Geochemical evolution of the suboceanic mantle

Abstract

The suboceanic mantle has on the basis of incompatible element and isotope ratios two chemically distinct units; the mantle source for ocean island basalts and the mantle source for ocean ridge basalts. Ocean island alkali basalts have an enriched light rare earth element (REE) source with K/Rb = 400, K/Ba = 28, Zr/Nb = 6.6. Ocean ridge tholeiites have a depleted light REE source, with K/Rb = 1060, K/Ba = no, and Zr/Nb = 37. 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios are generally lower in the ocean ridge basalts than in the basalts from ocean islands. It is suggested that these differences are not a result of simple partial melting, fractional crystallization, zone refining, residual minerals or isotopic disequilibrium between minerals and melt during melting. Instead, the differences between the sources are real, and have existed for some 1500 Ma or more. Nephelinite, alkali basalt, and tholeiites from ocean islands have similar isotopic and incompatible element ratios. The nephelinites and alkali basalts are, however, progressively much more enriched in the light REE than are the tholeiites, yet the light REE in nephelinites and alkali basalts appear to be incompatible. This may be explained by the occurrence of small (average 4 cm thick), closely spaced pyroxenite veins (within tens of cm) in the mantle source for ocean island basalts. The occurrence of a network of veins which melt preferentially may lead to melts which have apparent sources with a much greater enrichment in incompatible elements than the total mantle source and may also give the appearance of a source whose ratio for slightly compatible elements changes as a function of extent of melting.