**2.1 The steps of the modeling**

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

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

**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

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,

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

To find the degree of olivine � clinopyroxene fractionation quantitative estima-

With increasing degree of fractional crystallization of an alkaline basaltic melt

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 expo-

*Ni* , while the slope of the linear section depends on the *<sup>D</sup>*ð Þ� *ol*þ*cpx liq*

been shown on olivine and clinopyroxene phyric rock samples that the primitive

*Ni* . In accordance

*Ni* . It has

the Fo and Ni content of olivine crystals will decrease. As it is a minor/trace

crystallization of olivine and clinopyroxene draws a linear (diagonal or nearly horizontal) trend on the same plot. The slope of the exponential curve is determined

element, the concentration of Ni in olivine depends on the *Dol*�*liq*

nentially decreasing Ni-content with decreasing Fo-content, while co-

the source region of the basalts (e.g. [14, 19, 20]).

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

basalts from [22, 23].

mantle) (e.g. [7, 14, 17, 19, 24, 25]).

Mg# and mantle potential temperature [14].

parental melt can be given.

tions were performed.

by the *Dol*�*liq*

	- 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 compositions from the same rock can be used.
	- 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. %.

calculation composition of analyzed olivine crystals (being nearly in

b. Whereas the decrease of Fo content in olivines during fractionation has

assuming Rayleigh fractionation. Ni content of the most Ni-rich,

*Ni* of the investigated basalts. Based on [27] in the case of pure olivine fractionation the most primitive olivine crystals should cluster along the upper, steep part of an exponential curve on the Fo-Ni plot. The most Mg-rich analyzed olivine crystals (over the 75th percentile by Fo-content and or over Fo84) are compared to the calculated Fo-Ni curves (Step 2.) with the least squares method, and the best-fitting one

The hypothetical, pure olivine fractionation trajectory should be compared with the total olivine Fo-Ni dataset of the sample, whether they are fitting well, or at some point the initially exponential trend of analyzed olivine

this case the modeling has been ended here. Jump to Step 7.

b. If the trend of the analyzed olivine crystals can be divided into an

6.Modeling of the olivine+clinopyroxene co-crystallization (**Figure 2B**). The

a. Calculating the composition of the equilibrium melt ('intermediate melt') for the olivine that represents the end of the initial pure olivine fractionation (Step 4a.) like equilibrium melt calculations in Step 1.

b. Modeling the evolution of the "intermediate melt" by fractionation of olivine and clinopyroxene together. The decrease of olivines Ni concentration along the diagonal Fo-Ni trend has to be modeled by Rayleigh fractionation presuming different olivine/clinopyroxene ratios. As a thought experiment, the decrease of Ni concentration in olivines

could be modeled by a given quantity of olivine (X wt. %) or

a. If they are fitting well, only olivine fractionation happened. The olivine crystal with lowest Fo-Ni content will draw the degree of fractionation, which can be easily read from the modeled Fo-Ni curve (**Figure 1D**). In

exponential and a linear section, the first part represents the initial pure olivine fractionation and the linear part the later olivine-clinopyroxene co-crystallization. The degree of pure olivine fractionation is determined by the intersection of the modeled olivine Fo-Ni curve and the linear

*Ni* has been selected in Step 3, while for *<sup>D</sup>cpxliq*

*Ni* a constant

*Ni* of the investigated basalt (**Figure 1C**).

*olliq*

concentration in the fractionated olivine crystals and in the equilibrium

*FeMg* (0.3), the decrease of Ni

*Ni* value between 5 and 20,

equilibrium with the instantaneous melt) should be applied.

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

been calculated with a constant *KD*

4.Determination of the *Dolliq*

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

will be accepted for the *Dolliq*

applied value of *Dolliq*

**25**

value of 2.8 should be used [31].

melt have to be modeled for each *Dolliq*

analyzed olivine crystal is taken as starting data.

5.Estimation of the degree of the pure olivine fractionation.

crystals changes to a more straight, diagonal one.

section of the olivine dataset (**Figure 2**).

### **Figure 1.**

*An example of olivine fractionation modeling (BRE basanite olivine Fo-Ni plots). A) the most primitive olivine phenocryst is not in equilibrium with the bulk rock composition. The difference between the most Ni-rich olivine (large symbol, Fo86.7) and a hypothetical olivine being in equilibrium with the bulk rock (Fo87.6) can be explained by 2.6 wt. % olivine fractionation. B) Calculation of olivine fractionation curves with different values of Dol*�*liq Ni . Starting point for calculation is the most Ni-rich olivine (large symbol, 1807 ppm Ni, Fo86.7), from where the fractionation curve should be modeled by olivine addition (until reaching equilibrium with the bulk rock at Fo87.6 olivine) and by olivine subtraction (until reaching 20 wt. % olivine fractionation, lower end of the curves). The Ni-content of the hypothetical, most primitive olivine (Fo87.6) varies between 2061 and 2828 ppm, depending on the applied Dol*�*liq Ni value. C) Based on the composition of the most primitive olivine phenocrysts (olivine population over the 75th percentile by Fo-content or over Fo84; marked by gray background) the best fitting olivine fractionation curve should be selected to assign the Dol*�*liq Ni of the rock. In this example the olivine fractionation curve related to Dol*�*liq Ni* ¼ 10 *was selected. D) As the total olivine phenocryst population fits well to the selected fractionation curve, only olivine fractionation can be assumed. Green dots and numbers (wt. %) along the curve represent increasing olivine fractionation. The degree of olivine fractionation is 12.5 wt. %, it is marked by the most evolved crystal (Fo81.6, Ni = 707 ppm). For a detailed description of fractionation modeling see the appendix of [23].*

The amount of subtracted or added olivine (in wt. %) could represent the difference between the melt composition represented by the bulk rock and by the equilibrium melt related to the most primitive olivine in the sample. We call this 'initial olivine fractionation'.

3.Calculating hypothetical (pure) olivine fractionation curves for *Dol*�*liq Ni* ¼

<sup>5</sup> � 20 [29] on the Fo-Ni plot. The higher the *<sup>D</sup>ol*�*liq Ni* , the smoother the fractionation curve (**Figure 1B**).

a. Modeling the decrease of MgO and Fo content in the melt and in equilibrium olivines simultaneously due to fractionation. Starting melts composition (Step 1.) has to be in equilibrium with the most Ni-rich olivine. Fractionation can be modeled by subtracting olivine in portions of 0.1 wt. % from the melt until 20 wt. % of total fractionation. During

calculation composition of analyzed olivine crystals (being nearly in equilibrium with the instantaneous melt) should be applied.


