Feldspar crystallization and the behavior of fluorine in the Utagi trachyte, Oki-Dogo, Japan

Abstract

Anti-rapakivi feldspar phenocrysts of ternary Or–Ab–An compositions in a sheet trachyte from Utagi Pass, Oki-Dogo Island (Sea of Japan), show characteristic lamellar–wavy textures, commonly also with patchy texture. These textures have scales ranging from micrometers to decamicrometers, show narrow variation in composition, and preserve very fine cryptoperthites with high-temperature structural states. Several types of fluorite alignment are developed in the lamellar–wavy–patchy textures. Small fluorite grains (mostly <100 nm in size) are not distributed homogeneously or randomly but are heterogeneously or locally dispersed/clustered. Complex microtextures and alignment of fluorites in groundmass feldspars are also observed. In addition to these occurrences of fluorite, interstitial fluorite is found in the groundmass. Feldspars were examined using an electron microprobe analyzer, a scanning electron microscope, a transmission electron microscope, cathodoluminescence imager spectrometers and a Fourier transform-infrared spectrometer. It is inferred from the obtained data that the feldspar microtextures are products of complex, kinetically controlled magmatic crystallization with predominantly diffusion-controlled resorption reactions under H2O-poor or near-anhydrous conditions, and they are developed in association with the crystallization of groundmass during magma ascent to final emplacement. In addition, fluorite alignments in the phenocrysts and groundmass feldspars are estimated to have formed by diffusion-controlled replacement through interaction with residual melt during magma ascent. The interstitial fluorite was the final crystallization product of the residual melt. These various occurrences of fluorite constitute a valuable record of the behavior of F in natural volcanic systems.