Ultramafic inclusions in basaltic rocks from Hawaii

Abstract

Ultramafic inclusions and the enclosing basaltic rocks were collected from a number of localities in the Hawaiian Islands; these and other specimens were studied by standard petrographic techniques and with an electron microprobe. Emphasis was on determination of mineral assemblages, mineral compositions, and variations in composition. Sixty-eight inclusions and thirteen basaltic rocks are described, with partial chemical analyses (Ti, Al, Cr, Fe, Mn, Ni, Mg, Ca, Na, K) of olivines, orthopyroxenes, clinopyroxenes, and some feldspars and other minerals. Inclusions range from dunite to anorthosite, and basaltic hosts range from olivine nephelinite to olivine tholeiite. The inclusions are separable into three categories, which correlate with three groups basaltic hosts: Lherzolite inclusions are relatively poor in Fe, and the component minerals have limited ranges of composition. In Hawaii, lherzolite inclusions occur preferentially in extremely undersaturated hosts (olivine nephelinite, nepheline basanite, and ankaratrite). Other varieties of inclusions (dunite, wehrlite, feldspathic peridotite, pyroxenite) are relatively rich in Fe, and the component minerals have wider ranges of composition. These inclusions, together with gabbro, occur preferentially in hosts which are but moderately undersaturated (alkaline olivine basalt, hawaiite, and ankaramite). The sparse inclusions in nearly-saturated basalt (olivine tholeiite) are petrographically distinct from those in the other two categories. These correlations suggest that the inclusions and the enclosing basaltic rocks are genetically related. As the three suites of inclusions differ chemically, mineralogically, physically, and texturally, more than one origin is probable.