Based on [27] in the case of pure olivine fractionation the most primitive olivine crystals should cluster along the upper, steep part of an exponential curve on the Fo-Ni plot. The most Mg-rich analyzed olivine crystals (over the 75th percentile by Fo-content and or over Fo84) are compared to the calculated Fo-Ni curves (Step 2.) with the least squares method, and the best-fitting one will be accepted for the *Dolliq Ni* of the investigated basalt (**Figure 1C**).

5.Estimation of the degree of the pure olivine fractionation.

The hypothetical, pure olivine fractionation trajectory should be compared with the total olivine Fo-Ni dataset of the sample, whether they are fitting well, or at some point the initially exponential trend of analyzed olivine crystals changes to a more straight, diagonal one.

	- a. Calculating the composition of the equilibrium melt ('intermediate melt') for the olivine that represents the end of the initial pure olivine fractionation (Step 4a.) like equilibrium melt calculations in Step 1.
	- b. Modeling the evolution of the "intermediate melt" by fractionation of olivine and clinopyroxene together. The decrease of olivines Ni concentration along the diagonal Fo-Ni trend has to be modeled by Rayleigh fractionation presuming different olivine/clinopyroxene ratios. As a thought experiment, the decrease of Ni concentration in olivines could be modeled by a given quantity of olivine (X wt. %) or

The amount of subtracted or added olivine (in wt. %) could represent the difference between the melt composition represented by the bulk rock and by the equilibrium melt related to the most primitive olivine in the sample. We

*population fits well to the selected fractionation curve, only olivine fractionation can be assumed. Green dots and numbers (wt. %) along the curve represent increasing olivine fractionation. The degree of olivine fractionation is 12.5 wt. %, it is marked by the most evolved crystal (Fo81.6, Ni = 707 ppm). For a detailed*

*An example of olivine fractionation modeling (BRE basanite olivine Fo-Ni plots). A) the most primitive olivine phenocryst is not in equilibrium with the bulk rock composition. The difference between the most Ni-rich olivine (large symbol, Fo86.7) and a hypothetical olivine being in equilibrium with the bulk rock (Fo87.6) can be explained by 2.6 wt. % olivine fractionation. B) Calculation of olivine fractionation curves with different*

*from where the fractionation curve should be modeled by olivine addition (until reaching equilibrium with the bulk rock at Fo87.6 olivine) and by olivine subtraction (until reaching 20 wt. % olivine fractionation, lower end of the curves). The Ni-content of the hypothetical, most primitive olivine (Fo87.6) varies between 2061 and*

*phenocrysts (olivine population over the 75th percentile by Fo-content or over Fo84; marked by gray background) the best fitting olivine fractionation curve should be selected to assign the Dol*�*liq*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

*Ni . Starting point for calculation is the most Ni-rich olivine (large symbol, 1807 ppm Ni, Fo86.7),*

*Ni value. C) Based on the composition of the most primitive olivine*

*Ni* ¼ 10 *was selected. D) As the total olivine phenocryst*

a. Modeling the decrease of MgO and Fo content in the melt and in

equilibrium olivines simultaneously due to fractionation. Starting melts composition (Step 1.) has to be in equilibrium with the most Ni-rich olivine. Fractionation can be modeled by subtracting olivine in portions of 0.1 wt. % from the melt until 20 wt. % of total fractionation. During

*Ni* ¼

*Ni of the rock. In this*

*Ni* , the smoother the

3.Calculating hypothetical (pure) olivine fractionation curves for *Dol*�*liq*

call this 'initial olivine fractionation'.

*description of fractionation modeling see the appendix of [23].*

fractionation curve (**Figure 1B**).

**Figure 1.**

*values of Dol*�*liq*

**24**

*2828 ppm, depending on the applied Dol*�*liq*

*example the olivine fractionation curve related to Dol*�*liq*

<sup>5</sup> � 20 [29] on the Fo-Ni plot. The higher the *<sup>D</sup>ol*�*liq*

7.The total degree of olivine and clinopyroxene fractionation will be the sum of

9.Optional check: it is possible to calculate parental melt composition by another method based only on olivine addition, like the method of PRIMELT2 [14] or Fractionate-PT [19]. If significant amount (>10 wt. %) fractionation of clinopyroxene has been calculated by the presented method, the mentioned programs should calculate a more Mg-rich parental melt (**Figure 3** and

8.To find the major element composition of the parental melt (**Table 3**) the summerized amount of olivine or olivine+clinopyroxene have to be added to

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

**2.2 Remarks on limitations of the presented fractionation estimation method**

86–89 mol% [17]. It is common that even the most primitive olivine

clinopyroxene phenocrysts, no signs of amphibole or early pyroxene

**Sample BRE RAC2 SAN SOR** GPS N 48.403 46.032 45.811 48.226

SiO2 44,71 46,87 49,30 44,86 TiO2 2,46 1,55 2,21 2,29 Al2O3 13,18 15,8 14,80 15,81 Fe2O3 Total 11,6 9,57 10,79 8,86 MnO 0,17 0,16 0,15 0,14 MgO 10,91 9,68 8,50 9,44 CaO 10,08 9,85 8,59 10,49 Na2O 4,04 3,79 3,42 3,35 K2O 1,72 1,63 1,64 2,25 P2O5 0,77 0,39 0,60 0,6 LOI 0 0,2 0,80 1,4 Ni 219 216 170 153 Cr 342 431 247 212

*Composition of the basaltic samples applied for modeling. Oxides and LOI are given in wt. %, Ni and Cr in*

E 18.635 25.418 21.718 19.913

1.Alkaline basalts that are derivative melts of fertile mantle peridotites will have an MgO content of 8–13 wt. % and equilibrium olivines with a Fo content of

phenocrysts of an alkaline basalt seemingly suitable for modeling (olivine +/

crystallization or incorporation of mafic minerals) have quite low Fo and/or Ni-content (e.g. [34]). In this case a proper fractionation modeling is not possible, only a rough estimation can be performed based on a comparison with similar olivine datasets containing more Fo-rich olivines. An exact limit for proper calculations cannot be specified, only suggestions for it. Based on the empirical observations of [23] the fractionation modeling is more reliable if the olivine crystals over the 75th percentile by Fo-content draw a steep, semi

the volumes calculated in "Step 4b" and "Step 5e".

the bulk rock composition (**Tables 1** and **2**).

