2018 年 47 巻 2 号 p. 69-81
This paper reports detailed mineralogical and geochemical descriptions of the spherulite occurring in welded tuff from Aga town, Niigata Prefecture, Japan, and also discusses its formation process. Powder X-ray diffraction showed that this spherulite mainly consists of cristobalite, Na-rich sanidine, and Na-rich plagioclase without any secondary minerals such as clay and zeolite minerals produced by hydrothermal alteration. Both the chemical compositions of plagioclase phenocrysts included in spherulite and groundmass and the chondrite-normalized rare earth elements (REE) patterns of the spherulite and its host rock are similar, showing that the present spherulite crystallized from the same pyroclastic deposits as the host rock (strongly welded rhyolite tuff). Textural observation using back-scattered electron image and micro-Raman spectroscopy showed that the present spherulite has three layers structure: rim (dense texture), mantle (porous texture) and core (fan-shaped aggregates of fibrous crystals). The distribution of elements within spherulite was examined using electron-microprobe analyzer with wave-dispersive spectrometer (EMPA-WDS). The results demonstrated that both Ca and Na are enriched in both the rim and core of spherulite, suggesting that plagioclase distributes dominantly in these parts. Detailed observation with secondary electron microscopy showed that fibrous microcrystals (～1 μm in width) of both feldspar minerals and cristobalite were intergrown to constitute fan-shaped aggregates in the core, and porous aggregates of larger crystals (～5 μm) in the mantle. These chemical and textual evidence suggested that the present spherulite formed in a series of steps as follows: (1) Nucleation and rapid intergrowth of cristobalite and Na-rich plagioclase occurred in the core of spherulite, and these minerals made up into the fan-shaped aggregate. (2) From the surrounding glass, fibrous crystals of Na-rich sanidine, and cristobalite were intergrown radially, and the incompatible components such as H2O and Ca were expelled into the residual glass, creating enrichments of those elements along the spherulite-host boundary. (3) Saturation of H2O in the residual glass resulted in pore generation in the mantle part. (4) On cooling, cristobalite precipitated on the pore wall due to degassing of volatile components and the residual glass rich in Ca crystallized into Na-rich plagioclase in the rim part of spherulite.
