Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism

Abstract

Ultramafic xenoliths from Koolau Volcano on the island of Oahu, Hawaii, are divided into spinel lherzolite, pyroxenite, and dunite suites. On the basis of a study of the petrography and mineral compositions of 43 spinel lherzolites, 12 pyroxenites, and 20 dunites, the following characteristics of the dunites in relation to the other nodule types and to Hawaiian lavas emerge. (1) The forsterite content of olivines in the Koolau dunites (Fo82.6-Fo89 7) overlap those of Hawaiian tholeiitic and alkalic lavas and are generally lower than those in abyssal lherzolites and dunites and in Koolau spinel lherzolites. (2) Most of the dunites contain no orthopyroxene, all except two contain chrome spinel, and a few contain interstitial plagioclase and clinopyroxene. (3) Chrome spinels from the Koolau dunites are distinctly higher in Cr/(Cr +Al), lower in Mg/(Mg + Fe2+), and higher in TiO2 than those from abyssal basalts and peridotites. Chrome spinels in the dunites correspond closely in composition to chrome spinels in Hawaiian tholeiitic and alkalic lavas. (4) The abundance of dunite relative to other nodule types decreases outward from the central part of the volcano. The dunites are interpreted not as residues of partial fusion of the mantle but as crystal accumulations stored at shallow depths beneath the central part of Koolau Volcano and derived from picritic magmas parental to the shield-building tholeiitic lavas.