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

**Table 3**).

**Table 1.**

**27**

*ppm. GPS: Coordinates of sample locations.*

### **Figure 2.**

*An example of olivine and clinopyroxene fractionation modeling (SOR basanite olivine Fo-Ni plots). A) the most primitive olivine phenocryst (Fo87.95) is not in equilibrium with the bulk rock composition. Olivine fractionation curves were modeled as in Figure 1. The most primitive olivine phenocrysts (marked by gray background) suggest a Dol*�*liq Ni value of 13, however the geochemical diversity of the total olivine population cannot be explained by pure olivine fractionation, as olivine crystals (<Fo83.6) do not fit the curve. B) Based on the modeled Fo-Ni curve of Dol*�*liq Ni* ¼ 13 *an initial olivine fractionation of 5.5 wt. % can be assumed. The composition of the more evolved olivine crystals (< Fo83.6) suggest that besides olivine the fractionation of clinopyroxene has also begun [27]. The co-crystallization of the two minerals (i.e. the effect of clinopyroxene crystallization on olivine composition) can be modeled by the olivine population between the least primitive olivine crystal that fits to the calculated fractionation curve (Fo86.3, Ni = 1186 ppm) and the most evolved crystal (Fo83.4, Ni = 801 ppm). The decrease of Ni concentration (400 ppm) and Fo content (2.9 Mol%) in this olivine population have to be explained. If only olivine or clinopyroxene crystallization had occurred, the 400 ppm drop in Ni-concentration could be explained by 3.32 wt. % olivine or by 20.15 wt. % clinopyroxene fractionation (with a Dol*�*liq Ni* <sup>¼</sup> <sup>13</sup> *and Dcpx*�*liq Ni* ¼ 2*:*8 *[31]). If this had happened, at the end of the fractionation, the Fo concentration of olivine crystals would have been Fo84.3 (red dashed line and circle) or Fo80.4 (purple dashed line and circle), respectively. None of them match the composition of the most evolved olivine crystal (Fo83.4), therefore a co-crystallization of olivine and clinopyroxene must be assumed. The fractionation of the basaltic melt being in equilibrium with the Fo86.3 olivine has been modeled with several different olivine/clinopyroxene ratio and amount. Fractionation of 2 wt. % olivine and 8 wt. % clinopyroxene would create a basaltic melt that would be in equilibrium with the most evolved analyzed olivine crystal (Fo83.4). Because fractionation of 5.5 wt. % olivine had already taken place before this, the total degree of fractionation would be 7.39 wt. % olivine and 8.56 wt. % clinopyroxene (Initial olivine fractionation: 5.5 wt. %. Olivine and clinopyroxene co-crystallization:* 0*:*945 � 2 ¼ 1*:*89 wt*:*% *olivine and* 0*:*945 � 8 ¼ 7*:*56 wt*:*% *clinopyroxene. Altogether 7.39 wt. % olivine and 7.56 wt. % clinopyroxene fractionation.). For a detailed description of fractionation modeling see the appendix of [23].*

> clinopyroxene (Y wt. %) fractionation only, however it must be a cocrystallization of the two minerals. Therefore, the decrease of Ni by fractional crystallization has to be calculated for each ol/cpx ratio from X wt. % olivine +0 wt. % clinopyroxene to 0 wt. % olivine + Y wt. % olivine, changing the amount of each mineral by steps of 0.1 wt. %.


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


## **2.2 Remarks on limitations of the presented fractionation estimation method**

1.Alkaline basalts that are derivative melts of fertile mantle peridotites will have an MgO content of 8–13 wt. % and equilibrium olivines with a Fo content of 86–89 mol% [17]. It is common that even the most primitive olivine phenocrysts of an alkaline basalt seemingly suitable for modeling (olivine +/ clinopyroxene phenocrysts, no signs of amphibole or early pyroxene crystallization or incorporation of mafic minerals) have quite low Fo and/or Ni-content (e.g. [34]). In this case a proper fractionation modeling is not possible, only a rough estimation can be performed based on a comparison with similar olivine datasets containing more Fo-rich olivines. An exact limit for proper calculations cannot be specified, only suggestions for it. Based on the empirical observations of [23] the fractionation modeling is more reliable if the olivine crystals over the 75th percentile by Fo-content draw a steep, semi


### **Table 1.**

*Composition of the basaltic samples applied for modeling. Oxides and LOI are given in wt. %, Ni and Cr in ppm. GPS: Coordinates of sample locations.*

clinopyroxene (Y wt. %) fractionation only, however it must be a cocrystallization of the two minerals. Therefore, the decrease of Ni by fractional crystallization has to be calculated for each ol/cpx ratio from X wt. % olivine +0 wt. % clinopyroxene to 0 wt. % olivine + Y wt. % olivine, changing the amount of each mineral by steps of 0.1 wt. %.

*An example of olivine and clinopyroxene fractionation modeling (SOR basanite olivine Fo-Ni plots). A) the most primitive olivine phenocryst (Fo87.95) is not in equilibrium with the bulk rock composition. Olivine fractionation curves were modeled as in Figure 1. The most primitive olivine phenocrysts (marked by gray*

*cannot be explained by pure olivine fractionation, as olivine crystals (<Fo83.6) do not fit the curve. B) Based on*

*composition of the more evolved olivine crystals (< Fo83.6) suggest that besides olivine the fractionation of clinopyroxene has also begun [27]. The co-crystallization of the two minerals (i.e. the effect of clinopyroxene crystallization on olivine composition) can be modeled by the olivine population between the least primitive olivine crystal that fits to the calculated fractionation curve (Fo86.3, Ni = 1186 ppm) and the most evolved crystal (Fo83.4, Ni = 801 ppm). The decrease of Ni concentration (400 ppm) and Fo content (2.9 Mol%) in this olivine population have to be explained. If only olivine or clinopyroxene crystallization had occurred, the 400 ppm drop in Ni-concentration could be explained by 3.32 wt. % olivine or by 20.15 wt. % clinopyroxene*

*fractionation, the Fo concentration of olivine crystals would have been Fo84.3 (red dashed line and circle) or Fo80.4 (purple dashed line and circle), respectively. None of them match the composition of the most evolved olivine crystal (Fo83.4), therefore a co-crystallization of olivine and clinopyroxene must be assumed. The fractionation of the basaltic melt being in equilibrium with the Fo86.3 olivine has been modeled with several different olivine/clinopyroxene ratio and amount. Fractionation of 2 wt. % olivine and 8 wt. % clinopyroxene would create a basaltic melt that would be in equilibrium with the most evolved analyzed olivine crystal (Fo83.4). Because fractionation of 5.5 wt. % olivine had already taken place before this, the total degree of fractionation would be 7.39 wt. % olivine and 8.56 wt. % clinopyroxene (Initial olivine fractionation: 5.5 wt. %. Olivine and clinopyroxene co-crystallization:* 0*:*945 � 2 ¼ 1*:*89 wt*:*% *olivine and* 0*:*945 � 8 ¼ 7*:*56 wt*:*% *clinopyroxene. Altogether 7.39 wt. % olivine and 7.56 wt. % clinopyroxene fractionation.). For a detailed*

*Ni value of 13, however the geochemical diversity of the total olivine population*

*Ni* ¼ 13 *an initial olivine fractionation of 5.5 wt. % can be assumed. The*

*Ni* ¼ 2*:*8 *[31]). If this had happened, at the end of the*

c. Estimating the compositional change of the intermediate melt for all ratios of ol/cpx by subtracting these minerals from it. Subtraction of minerals is the same as in Step 1a. The calculated melt compositions are

d. Estimating the Fo-content of equilibrium olivine for each final melt composition and compare them to the analyzed olivine crystals.

e. That olivine/clinopyroxene fractionation ratio and amount will be

accepted, for which the related final melts equilibrium olivine will have the same Fo-content as the lower end of the diagonal Fo-Ni trend of

considered "final melts".

