Some Chemical Features of Lavas from the Manu'a Islands, Samoa

Abstract

The lavas of the Manu'a Islands have chemical compositions typical of oceanic island alkali lavas. They have rare earth abundances with chondrite normalized lanthanum:ytterbium ratios around 10 and ytterbium concentrations about 2.2 ppm. Strontium concentrations are greater than 400 ppm and within the range of strontium values for Hawaiian alkalic lavas. Potassium:rubidium ratios are generally between 300 and 400 but three samples have potassium: rubidium ~700, suggesting heterogeneity of source materials. The 87strontium: 86strontium ratios average 0.7046 ± .0003, and are the highest known for oceanic islands. Low pressure differentiation is controlled by olivine and plagioclase. These lavas were segregated from a normal oceanic upper mantle at >40 km depth and the percentage of partial melting was in the range of 3 to 7 percent. The chemical composition of the probable original magma is estimated using combined trace element, major element, and partition coefficient data.