5:15 PM - 6:45 PM
[SGC32-P10] Primary magma composition of the Abu monogenetic volcano group using olivine melt inclusions in scoria
Keywords:Abu Monogenetic Volcano Group
Quaternary igneous activity in southwestern Japan is mainly caused by the formation of mantle wedges by the subducting Philippine Sea (PHS) plate and the contribution of dehydrated fluids from the PHS slab. In the Chugoku area, island-arc basalt (IAB) from PHS slab subduction is abundant, but some of ocean-island basalt (OIB) are also sporadically produced. Some studies show that Kannabe, Oki, and Abu volcanoes, which are island arcs but OIB eruptions. That were formed by mantle uplift of a different origin than PHS slab subduction, but the volatile composition of the uplifted mantle is not well characterized, and thus the estimation of mantle melting conditions remains a problem. In this study, scoria samples were collected near Mt. Takamure in Abu County, Yamaguchi Prefecture, Japan, which belongs to the Abu volcanic group, and melt inclusions in the olivine were analyzed to constrain the volatile composition of the primary magma. In this presentation, we report the results of the chemical composition analysis of the whole rock of the scoria, the chemical composition analysis of the phenocryst, and the progress of the olivine crystal separation work. Those works were done as a preparatory step for the study of melt inclusions.
The melt inclusions are thought to be the result of the incorporation of surrounding melts when the host mineral crystallized. Therefore, the effect of degassing at shallow depths should be smaller for mineral melt inclusions that crystallized earlier. In general, olivine is the first mineral fractionated from basaltic magma, but it is easy to be dehydrated by diffusion. Also melt inclusions of olivine in lava formed after slow cooling do not retain their primary water content. On the other hand, since scoria is formed by rapid cooling at the time of eruption, less water is lost from olivine by diffusion than in lava. Thus, it is thought to retain more primitive information. In this study, we estimate the amount of primary volatile components in the melt inclusions of olivine.
The Abu Monogenetic Volcano Group is a group of about 40 monogenetic volcanoes with an active period of 2.0 Ma to 8,800 years ago. The active period is divided into an early stage (1.6 to 2.0 Ma), a dormant period of about 800,000 years, and a late stage (0.8 Ma to 8800 years ago). In the early stage, alkali basalts are mainly erupted. In the late stage, from calc-alkali andesite to dacite are erupted with alkali basalts. The K-Ar age of the Takamure-yama basalt, which is the source of the scoria sampled in this study, is reported to be 0.18 ± 0.01 Ma. The scoria contains mainly olivine and clinopyroxene as phenocryst. Also, it has quartz that appears to be a xenolith with a reaction rim of clinopyroxene around it. The olivine crystals were up to 3.0 mm in diameter with spinel, and the interior was fresh.
All olivine samples were analyzed by electron probe microprobe analyzer (EPMA), and Fo# ranged from 85 to 88. Whole rock chemical composition analysis by X-ray fluorescence spectrometer (XRF) showed that SiO2 is about 45wt% MgO is about 13wt%, and FeO*/MgO is 0.9-1.1. Those results are the closest composition to undifferentiated among other reported rocks of the Abu Volcano Group. On the other hand, although the rock is classified as alkali basalt Na2O+K2O was about 2.0 wt%, which is low content.
The scoria samples were coarsely crushed using a high-voltage pulse selective crusher (Selfrag Lab), and olivine phenocryst were separated by observation under a stereomicroscope. The olivine was fixed with resin and polished until melt inclusions appeared on the surface. Most of the olivine crystals are dendritic and have a reddish brown, as well as internal microstructures and associated fractures due to rapid growth. This makes it difficult to observe the pure portions under a transmission microscope and to find melt inclusions. The average grain size of melt inclusions is approximately 20 μm, and the maximum is approximately 30μm. The percentage of melt inclusions above the analytical limit of 10 μm is about 10%, and most melt inclusions contain bubbles. Further analysis of volatile components, trace elements, and isotope ratios is planned as soon as the number of melt inclusions is collected.
The melt inclusions are thought to be the result of the incorporation of surrounding melts when the host mineral crystallized. Therefore, the effect of degassing at shallow depths should be smaller for mineral melt inclusions that crystallized earlier. In general, olivine is the first mineral fractionated from basaltic magma, but it is easy to be dehydrated by diffusion. Also melt inclusions of olivine in lava formed after slow cooling do not retain their primary water content. On the other hand, since scoria is formed by rapid cooling at the time of eruption, less water is lost from olivine by diffusion than in lava. Thus, it is thought to retain more primitive information. In this study, we estimate the amount of primary volatile components in the melt inclusions of olivine.
The Abu Monogenetic Volcano Group is a group of about 40 monogenetic volcanoes with an active period of 2.0 Ma to 8,800 years ago. The active period is divided into an early stage (1.6 to 2.0 Ma), a dormant period of about 800,000 years, and a late stage (0.8 Ma to 8800 years ago). In the early stage, alkali basalts are mainly erupted. In the late stage, from calc-alkali andesite to dacite are erupted with alkali basalts. The K-Ar age of the Takamure-yama basalt, which is the source of the scoria sampled in this study, is reported to be 0.18 ± 0.01 Ma. The scoria contains mainly olivine and clinopyroxene as phenocryst. Also, it has quartz that appears to be a xenolith with a reaction rim of clinopyroxene around it. The olivine crystals were up to 3.0 mm in diameter with spinel, and the interior was fresh.
All olivine samples were analyzed by electron probe microprobe analyzer (EPMA), and Fo# ranged from 85 to 88. Whole rock chemical composition analysis by X-ray fluorescence spectrometer (XRF) showed that SiO2 is about 45wt% MgO is about 13wt%, and FeO*/MgO is 0.9-1.1. Those results are the closest composition to undifferentiated among other reported rocks of the Abu Volcano Group. On the other hand, although the rock is classified as alkali basalt Na2O+K2O was about 2.0 wt%, which is low content.
The scoria samples were coarsely crushed using a high-voltage pulse selective crusher (Selfrag Lab), and olivine phenocryst were separated by observation under a stereomicroscope. The olivine was fixed with resin and polished until melt inclusions appeared on the surface. Most of the olivine crystals are dendritic and have a reddish brown, as well as internal microstructures and associated fractures due to rapid growth. This makes it difficult to observe the pure portions under a transmission microscope and to find melt inclusions. The average grain size of melt inclusions is approximately 20 μm, and the maximum is approximately 30μm. The percentage of melt inclusions above the analytical limit of 10 μm is about 10%, and most melt inclusions contain bubbles. Further analysis of volatile components, trace elements, and isotope ratios is planned as soon as the number of melt inclusions is collected.