Composition and origin of Hawaiian lavas

Abstract

New chemical analyses for major elements in 76 Hawaiian rocks are presented and bring the total number of such modern analyses to about 470. Many determinations of minor elements also are becoming available. Hawaiian petrology is discussed against this total background. The three major rock suites, tholeiitic, alkalic, and nephelinic, are chemically intergradational. The main mass of the volcanoes is tholeiitic, followed by a relatively small volume (generally less than 1 percent) of alkalic lavas; the two types of lavas are interbedded in a thin transitional zone. The nephelinic lavas are separated from the others by a long time interval that is marked by a profound erosional unconformity. Variations within the rock suites are largely the result of crystal differentiation. All three rock suites probably are derived from a single type of parent magma, which varies slightly from one volcanic center to another, of olivine tholeiite composition. Crystallization of this magma in shallow magma chambers leads to eruptible magmas of tholeiitic composition. In the last stages of volcanism, consolidation of the upper part of the magma body leads to crystallization at deeper levels under higher pressure and to production of alkalic magmas. Finally, crystallization at depths of several tens of kilometers produces nephelinic magmas that are erupted after a long period of volcanic quiescence.