The major element variation between the basalt and melanephelinite lava indicate that; this two lava series cannot be related through fractional crystallization from a common parent. This inconsistency evidenced by, major element oxides (e.g. TiO2, CaO, Fe2O3, Na2O and Al2O3see figure 3) variation diagram of the two groups as a function of MgO do not show a coherent correlation trend with one another.
The major element content of basaltlavas from Hirna southeastern Ethiopia define a coherent segmented trend which represent modification of the primary magma during fractional crystallization. The variation diagram of the basalt lavas shows that there is a strong negative correlation between MgO, SiO2, K2O and Na2O; with little variation of SiO2, MgO drop rapidly this suggests that differentiation of the melt is controlled by subtraction of olivine from the system. On the other hand Fe2O3 and TiO2against MgO show positive correlation trend on the liquid line of descent. This correlation is a suggestive of crystal liquid control by the fractionation of clinopyroxene and iron-titanium oxides from the melt.CaO/Al2O3 with MgO plot of the basalt (silica saturated) lavas show inflected trend on a variation diagram. Clinopyroxene has a high CaO/Al2O3 ratio and MgO content and therefore fractionation of clinopyroxene rapidly drive the residual melt to lower CaO/Al2O3 and MgO. And conversely the removal of plagioclase from the magma increase both CaO/Al2O3 and MgO in the residual melt. On the contrary olivine fractionation decrease MgO rapidly but leaves CaO/Al2O3 unchanged. Hirna basalts lava reflect similar inflected trend, showing fractionation of olivine up to ~5.8 wt. % of MgO and then theplot supports the onset of fractionation of clinopyroxene (together with plagioclase and olivine).
All the sample in the area are characterized by low Ni (<45 ppm) and Cr (< 59 ppm) as well as low to moderate MgO content (4.55 to 10.67 wt.%). These values are very low compared to primary magma formed from upper mantle (Ni: > 400 to 500 ppm, Cr: > 1000ppm and MgO: 10 to 15 wt.%, Frey et al., 1978; Hess, 1992).These trace elements in all (melanephelinite and basalt) samples have undergone large fractionation since initially forming as partial melts from the mantle indicating the major fractionation of olivine and/or clinopyroxene from primary mantle melts. Crystal fractionation is the preferred mechanism for such Cr and Ni depletion as its abundance is not expected to vary under conditions of increasing partial melting.
Generally it is concluded that allof the lavas from Hirna area largely undergo crystal fractionation involving olivine, clinopyroxene, iron-titanium oxides, and ± plagioclase, which is consistent with the observed phenocrysts assemblage.However, difference in major and trace element between the two lavas of the area (melanephelinite and basalt) may reflect variation in source region enrichment history and/orvariable degreeof crustal contribution to the melt.