Experimental and major element constraints on the evolution of lavas from Lihir Island, Papua New Guinea

Abstract

The major element chemistry of SiO₂-undersaturated arc lavas from Lihir Island, Papua New Guinea, and 1 atmosphere experiments on an alkali basalt from this island show complex polybaric fractionation affected this suite of lavas. Low Ni and MgO are typical of these arc lavas and result from olivine fractionation, probably at high pressure. Fractionation at low pressure (<5 kb) produces two evolutionary trends. Separation of clinopyroxene, plagioclase and minor olivine from the primitive lavas results in increasing normative nepheline contents and major element trends similar to those of the experiments. In contrast, addition of magnetite and amphibole to the fractionating assemblage in the evolved lavas results in decreasing normative nepheline and major element trends which are markedly different from those of the experiments. The composition of experimental glasses and 1 atmosphere liquid lines of descent, derived from anhydrous melting experiments run at the fayalite-magnetite-quartz (FMQ) buffer and at higher oxygen fugacities, are displaced from the lavas on oxide-oxide plots. HighfO₂ produces high Fe³⁺/Fe²⁺ and the early crystallization of abundant magnetite, and high H₂O contents are responsible for crystallization of amphibole. Crystal fractionation of these phases and the high Fe³⁺/Fe²⁺ are responsible for the displacement of the lavas and experimental glasses in mineral projection schemes from the 1 atmosphere olivine-clinopyroxene-plagioclase saturation boundary of Sack et al. (1987).