*description of fractionation modeling see the appendix of [23].*

*Ni* <sup>¼</sup> <sup>13</sup> *and Dcpx*�*liq*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**Figure 2.**

*background) suggest a Dol*�*liq*

*fractionation (with a Dol*�*liq*

*the modeled Fo-Ni curve of Dol*�*liq*

analyzed olivines.


### **Table 2.**

*The amount and average composition of olivine and clinopyroxene added to the bulk rock composition (m/m %) during parental melt calculations. Olivine and pyroxene addition are based on fractionation modeling (Figure 8).*

vertical trend on the Fo-Ni plot and they have 89–82 mol% Fo and 2500–

*Primitive melt compositions calculated by the PRIMELT2 software [14] and by addition of olivine and clinopyroxene to the bulk rock based on fractionation modeling presented in this paper. Regardless of the applied method, the Mg# of the primitive melts will be quite similar in the case of olivine phyric rocks (BRE, RAC2). On the contrary, the Mg# of the primitive melts can depend on the applied method, if there were a significant amount of clinopyroxene in the sample (SAN). This means, that only by olivine addition – as the PRIMELT2 and similar*

**Calculation based on PRIMELT2 software Fractionation modeling presented in this**

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

**Sample BRE RAC2 SAN SOR BRE RAC2 SAN SOR** SiO2 44,68 47,07 47,83 45,75 44,13 46,21 48,06 44,71 TiO2 2,17 1,46 1,78 2,21 2,15 1,39 1,77 2,1 Al2O3 11,57 14,88 11,86 15,22 11,53 14,17 12,63 14,01 Fe2O3 1,08 0,73 0,88 0,69 1,21 1,02 1,09 1,14 FeO 9,47 8,18 9,12 7,60 9,52 8,1 9,14 7,38 MnO 0,17 0,16 0,15 0,14 0,18 0,18 0,16 0,15 MgO 16,17 12,65 16,80 12,20 15,39 13,47 14,18 12,23 CaO 8,87 9,29 6,91 10,11 8,85 8,85 7,69 10,61 Na2O 3,54 3,57 2,73 3,22 3,54 3,4 2,63 2,89 K2O 1,51 1,53 1,31 2,16 1,51 1,46 1,53 1,91 P2O5 0,68 0,37 0,48 0,58 0,67 0,35 0,38 0,51 NiO 0,07 0,05 0,12 0,04 0,05 0,04 0 0,03 Cr2O3 0,05 0,06 0,03 0,03 0,05 0,06 0 0,03 **Mg# 75,27 73,39 76,65 74,00 74,24 74,77 73,44 74,54**

**article**

composition of later crystallized olivines, while the presented method can reveal clinopyroxene crystallization only if it occurred together with olivine fractionation. Therefore, it cannot indicate the precipitation of clinopyroxene onto the wall rocks at great depth. To identify any modification of primary melts by reaction with mantle minerals is also beyond the limits of the

3. Since there are several variable parameters during fractionation modeling (the

*Ni* value, the applied Fe3+/Fe2+ ratio for the bulk rock, the amount and ratio of olivine and clinopyroxene, the composition of the intermediate melt), the

2.The deep-seated clinopyroxene fractionation [25] will not affect the

*softwares [14, 19, 20, 24] work – the Mg# of the primary melt will be over calculated.*

1200 ppm Ni content.

Olivin add. by Primelt2 14,20 7,10 23,40 6,60

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

presented method.

calculation is a little bit cumbersome.

**3. Case studies from the Carpathian-Pannonian region**

From a volcanological and geodynamical point of view – based on its Neogene-Quaternary volcanism – the Carpathian-Pannonian Region (**Figure 4**) is a related to the broader Mediterranean area in terms of geodynamic setting [37].

*Dolliq*

**Table 3.**

**3.1 Geological setting**

**29**

### **Figure 3.**

*Estimation of primary melts' composition - the clinopyroxene problem. Figure modified after [32]. OJP – Ontong-Java plateau basalts, MORB – Siqueiros fracture zone basalts [32], dark blue haxagons represent the compositional diversity of the monogenetic volcano of Kissomlyó (West-Hungary) [33], while the pale blue rectangle the total diversity of a volcanic field (Novohrad-Gemer/Nógrád-Gömör VF, northern Hungary and southern Slovakia) [10]. FeO and MgO are in wt. %, gray area: Primary melts' composition after accumulated fractional melting of depleted mantle peridotite. Thick red line represents the solidus, thin red lines represent the degree of melting (%). Basaltic rocks to the left of the solidus are derivatives of primary melts, while rocks to the right are richer in MgO because of accumulation of MgO-rich minerals (typically olivine). If a basaltic magma was formed only by olivine fractionation/accumulation, the primary melts composition can be estimated from the bulk rock composition using olivine addition or subtraction (gray dotted curves) [24, 32]. The pale brown circle (A) represents a basaltic rock that was formed by olivine fractionation, therefore its primary melt composition (dark brown circle, A') can be calculated by olivine addition. The pale green circle (B0) represents a basaltic rock that was formed by olivine and clinopyroxene fractionation. Its composition can be modified by adding the amount of fractionated clinopyroxene – to reach B1 (the pale green circle with a black rim). Adding olivine to the B1 melt its primary melt composition (B1, dark green circle with black rim) can be calculated. If the clinopyroxene fractionation had been ignored, the primary melts estimation by olivine addition would have been misleading (B0'). The too high MgO- and FeO-content of B0' melt would make all further estimations (e.g. temperature of the primary melt, degree of melting) inaccurate.*


