup>=</sup> Mg* <sup>þ</sup> *Fe*<sup>2</sup><sup>þ</sup> � 100, where Mg and Fe2+ are cation fractions. A different Fe3+/Fe2+ ratio (and Mg#) of the rock would modify later calculations (e.g. the amount of olivine that is calculated in the 2nd step) as they will lead to another equilibrium olivine composition. Therefore highly oxidized samples cannot be used as they have significantly higher Fe3+ content and lower Mg#rock. If the exact ratio of Fe3+/Fe2+ is known, it must be used instead of 0.15.

2.Calculating the Fo content of a hypothetical olivine (Fo\*) being in equilibrium with the bulk rock composition, using the equation of Fo (mol%) = 100-

= 0.3. If the difference between the calculated equilibrium Fo value (= Fo\*) and the Fo content of the most primitive (richest in Ni and probably in Fo) olivine (*FoolNi*\_ *max* ) exceeds 0.1 mol%: the bulk rock composition has to be modified by olivine addition or subtraction in portions of 0.1 wt. % to reach

a. If Fo\* < *FoolNi*\_ *max* : the composition of an olivine crystal (being in equilibrium with the instantaneous melt) has to be added to the bulk rock composition in steps of 0.1 wt. %. For this, real analyzed olivine

b. If Fo\* <sup>&</sup>gt; *FoolNi*\_ *max* : the composition of the most primitive olivine (*olNi*\_ *max* ) has to be subtracted from the bulk rock composition in steps of 0.1 wt. %.

compositions from the same rock can be used.

*ol*�*liq*

tot content into FeO and Fe2O3

*Fe*�*Mg* ]-[*Mg*#*rock*])/ *Mg*#*rock* and *KD*

*ol*�*liq Fe*�*Mg*

can modify the Fo-Ni pattern of olivines than crystallization of olivine and clinopyroxene can be ruled out, the degree of fractional crystallization of the two mineral phases can be estimated based on the composition of the olivine crystals and the bulk rock. The Fo-Ni pattern of fresh, unaltered olivine crystals depends on the Mg# and/or the Ni-content of the basaltic melt. Significant change in Mg# of alkaline basaltic melts – besides olivine � clinopyroxene fractionation – could be caused by fractionation of amphibole or by incorporation of Mg/Fe rich minerals (olivine, clinopyroxene, amphibole and magnetite). The Ni-content of olivines during fractional crystallization could be changed by precipitation of minerals with

cantly different petrographic or geochemical characteristics.

LOI-free raw data: redistributing the Fe2O3

(X/(1 + X)\*100), where X = ([100\**KD*

equilibrium as follows (**Figure 1A**).

**2.1 The steps of the modeling**

*DOI: http://dx.doi.org/10.5772/intechopen.94808*

*D min* �*liq*

**23**

To find primary melt compositions we have to study their ascent history and characterize the melt evolution from the source to the surface. Most common methods are based on the main and trace elements composition of the bulk rock (e.g. [14, 18–21]), and these methods are often less accurate if clinopyroxene fractionation occurred or if the source rock contained pyroxenite. To avoid the "pyroxene problem" a new olivine +/� clinopyroxene fractionation method has been developed by [22] and first published by [23] in Hungarian. Here we will show the method accuracy and some fractionation calculation results of alkaline basalts from [22, 23].
