Ge-Si and Ga-Al fractionation in Hawaiian volcanic rocks

Abstract

Ge and Ga abundances in 19 petrologically and geochemically well-documented Hawaiian volcanic rocks were determined simultaneously by radiochemical neutron activation analysis. The data demonstrate a close affinity of Ga for AI and Ge for Si over the entire range of the nephelinitic, alkalic and tholeiitic lava series studied. Ga and Ge become progressively enriched in the final products of crystallization of the alkali series. No systematic pressure effect on the Ga/Al and Ge/Si ratios could be detected over the pressure range at which nephelinites, alkali olivine basalts and tholeiites have presumably been generated. Ga/Al ratio varies by less than 62% within the three Hawaiian series, and decrease slightly toward residual melts of the alkali series. Ge/Si ratio varies generally by less than 20% within the three Hawaiian series, and can, within experimental errors, be considered independent of degree of partial melting and extent of fractional crystallization effective in the generation of these lavas. We suggest that the Ge/Si ratio in basalts may be a useful mantle source indicator and should help mapping mantle heterogeneities.