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

### **Table 3.**

**Olivine Clinopyroxene**

**5,41 7,56**

**BRE RAC2 SAN SOR SAN SOR**

**12,4 10,35 16,53 7,39**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

SiO2 40,09 40,57 39,31 39,93 48,70 47,73 TiO2 0,00 0,00 0,05 1,86 2,03 Al2O3 0,01 0,04 0,03 0,16 4,69 7,34 Fe2O3 3,19 3,21 FeO 11,74 11,27 16,73 15,25 4,27 3,65 MnO 0,24 0,19 0,24 0,26 0,13 0,14 MgO 46,76 47,88 43,22 44,31 14,00 13,52 CaO 0,27 0,20 0,24 0,31 22,40 22,07 Na2O 0,40 0,59 NiO 0,24 0,29 0,18 0,14 0,03 Cr2O3 0,04 0,09 0,05 0,01 0,03 0,03

*The amount and average composition of olivine and clinopyroxene added to the bulk rock composition (m/m %) during parental melt calculations. Olivine and pyroxene addition are based on fractionation modeling*

*Estimation of primary melts' composition - the clinopyroxene problem. Figure modified after [32]. OJP – Ontong-Java plateau basalts, MORB – Siqueiros fracture zone basalts [32], dark blue haxagons represent the compositional diversity of the monogenetic volcano of Kissomlyó (West-Hungary) [33], while the pale blue rectangle the total diversity of a volcanic field (Novohrad-Gemer/Nógrád-Gömör VF, northern Hungary and southern Slovakia) [10]. FeO and MgO are in wt. %, gray area: Primary melts' composition after accumulated fractional melting of depleted mantle peridotite. Thick red line represents the solidus, thin red lines represent the degree of melting (%). Basaltic rocks to the left of the solidus are derivatives of primary melts, while rocks to the right are richer in MgO because of accumulation of MgO-rich minerals (typically olivine). If a basaltic magma was formed only by olivine fractionation/accumulation, the primary melts composition can be estimated from the bulk rock composition using olivine addition or subtraction (gray dotted curves) [24, 32]. The pale brown circle (A) represents a basaltic rock that was formed by olivine fractionation, therefore its primary melt composition (dark brown circle, A') can be calculated by olivine addition. The pale green circle (B0) represents a basaltic rock that was formed by olivine and clinopyroxene fractionation. Its composition can be modified by adding the amount of fractionated clinopyroxene – to reach B1 (the pale green circle with a black rim). Adding olivine to the B1 melt its primary melt composition (B1, dark green circle with black rim) can be calculated. If the clinopyroxene fractionation had been ignored, the primary melts estimation by olivine addition would have been misleading (B0'). The too high MgO- and FeO-content of B0' melt would make all further estimations (e.g.*

*temperature of the primary melt, degree of melting) inaccurate.*

**Sample**

**Table 2.**

*(Figure 8).*

**Figure 3.**

**28**

**olivine addition cpx addition**

> *Primitive melt compositions calculated by the PRIMELT2 software [14] and by addition of olivine and clinopyroxene to the bulk rock based on fractionation modeling presented in this paper. Regardless of the applied method, the Mg# of the primitive melts will be quite similar in the case of olivine phyric rocks (BRE, RAC2). On the contrary, the Mg# of the primitive melts can depend on the applied method, if there were a significant amount of clinopyroxene in the sample (SAN). This means, that only by olivine addition – as the PRIMELT2 and similar softwares [14, 19, 20, 24] work – the Mg# of the primary melt will be over calculated.*

vertical trend on the Fo-Ni plot and they have 89–82 mol% Fo and 2500– 1200 ppm Ni content.


### **3. Case studies from the Carpathian-Pannonian region**

### **3.1 Geological setting**

From a volcanological and geodynamical point of view – based on its Neogene-Quaternary volcanism – the Carpathian-Pannonian Region (**Figure 4**) is a related to the broader Mediterranean area in terms of geodynamic setting [37].

or to lithospheric delamination while the sporadic volcanism in the central parts of the basin could have been related to strike-slip tectonics [10, 22, 37]. 2) The other theory comes from [50], who suggested that basaltic melt generation was caused by rather a regional, mostly horizontal mantle flow related to the north-eastward push

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

Whole-rock compositions were analyzed at Acme Labs (Vancouver, Canada). Major and minor elements were analyzed by ICP-ES, trace elements by ICP-MS. Petrographic descriptions were done with a Nikon YS2-T polarizing microscope using NIS-Elements Br software and an AMRAY 1830 I/T6 scanning electron microscope at the Department of Petrology and Geochemistry, Eötvös Loránd University (Budapest, Hungary). The polished and carbon-coated thin sections were analyzed with a CAMECA SX100 EMPA, equipped with one energy-dispersive and four wavelength-dispersive spectrometers at the Department of Lithospheric Research, University of Vienna (Austria). An accelerating potential of 15 kV and a beam current of 20 nA were used with at least 20 s counting time on peak position and 1 μm beam diameter. PAP correction procedure was applied on raw data. Analyses were done against natural and synthetic mineral standards: olivine (Mg), corundum (Al), quartz (Si), apatite (P), wollastonite (Ca), rutile (Ti), Mg-chromite

**BRE**: a massive basanitic lava rock sample (**Figures 5** and **6**) from the Putikov Vŕšok volcano, Štiavnica (Selmec) Volcanic Field. The small scoria cone and related lava flow is the youngest alkaline basaltic volcano in the Carpathian-Pannonian Region with its age of 102 11 ka [51]. Sample data were taken from [22, 23]. **RAC2**: a trachybasaltic vesicular scoria clast (**Figures 5** and **6**) from the Racoş (Rákos) scoria cone, Perşani (Persányi) Mts. Volcanic Field. Its age is 1221 11 ka

**SAN**: a massive trachybasaltic lava rock sample (**Figures 5** and **6**) from the Lucaret-Sanoviţa (Lukácskő-Sziklás) volcano, Banat Volcanic Field. Its age is

**SOR**: a massive basanite (**Figures 5** and **6**) from a dyke cutting the maar diatreme rocks at Šurice (Sőreg), Novohrad-Gemer/Nógrád-Gömör VF. Its age is

BRE: an olivine phyric massive basanite sample with porphyritic-intergranular texture (**Figure 5**). Clinopyroxene phenocrysts are very rare (olivine/clinopyroxene ratio is 19/1, total phenocryst content: 6%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphic-idiomorphic with an average size of 650–700 μm, they contain idiomorphic magnetite and Cr-spinel inclusions dominantly in the crystal rims. Largest grains are often resorbed and iddingsitized along crystal rims and cleavage tracks. Hypidiomorphic-idiomorphic pale brown clinopyroxene microphenocrysts are dominantly normal-, rarely sector zoned crystals with a maximal size of 400–500 μm. They often contain idiomorphic magnetite inclusions.

(Cr), spessartine (Mn), almandine (Fe), Ni-oxide (Ni).

**3.3 The investigated alkaline basalts**

[52, 53]. Sample data were taken from [22, 53].

between 2.5–2.6 Ma [54]. Sample data were taken from [23].

