Characteristic Textures of Recrystallized, Peritectic, and Primary Magmatic Olivine in Experimental Samples and Natural Volcanic Rocks

Abstract

Olivine textures are potentially important recorders of olivine origin and crystallization conditions. Primary magmatic and xenocrystic origins are commonly considered for olivine from ultramafic to intermediate magmas, whereas secondary olivine origins (i.e. crystals formed by recrystallization or peritectic reaction) are rarely considered in the interpretation of magmatic phenocrysts. The main aim of our study was to determine textures that are characteristic for secondary magmatic olivine and non-characteristic, or at least rare, for primary magmatic olivine. To characterize the textures of the different olivine types, we review previous experimental work and present new textural data for olivine from four melanorite melting and eight basalt crystallization experiments. We qualitatively characterize olivine textures using transmitted light and back-scattered electron microscopy, and semi-quantitatively characterize the 2D surface area of olivine branches and single crystals, their 2D and calculated 3D shapes, and 2D grain boundary segment lengths. Olivine recrystallization yields crystals with dendritic branches, whereas peritectic reaction produces olivine clusters with randomly oriented crystals. In agreement with previous studies, we find that olivine crystal and branch size, grain boundary segmentation, and inclusion relations cannot unequivocally distinguish between olivine of secondary and primary origins. However, recrystallized olivine typically has short prismatic branches, whereas primary magmatic olivine dendrites commonly have elongated branches. The peritectic crystals are closely comparable with glomerocrysts and single polyhedral primary magmatic crystals, but they commonly form small groups of touching crystals and clusters with crystal-poor cores. Spatial association and a comparison with xenocryst textures may further guide the interpretation of their origin, but detailed analyses of core compositions and zoning patterns appear necessary to firmly distinguish peritectic from primary magmatic crystals. Our comparison of experimental and natural samples suggests that dendritic olivine crystals and clusters of polyhedral olivine that commonly occur in mafic and ultramafic igneous rocks should be evaluated for possible primary as well as secondary magmatic origins.