Abstract

Large crystals of olivine, pyroxene and plagioclase in a hyalocrystalline mesostasis of alkali basalt composition are believed to represent intratelluric products of the early crystallization phases of a newly formed basalt magma. The habit of the crystals as well as the optical and chemical data of the crystalline and melt phases indicate that a moderate amount of crystal fractionation occurred as part of the cooling history of the magma. The physical distribution of the crystalline phases, on the other hand, suggests that eruption and chilling of the magma were rapid enough to prevent notable differentiation by crystal settling.

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