**2. Concept of olivine and clinopyroxene fractionation modeling**

Primitve magma compositions derived from less fractionated alkaline basalts are key tools to characterize the source region of upper mantle melts (i.e. petrology and geochemistry, depth and degree of melting, potential temperature of the upper mantle) (e.g. [7, 14, 17, 19, 24, 25]).

Most well-known primitive melt calculations are based on olivine addition or subtraction from bulk rock compositions of alkaline basalts from OIBs and LIPs [14, 19, 20, 24]. These methods could be misleading if the source rock of the basalt wasn't purely peridotitic and especially if not only olivine but clinopyroxene fractionation occurred [14]. Olivine addition to rocks that were generated from eclogitic/pyroxenitic mantle or went through clinopyroxene fractionation can produce unrealistically primitive melt compositions with too high MgO content, Mg# and mantle potential temperature [14].

Even primitive, high Mg# intracontinental alkaline basaltic rocks could be derivative melts after olivine and deep-seated clinopyroxene fractionation [18]. To develop a more realistic fractionation modeling method instead of pure olivine addition/subtraction the degree of clinopyroxene fractionation should be taken into account. During fractional crystallization both melt and newly-formed crystals' composition will change continuously. If this process can be modeled, an estimate for the degree of crystallization and for the main elements composition of the parental melt can be given.

To find the degree of olivine � clinopyroxene fractionation quantitative estimations were performed.

With increasing degree of fractional crystallization of an alkaline basaltic melt the Fo and Ni content of olivine crystals will decrease. As it is a minor/trace element, the concentration of Ni in olivine depends on the *Dol*�*liq Ni* . In accordance with results of [26] it was suggested by [27] that on the Fo-Ni plot of olivines from an alkaline basalt the olivine-only fractionation would appear as a curve of exponentially decreasing Ni-content with decreasing Fo-content, while cocrystallization of olivine and clinopyroxene draws a linear (diagonal or nearly horizontal) trend on the same plot. The slope of the exponential curve is determined by the *Dol*�*liq Ni* , while the slope of the linear section depends on the *<sup>D</sup>*ð Þ� *ol*þ*cpx liq Ni* . It has been shown on olivine and clinopyroxene phyric rock samples that the primitive

*Modeling of Olivine and Clinopyroxene Fractionation in Intracontinental Alkaline Basalts… DOI: http://dx.doi.org/10.5772/intechopen.94808*

(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 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 *D min* �*liq 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 significantly different petrographic or geochemical characteristics.
