Iron-rich lherzolitic xenoliths from Oahu: origin and implications for Hawaiian magma sources

Abstract

Petrographic studies and mineral chemical analyses support the hypothesis that garnet lherzolites and websterites from Oahu, Hawaii were produced by mechanical disintegration (stoping), chemical alteration (metasomatism) and mechanical mixing of spinel lherzolite wall rock (i.e., lithosphere) with intrusive clinopyroxenite veins. This conclusion is inconsistent with two prevalent hypotheses concerning the origin of these rocks, one of which proposes that the garnet lherzolites represent the upper mantle source for Hawaiian tholeiitic magmas, and the other that invokes origin of these xenolith lithologies by high-pressure crystallization of Hawaiian alkalic magmas. Although the undepleted mantle source of Hawaiian tholeiitic magmas may be chemically similar to some of the garnet lherzolite xenoliths, it is physically impossible for these xenoliths to represent actual source rocks.