The Hirna lava show enrichment in the highly incompatible trace element relative to OIB and MORB and the primitive mantle normalized pattern (spider diagram) are not smooth (fig. 5) which may either be a consequence of crustal contamination or derivation from enriched mantle source.
The basalt samples of the area have Ce/Pb ratio between 20 and 25 within the range of OIB and MORB (25±5 Hofmann et al., 1986) which explain crustal contamination is not prominent factor in the modification of the mantle derived magma. However, they exhibit elevated Sr/Ce ratio (10 to 16) compared to OIB and MORB (5 to 8 Furman et al., 1999) and La/Nb ratio (>1) seems to be contaminated by crustal material. The elevated values of Sr/Ce ratio with slight positive Eu anomalies of these samples are resulting from the incorporation of plagioclase feldspar from the crustal material during its ascent to the surface. It is supported by the evident that strong fractionated Zr/Nb ratio (7 – 9) in the basalt samples indicate that contamination with crustal material with high Zr/Nb ratio and low Nb concentration.
Melanephelinite sample on the other hand show no evidence of crustal contamination. These lavas have unusually high abundance of incompatible trace elements which make them unlikely to be contaminated easily during ascent or in crustal chamber. For example, the samples are characterized by elevated values of Ce/Pb ratio (66 to 99) which is greater than OIB and MORB (25±5 Hofmann et al., 1986), Sr/Ce ratio within the range of MORB and OIB (5 to 8 Furman et al., 1999) and La/Nb ratio (<1) which implying the high level of crustal contamination process did not play a major role in the evolution of the rock. Thus, the observed trace element ratio and highly enrichment of incompatible trace element of these samples can be attributed to an enriched mantle source rather than crustal contamination. However, the effect of contamination cannot be entirely neglected since these lavas have traveled across the crust.