Geochemistry of Kauai volcanics and a mixing model for the origin of Hawaiian alkali basalts |
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| Authors: | Mark D Feigenson |
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| Institution: | (1) Department of Geological Sciences, Rutgers University, 08903 New Brunswick, N.J., USA |
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| Abstract: | A comprehensive model is developed to explain the major, trace element and strontium and neodymium isotopic characteristics
of alkali basalts from Hawaii. The model is similar to that of Chen and Frey (1983) in that it requires mixing of a small
melt fraction of MORB-source material with another component to generate the alkalic suite of a particular Hawaiian volcano.
It differs from the Chen and Frey model in that the other end-member must be different from primitive mantle if it is to be
consistent with both trace element and isotopic data. Alkali basalts and tholeiites from Kauai analyzed in this study show
a nearly complete transition in Sr and Nd isotopes. There is a relatively well-constrained array on a Nd-Sr isotope correlation
plot that can be explained by two-component mixing of Kauai tholeiite magma and a small amount of melt of East Pacific Rise
source rock. After corrections are made for fractional crystallization (involving primarily clinopyroxene and olivine), the
Sr and Ba concentrations of Kauai lavas plot along mixing curves defined by the above sources, providing positive tests of
the mixing hypothesis. Implications of this model are: (1) the main source of Hawaiian shield-building tholeiites is a mixture
of subducted crust, primitive mantle and depleted asthenosphere that has been homogenized prior to melting, (2) early alkalic
volcanism (as at Loihi seamount) will be characterized by greater isotopic heterogeneity than will late-stage alkali basalt
production, and (3) there are two fundamentally distinct types of alkalic lavas erupted towards the end of magmatism at a
given Hawaiian volcano. One represents smaller degrees of melting of the same source that generated shield-building tholeiites
(Kohala-type); the other derives from the mixed source discussed in this paper (Haleakala-, Kauai-type). |
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