Raman microspectroscopic study of reference clay minerals and alteration minerals in volcanic ejecta from the 7 March 2012 phreatic eruption on Ioto Island (Iwo-jima), Izu-Bonin arc, Japan

Highlights

• Volcanic ejecta were studied from the 7 March 2012 eruption on Ioto Island, Japan.

• Alteration minerals in the volcanic ejecta were identified using Raman spectroscopy.

• The alteration minerals were derived from the acidic alteration zone (150–190 °C).

• Five reference clay minerals were analyzed using Raman spectroscopy.

Abstract

We used micro-Raman spectroscopy to analyze volcanic ejecta from the 7 March 2012 phreatic eruption at Old Crater (Million Dollar Hole) on Ioto Island, Izu-Bonin arc, Japan, to assess the ability of this technique to identify clay and other alteration minerals in volcanic ejecta. We also analyzed five reference clay minerals (kaolinite, dickite, pyrophyllite, montmorillonite, and saponite) using the same technique. Dickite, montmorillonite, gypsum, pyrite, marcasite, quartz (α-quartz), and anatase were identified by comparison with spectra of the reference materials and published databases. These minerals occur in close spatial association with each other. Marcasite, quartz, and anatase in aliquots of the sample were not observed by a previous powder X-ray diffraction (XRD) study. The presence of dickite, montmorillonite, gypsum, quartz, marcasite, and anatase indicates that these alteration minerals were likely derived from the acidic alteration zone (150–190 °C) below the crater on Ioto Island. Our results demonstrate that Raman microspectroscopy, compared with conventional XRD, is a more sensitive technique for identifying and characterizing alteration minerals present in the volcanic ejecta. However, owing to the strong fluorescence from clay minerals during Raman analysis, the presence of other minerals in the sample may be overlooked. Therefore, the use of the two techniques in combination can be a powerful tool for obtaining a full understanding of mineral phases in volcanic ejecta from phreatic eruptions, thereby providing more accurate information on the hydrothermal conditions below volcanic craters.

Introduction

An understanding of hydrothermal conditions (e.g., temperature and pH) below a volcanic crater is important for constraining volcanic and hydrothermal processes within the volcano. Sulfur and oxygen isotopic compositions of alteration minerals in volcanic ejecta from phreatic or phreatomagmatic eruptions have been used as temperature indicators for magmatic–hydrothermal environments within the unexposed interior of a volcano [[1], [2], [3], [4]]. Assemblages of alteration minerals in volcanic ejecta also provide basic information on the temperature and pH conditions of the hydrothermal systems [[5], [6], [7]]. Raman microspectroscopy is a rapid and nondestructive analytical technique that requires only small sample volumes (down to approximately 1 μm in diameter) and minimal sample preparation, meaning that there has been a broadening range of geological and archaeological applications of Raman spectroscopy for the identification and characterization of minerals in various samples [8,9]. Several micro-Raman studies have been conducted to identify mineral particles of volcanic origin in ancient pigments and volcanic products [[10], [11], [12], [13], [14], [15], [16], [17]]. Micro-Raman spectroscopy revealed the presence of anatase, atacamite, jarosite, and langite in red and yellow ochre raw pigments from the archaeological site of Pompeii (Italy) [10]. The anatase and atacamite were not detected by conventional powder X-ray diffraction (XRD), due to their low content. Marcaida et al. [10] suggested that the possibility of the local origin of the pigments used by ancient artists because these four minerals have been found in volcanic area of Vesuvius volcano. Major primary rock-forming minerals (e.g., olivine, pyroxene, plagioclase, quartz, magnetite, and hematite) in volcanic rocks and tephras ejected by magmatic eruptions were successfully identified in situ by Raman spectroscopy [[11], [12], [13], [14], [15], [16]]. Secondary minerals (e.g., oxides, carbonates, sulfates, and phosphates) on the samples were also detected using the Raman technique, providing important information about weathering processes of volcanic materials [15,16]. The spectral characteristics and measurement condition of these minerals in terrestrial volcanic environments are useful for the evaluation of Raman spectra of primary and secondary minerals on the surface of other planets such as Mars [12,15,16]. However, to date, few studies have been published on the identification of clay minerals in volcanic ejecta using Raman spectroscopy [17]. Clay minerals are commonly found in tephras ejected by phreatic eruptions [1,3,4,18]. During this study, micro-Raman spectroscopy was performed on volcanic ejecta of the 7 March 2012 phreatic eruption on Ioto Island (formerly Iwo‑jima), Izu-Bonin arc, Japan [[19], [20], [21]], to assess the ability of this technique to identify various alteration minerals in volcanic ejecta. The results are compared with mineral compositions of the ejecta determined by XRD [19]. Micro-Raman analyses on five reference clay minerals (kaolinite, dickite, pyrophyllite, montmorillonite, and saponite) were also conducted to optimize analytical conditions and to obtain reference spectra. These spectra serve as reference data for the interpretation of Raman spectra of various clay-bearing materials.