4 0.29 Ma [54]. Sample data were taken from [22].

*3.3.1 Samples*

*3.3.2 Petrography*

**31**

of the Adria microplate.

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

**3.2 Analytical methods**

### **Figure 4.**

*Map of the Carpathian-Pannonian region after [35, 36]. 1: Pre-Cenozoic basement, 2: Neogene–Quaternary sediments, 3: Neogene calc-alkaline volcanic rocks on the surface, 4: Pieniny Klippen Belt, 5: Alp–Carpathian Flysch Belt, 6: Neogene–Quaternary alkaline basaltic volcanic fields on the surface, 7: Alp–Carpathian Molasse Belt. Sample locations. A: Putikov Vŕšok, B: Šurice (Sőreg), C: Lucaret-Sanoviţa (Lukácskő-Sziklás), D: Racoş (Alsórákos). The map insert on the upper right corner shows the position of the Carpathian-Pannonian region within Europe.*

The post-Paleogene volcanic formations of the region can be divided by age and geochemical features into four groups [38]: 1) Miocene Si-rich-; 2) Miocene-Quaternary K- and high-K-; 3) Miocene-Quaternary calc-alkaline- and 4) late Miocene-Quaternary alkaline type. The latter group is represented by several alkaline basaltic monogenetic volcanic fields, some of which are deeply buried by late Miocene sediments [10, 39–41]. The most intense period of alkaline basaltic volcanism took place between 5 and 3 Ma ago. At that time numerous volcanoes formed, for example, in the Bakony-Balaton Highland Volcanic Field (e.g. [42]), in the Nógrád-Gömör/Novohrad-Gemer Volcanic Field (e.g. [40]) and in the Styrian Basin Volcanic Field (e.g. [35]). The alkaline rocks show a wide range in composition from the primitive, olivine-phyric basanites towards the more differentiated, clinopyroxene-rich phonotephrites (e.g. [10, 43, 44]). Based on their trace element content they show similarities both with the Neogene-Quaternary alkaline rocks of western and central Europe (e.g. [45, 46]) and with alkaline basaltic rocks of the Mediterranean area (e.g. [47, 48]). The driving force of the alkaline basaltic melt generation is still subject of debate as the Neogene-Quaternary basaltic volcanism of the region dominantly postdates the rifting of the Pannonian Basin and took place in the thermal inversion phase of the basin, in a compressional geodynamic regime [10, 37, 49]. There are two dominant theories for basalt generation in the area, both associate it with regional mantle flows, the differences are in the driving forces. 1) In the outer parts of the Pannonian Basin melting could have been associated with mantle upwelling related to the so-called thin-spot event, a dominantly vertical asthenospheric flow from under the deep roots of the surrounding mountain chains *Modeling of Olivine and Clinopyroxene Fractionation in Intracontinental Alkaline Basalts… DOI: http://dx.doi.org/10.5772/intechopen.94808*

or to lithospheric delamination while the sporadic volcanism in the central parts of the basin could have been related to strike-slip tectonics [10, 22, 37]. 2) The other theory comes from [50], who suggested that basaltic melt generation was caused by rather a regional, mostly horizontal mantle flow related to the north-eastward push of the Adria microplate.

### **3.2 Analytical methods**

Whole-rock compositions were analyzed at Acme Labs (Vancouver, Canada). Major and minor elements were analyzed by ICP-ES, trace elements by ICP-MS. Petrographic descriptions were done with a Nikon YS2-T polarizing microscope using NIS-Elements Br software and an AMRAY 1830 I/T6 scanning electron microscope at the Department of Petrology and Geochemistry, Eötvös Loránd University (Budapest, Hungary). The polished and carbon-coated thin sections were analyzed with a CAMECA SX100 EMPA, equipped with one energy-dispersive and four wavelength-dispersive spectrometers at the Department of Lithospheric Research, University of Vienna (Austria). An accelerating potential of 15 kV and a beam current of 20 nA were used with at least 20 s counting time on peak position and 1 μm beam diameter. PAP correction procedure was applied on raw data. Analyses were done against natural and synthetic mineral standards: olivine (Mg), corundum (Al), quartz (Si), apatite (P), wollastonite (Ca), rutile (Ti), Mg-chromite (Cr), spessartine (Mn), almandine (Fe), Ni-oxide (Ni).

### **3.3 The investigated alkaline basalts**

### *3.3.1 Samples*

The post-Paleogene volcanic formations of the region can be divided by age and geochemical features into four groups [38]: 1) Miocene Si-rich-; 2) Miocene-Quaternary K- and high-K-; 3) Miocene-Quaternary calc-alkaline- and 4) late Miocene-Quaternary alkaline type. The latter group is represented by several alkaline basaltic monogenetic volcanic fields, some of which are deeply buried by late Miocene sediments [10, 39–41]. The most intense period of alkaline basaltic volcanism took place between 5 and 3 Ma ago. At that time numerous volcanoes formed, for example, in the Bakony-Balaton Highland Volcanic Field (e.g. [42]), in the Nógrád-Gömör/Novohrad-Gemer Volcanic Field (e.g. [40]) and in the Styrian Basin Volcanic Field (e.g. [35]). The alkaline rocks show a wide range in composition from the primitive, olivine-phyric basanites towards the more differentiated, clinopyroxene-rich phonotephrites (e.g. [10, 43, 44]). Based on their trace element content they show similarities both with the Neogene-Quaternary alkaline rocks of western and central Europe (e.g. [45, 46]) and with alkaline basaltic rocks of the Mediterranean area (e.g. [47, 48]). The driving force of the alkaline basaltic melt generation is still subject of debate as the Neogene-Quaternary basaltic volcanism of the region dominantly postdates the rifting of the Pannonian Basin and took place in the thermal inversion phase of the basin, in a compressional geodynamic regime [10, 37, 49]. There are two dominant theories for basalt generation in the area, both associate it with regional mantle flows, the differences are in the driving forces. 1) In the outer parts of the Pannonian Basin melting could have been associated with mantle upwelling related to the so-called thin-spot event, a dominantly vertical asthenospheric flow from under the deep roots of the surrounding mountain chains

*Map of the Carpathian-Pannonian region after [35, 36]. 1: Pre-Cenozoic basement, 2: Neogene–Quaternary sediments, 3: Neogene calc-alkaline volcanic rocks on the surface, 4: Pieniny Klippen Belt, 5: Alp–Carpathian Flysch Belt, 6: Neogene–Quaternary alkaline basaltic volcanic fields on the surface, 7: Alp–Carpathian Molasse Belt. Sample locations. A: Putikov Vŕšok, B: Šurice (Sőreg), C: Lucaret-Sanoviţa (Lukácskő-Sziklás), D: Racoş (Alsórákos). The map insert on the upper right corner shows the position of the Carpathian-Pannonian region*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**Figure 4.**

*within Europe.*

**30**

**BRE**: a massive basanitic lava rock sample (**Figures 5** and **6**) from the Putikov Vŕšok volcano, Štiavnica (Selmec) Volcanic Field. The small scoria cone and related lava flow is the youngest alkaline basaltic volcano in the Carpathian-Pannonian Region with its age of 102 11 ka [51]. Sample data were taken from [22, 23].

