Mid-Oceanic Ridge Basalt (MORB) samples collected from southern East Pacific Rise (SEPR) have been investigated. These highly phyric plagioclase basalts (HPPB) and moderately phyric plagioclase basalts (MOPB) show rare cumulate and vitrophyric textures with plagioclase (> 10% as phenocryst) and abundant glass (> 72%). Electron Probe Micro Analysis (EPMA) showed large compositional variations in the megacrysts as well as microcrysts of plagioclase (An(62) to An(82)), olivine (Fo(78) to Fo(87)), pyroxene (ferroaugite to augite) and iron oxides, mostly titaniferous magnetite. Olivine grains show high Mg# (> 80%) and distinctly low in NiO (0.01-0.2%). Ferroan trevorite (NiO =16.22 and FeO((t)) =83.06) a characteristic meteoritic mineral has been identified from the olivine megacrysts of MORB, possibly attributed to Ni-enrichment, resulted from heterogeneity of the lower mantle. Wide range of An composition in plagioclase is indicative of large pressure range of crystal nucleation under decompression at a depth of similar to 70 km (An(82)) up to the ocean spreading centre. Absence of zoning observed in all the minerals present in the MORB samples, possibly attributed to unmixing and dominant fractionation process.
