[S-MP47_1PO1] Physics and Chemistry of Minerals

[SMP47-P20] Cathodoluminescence study of metasomatic feldspar in aegirine syenite from Iwaki Island, Ehime Prefecture

*Seiya MAKI¹, Hirotsugu NISHIDO¹, Masahiro KAYAMA² (1.Okayama University of Science, 2.Hiroshima University)

In Iwaki Island, aegirine syenite was emplaced in the Ryoke granite during late Cretaceous time by alkali-rich hydrothermal metasomatism. The syenite and related rocks show various types of petrographic textures in response to the process of hydrothermal alteration, e.g. feldspar minerals. Feldspar exhibits a variety of cathodoluminescence (CL) colors depending on kinds of impurity elements and their concentrations, and defect densities related to Si-Al ordering and other structural disorder. Recently, the deconvolution method of CL spectra enables to assign the luminescence centers characteristic of the feldspar with satisfactory reliability (Kayama et al., 2010). In this study, we have conducted to clarify the metasomatic process through granite to syenite by CL spectral analyses for various types of feldspar.Polished thin sections of the rock samples collected from granite, altered granite and syenite were employed for petrographic observations under a polarizing light microscope, CL measurements, and electron microprobe analyses (EPMA). Color CL images were obtained using a cold-cathode type Luminoscope with a cooled-CCD camera. CL spectroscopy was made by a SEM-CL system, which is comprised of SEM (JEOL: JSM-5410LV) combined with a grating (OXFORD: Mono CL2). The CL emitted from the samples was dispersed by a grating monochromator (1200 grooves/mm), and recorded by a photon counting method using a photomultiplier tube. All CL spectra were corrected for total instrumental response, which was determined using a calibrated standard lamp.The feldspar in the unaltered granite shows apple green and blue CL emissions. The former is identified to plagioclase (Ab80, An20) characterized by divalent Mn activator at 556 nm, and the latter to alkali feldspar (Or90, Ab10) by defect center at 417 nm related to Al-O-Al. Altered granite has albite with red CL emission at around 750 nm, and alkali feldspar with inhomogeneous color of red to violet-blue emissions at around 400 nm and 720 nm. These CL emissions in a red region can be assigned to trivalent Fe activator in tetrahedral sites. The feldspar in syenite are mostly altered to albite with enhanced red emission at 748 nm, but minor alkali feldspar as residual after hydrothermal alteration exhibits dull red emission at 722 nm. The results of the spectral deconvolution reveals oxygen defect centers associated with Al-O-Al and Al-O-Ti bridges and impurity centers of trivalent Fe ions substituted for tetrahedral Al sites according to Kayama et al. (2010). Kayama et al. (2010) investigated the peak changes of a blue emission peak at 420 nm in alkali feldspar and they found that the elimination of Al-O-Al defect center was affected by hydrothermal metasomatism possibly at 250 ^oC. Therefore, the disappearance of blue emission in alkali feldspar in syenite implies that alkali-rich (sodium-rich) hydrothermal metasomatism for the formation of syenite could act at relatively high temperature above 250 ^oC successively after granitic magmatism.