**RAC2**: a trachybasaltic vesicular scoria clast (**Figures 5** and **6**) from the Racoş (Rákos) scoria cone, Perşani (Persányi) Mts. Volcanic Field. Its age is 1221 11 ka [52, 53]. Sample data were taken from [22, 53].

**SAN**: a massive trachybasaltic lava rock sample (**Figures 5** and **6**) from the Lucaret-Sanoviţa (Lukácskő-Sziklás) volcano, Banat Volcanic Field. Its age is between 2.5–2.6 Ma [54]. Sample data were taken from [23].

**SOR**: a massive basanite (**Figures 5** and **6**) from a dyke cutting the maar diatreme rocks at Šurice (Sőreg), Novohrad-Gemer/Nógrád-Gömör VF. Its age is 4 0.29 Ma [54]. Sample data were taken from [22].

### *3.3.2 Petrography*

BRE: an olivine phyric massive basanite sample with porphyritic-intergranular texture (**Figure 5**). Clinopyroxene phenocrysts are very rare (olivine/clinopyroxene ratio is 19/1, total phenocryst content: 6%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphic-idiomorphic with an average size of 650–700 μm, they contain idiomorphic magnetite and Cr-spinel inclusions dominantly in the crystal rims. Largest grains are often resorbed and iddingsitized along crystal rims and cleavage tracks. Hypidiomorphic-idiomorphic pale brown clinopyroxene microphenocrysts are dominantly normal-, rarely sector zoned crystals with a maximal size of 400–500 μm. They often contain idiomorphic magnetite inclusions.

in crystal rims. Hypidiomorphic-idiomorphic pale brown clinopyroxene microphenocrysts often form glomerocrysts. They are dominantly sector-, rarely normal zoned crystals with an average size of 300–350 μm. The groundmass contains plagioclase, clinopyroxene, olivine, glass, nepheline and accessory magnetite and

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

SAN: an olivine and clinopyroxene phyric trachybasaltic lava rock sample with porphyritic-intergranular texture (**Figure 5**). Clinopyroxene phenocrysts are quite common (olivine/clinopyroxene ratio is 4/1, total phenocryst content: 20%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphic-idiomorphic, often resorbed; with an average size of 500 μm, largest crystals reach 1200 μm. They are often iddingsitized along crystal rims and cleavage tracks. Idiomorphic inclusions of Cr-spinel are common in crystal rims. Hypidiomorphic-idiomorphic pale brown clinopyroxene phenocrysts are sector zoned crystals with an average size of 300– 400 μm, largest grains reach 1000 μm. They contain magnetite inclusions and they often form glomerocrysts. The groundmass contains plagioclase, clinopyroxene,

SOR: an olivine and clinopyroxene phyric basanitic dyke sample with slightly vesicular, porphyritic-intergranular/intersertal texture (**Figure 5**). Clinopyroxene phenocrysts are common (olivine/clinopyroxene ratio is 5/5, total phenocryst content: 8%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphicidiomorphic, often resorbed; with an average size of 650 μm, largest crystals reach 3000 μm. Idiomorphic inclusions of Cr-spinel and magnetite are common

in crystal rims. Idiomorphic-hypidiomorphic pale brown clinopyroxene phenocrysts are sector-, normal- or oscillatory zoned minerals with an average size of 470 μm, largest grains reach 2000 μm. They contain magnetite inclusions.

Some clinopyroxene phenocrysts have an olive-green core. Few grains of completely opacitized amphibole megacrysts can be observed. The groundmass contains plagioclase, clinopyroxene, olivine, glass, nepheline and accessory

Geochemical data (**Tables 1** and **2**, **Figures 6** and **7**) were taken from

**4. Results of olivine and clinopyroxene fractionation modeling**

From two samples (BRE and RAC2) olivine crystals are fitting well to the modeled pure olivine fractionation curves that were calculated with *Dolliq*

12 and 13 were applied for SAN and SOR, respectively. The calculated degree of olivine fractionation is 9.5 wt. % for SAN and 5.5 wt. % for SOR during the initial, pure olivine crystallization period. Based on the olivine crystals that are falling onto the diagonal trend it is obvious that the evolution of both melts switched at some

clinopyroxene. The calculated degree of fractionation for olivine and clinopyroxene

point from pure olivine fractionation to co-crystallization of olivine and

Based on this the calculated degree of olivine fractionation is 12.5 wt. % and 10.35 wt. %, respectively (**Figure 8A** and **B**). Adding this amount of olivine to the bulk rock composition the parental melt would have an Mg# of 74.24 and 74.77. In the case of SAN and SOR only the most primitive, Mg-rich olivine crystals follow the modeled exponential olivine fractionation curve on the Fo-Ni plot and the more evolved ones draw a diagonal trend (**Figure 8C** and **D**). A *Dolliq*

*Ni* = 10.

*Ni* value of

ilmenite.

magnetite.

*3.3.3 Geochemistry*

[22, 23, 53, 55].

**33**

olivine and accessory magnetite and ilmenite.

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

### **Figure 5.**

*Photomicrographs of the investigated samples. BRE: Olivine phyric intergranular basanite (xpl), RAC2: Olivine phyric, intersertal and moderately vesicular trachybasalt (xpl), SAN: Olivine and clinopyroxene phyric, intergranular trachybasalt (ppl), SOR: Olivine and clinopyroxene phyric, intersertal basanite (ppl). Abbreviations: Ol – Olivine, cpx – Clinopyroxene, pl. – Plagioclase, v - vesicle.*

### **Figure 6.**

*A: Nomenclature of the investigated alkaline basalts (s.l.). B: They have a high or moderate Mg# (BRE: 67.9, RAC2: 69.46, SOR: 70.54, SAN: 63.9). Mg# = Mg/(Mg + Fe2+) 100. By calculations of Mg# a Fe2O3/FeO ratio of 0.15 has been applied.*

The groundmass contains plagioclase, clinopyroxene, olivine, nepheline and accessory magnetite and ilmenite.

