Polygenetic olivine phenocrysts in Okete basanite, New Zealand.

Abstract

Generation of polygenetic olivine phenocrysts is discussed for a basanite from Okete Quarry, New Zealand, based on descriptions of its olivine and spinel and the reaction product between the basanite and its peridotite xenoliths. The olivine phenocrysts can be divided into two types, I and II. The phenocrysts (I) are relatively small, subhedral to euhedral and has minute spinel inclusions sparsely throughout the grain. They are free of the textures indicating deformation and fracture. The phenocrysts (II) are relatively large and have spinel inclusions only in the rim. Their core has deformation textures and/or fluid inclusion trails. The olivine phenocrysts (I) are normally zoned: the core ranges from Fo (sub 85) to Fo (sub 76) and the rim has Fo (sub 76). The spinel inclusions are highly variable in Cr/(Cr+Al+Fe (super 3+)) atomic ratio (0.02-0.5), Cr= Cr/[Cr+Al] atomic ratio) (0.1-0.6), TiO (sub 2) content (17-1 wt%) and Fe3+/(Cr+Al+Fe (super 3+)) atomic ratio (= Fe (super 3+) ratio) (0.7-0.2), and can be grouped into two, low-Fe (super 3+) (Fe (super 3+) ratio <0.5) and high-Fe (super 3+) (Fe (super 3+) ratio >0.5) spinels. The Cr ratio is correlated positively with the Crnd negatively with the TiO (sub 2) content and Fe (super 3+) ratio. The phenocrysts (II) frequently have compositionally flat cores, which are relatively Mg-rich (up to Fo (sub 90)), and zoned rims (Fo (sub 85-76)). Their spinel inclusions mostly belong to the high-Fe (super 3+) group. The generation of olivine crystals and their spinel inclusions of various origins can be observed at the boundaries between the basanite and its peridotite xenoliths. Orthopyroxene in the peridotite xenoliths was decomposed into olivine, clinopyroxene, spinel and melt (glass) by the reaction with the basanite melt. The spinel from the reaction product is relatively high-CrSubhedral olivine which includes chromian spinels with a wide compositional range has overgrown on the mantle olivine, which has been gradually disintegrated into the basanite matrix. The olivine phenocrysts (I) with low-Fe (super 3+) spinel inclusions were derived from the orthopyroxene/melt reaction product, and those with high-Fe (super 3+) spinels were precipitated from the basanite melt. The olivine phenocrysts (II) are of xenocrystal origin and have been modified by the melt to various degrees. The olivine phenocrysts in olivine-rich magmas such as picrite basalts and alkali olivine basalts can be polygenetic due to the reaction between the mantle peridotite and more primitive magmas.