Petrography of the Manu'a Islands,Samoa

The rocks of the Manu'a Islands are predominantly olivine basalt with lesser amounts of picrite basalt (both ankaramite and oceanite), basalt, hawaiite, olivine gabbro, basaltic lapilli tuff, and ash. Alkali silica ratios obtained from chemical analyses of twenty widely selected samples place the rocks in the alkali basalt suite. All of the samples are also high in titanium. Plots of the chemical analyses, including samples from nearby Tutuila Island, show a progressive enrichment in alkalis. The occurrence of hawaiite and picrite basalt indicates that a primitive alkalic olivine basalt magma was undergoing differentiation. The most important factor in this process was crystal settling, especially of olivine. The dunite xenoliths in the late-stage rocks of Ta'u Island probably came from a residual olivine layer near the bottom of the magma chamber. The magma did not become sufficiently silicic to produce the trachytic end-member of the series, which probably would have contained normative and possibly some modal quartz, as on Tutuila Island. With increasing silica content, iron and titanium generally decrease slightly, whereas alkalis increase.Hawaii Institute of Geophysics Contribution No. 237.