RAC2: an olivine phyric trachybasaltic scoria clast with vesicular, porphyritic-intersertal texture (**Figure 5**). Clinopyroxene microphenocrysts are very rare (olivine/clinopyroxene ratio is 19/1, total phenocryst content: < 5%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphicidiomorphic, sometimes skeletal; with an average size of 500 μm, largest crystals reach 1250 μm. They are often strongly iddingsitized. Most oxidized grains contain few μm thick iron-oxide needles. Idiomorphic inclusions of Cr-spinel are common

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

in crystal rims. Hypidiomorphic-idiomorphic pale brown clinopyroxene microphenocrysts often form glomerocrysts. They are dominantly sector-, rarely normal zoned crystals with an average size of 300–350 μm. The groundmass contains plagioclase, clinopyroxene, olivine, glass, nepheline and accessory magnetite and ilmenite.

SAN: an olivine and clinopyroxene phyric trachybasaltic lava rock sample with porphyritic-intergranular texture (**Figure 5**). Clinopyroxene phenocrysts are quite common (olivine/clinopyroxene ratio is 4/1, total phenocryst content: 20%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphic-idiomorphic, often resorbed; with an average size of 500 μm, largest crystals reach 1200 μm. They are often iddingsitized along crystal rims and cleavage tracks. Idiomorphic inclusions of Cr-spinel are common in crystal rims. Hypidiomorphic-idiomorphic pale brown clinopyroxene phenocrysts are sector zoned crystals with an average size of 300– 400 μm, largest grains reach 1000 μm. They contain magnetite inclusions and they often form glomerocrysts. The groundmass contains plagioclase, clinopyroxene, olivine and accessory magnetite and ilmenite.

SOR: an olivine and clinopyroxene phyric basanitic dyke sample with slightly vesicular, porphyritic-intergranular/intersertal texture (**Figure 5**). Clinopyroxene phenocrysts are common (olivine/clinopyroxene ratio is 5/5, total phenocryst content: 8%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphicidiomorphic, often resorbed; with an average size of 650 μm, largest crystals reach 3000 μm. Idiomorphic inclusions of Cr-spinel and magnetite are common in crystal rims. Idiomorphic-hypidiomorphic pale brown clinopyroxene phenocrysts are sector-, normal- or oscillatory zoned minerals with an average size of 470 μm, largest grains reach 2000 μm. They contain magnetite inclusions. Some clinopyroxene phenocrysts have an olive-green core. Few grains of completely opacitized amphibole megacrysts can be observed. The groundmass contains plagioclase, clinopyroxene, olivine, glass, nepheline and accessory magnetite.

### *3.3.3 Geochemistry*

Geochemical data (**Tables 1** and **2**, **Figures 6** and **7**) were taken from [22, 23, 53, 55].

## **4. Results of olivine and clinopyroxene fractionation modeling**

From two samples (BRE and RAC2) olivine crystals are fitting well to the modeled pure olivine fractionation curves that were calculated with *Dolliq Ni* = 10. Based on this the calculated degree of olivine fractionation is 12.5 wt. % and 10.35 wt. %, respectively (**Figure 8A** and **B**). Adding this amount of olivine to the bulk rock composition the parental melt would have an Mg# of 74.24 and 74.77.

In the case of SAN and SOR only the most primitive, Mg-rich olivine crystals follow the modeled exponential olivine fractionation curve on the Fo-Ni plot and the more evolved ones draw a diagonal trend (**Figure 8C** and **D**). A *Dolliq Ni* value of 12 and 13 were applied for SAN and SOR, respectively. The calculated degree of olivine fractionation is 9.5 wt. % for SAN and 5.5 wt. % for SOR during the initial, pure olivine crystallization period. Based on the olivine crystals that are falling onto the diagonal trend it is obvious that the evolution of both melts switched at some point from pure olivine fractionation to co-crystallization of olivine and clinopyroxene. The calculated degree of fractionation for olivine and clinopyroxene

The groundmass contains plagioclase, clinopyroxene, olivine, nepheline and acces-

*A: Nomenclature of the investigated alkaline basalts (s.l.). B: They have a high or moderate Mg# (BRE: 67.9, RAC2: 69.46, SOR: 70.54, SAN: 63.9). Mg# = Mg/(Mg + Fe2+) 100. By calculations of Mg# a Fe2O3/FeO*

*Photomicrographs of the investigated samples. BRE: Olivine phyric intergranular basanite (xpl), RAC2: Olivine phyric, intersertal and moderately vesicular trachybasalt (xpl), SAN: Olivine and clinopyroxene phyric, intergranular trachybasalt (ppl), SOR: Olivine and clinopyroxene phyric, intersertal basanite (ppl).*

*Abbreviations: Ol – Olivine, cpx – Clinopyroxene, pl. – Plagioclase, v - vesicle.*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

RAC2: an olivine phyric trachybasaltic scoria clast with vesicular, porphyritic-intersertal texture (**Figure 5**). Clinopyroxene microphenocrysts are very rare (olivine/clinopyroxene ratio is 19/1, total phenocryst content: < 5%). Olivine phenocrysts are normal zoned, dominantly hypidiomorphicidiomorphic, sometimes skeletal; with an average size of 500 μm, largest crystals reach 1250 μm. They are often strongly iddingsitized. Most oxidized grains contain few μm thick iron-oxide needles. Idiomorphic inclusions of Cr-spinel are common

sory magnetite and ilmenite.

*ratio of 0.15 has been applied.*

**Figure 5.**

**Figure 6.**

during this phase are 7 and 5.5 wt. % for SAN and 1.89 and 7.56 wt. % for SOR. The total degree of olivine and clinopyroxene fractionation is 16.5 and 5.5 wt. % for SAN and 7.39 and 7.56 wt. % for SOR. Adding this amount of the two mineral phases to the bulk rock composition the parental melt would have an Mg# of 73.44 and 74.54,

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

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

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#

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

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

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

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

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

respectively (**Table 3**).

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

SAN and 7.4/7.6 for SOR).

(BRE – 75.13, RAC2–73.27).

(7.39 wt. %).

melts' Mg#.

calculations.

**35**

a warning about the possible pyroxene fractionation.

the fractionation process of alkaline basaltic melts.

**5. Conclusions**

### **Figure 7.**

*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.*

### **Figure 8.**

*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). Calculated olivine fractionation curves are based on the indicated Dolliq Ni values, black numbers indicate 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.*

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

during this phase are 7 and 5.5 wt. % for SAN and 1.89 and 7.56 wt. % for SOR. The total degree of olivine and clinopyroxene fractionation is 16.5 and 5.5 wt. % for SAN and 7.39 and 7.56 wt. % for SOR. Adding this amount of the two mineral phases to the bulk rock composition the parental melt would have an Mg# of 73.44 and 74.54, respectively (**Table 3**).
