1:45 PM - 3:15 PM
[O08-P26] Experimental study on the liquid-solid transition of phenocryst-rich basaltic lava: A case of Suijin lava, Fuji volcano, Japan
Keywords:Mt.Fuji, basalt, liquid-solid transition
Mt. Fuji is an active volcano that has been active since 100 ka and has erupted more than 40 times in the past 2000 years. Through its history, most of the erupted magma is basalt lava. Because of high temperature and low viscosity, basaltic lava often flows long distances. Therefore, for hazard mitigation, it is important to understand the flow properties of lava, especially the criteria at which the liquid-solid transition occurs. The liquid-solid transition is induced by the increase in crystal content in the lava, and the critical crystal content at which the transition occurs depends on various textural factors. Therefore, an experimental approach using the real lava sample is critical to directly constrain the criteria. In this study, high-temperature melting and deformation experiments were carried out for the phenocryst-rich basaltic lava from Mt. Fuji to constrain the temperature-crystal content conditions at which liquid-solid transition occurs.
The phenocryst-rich basaltic lava from Mt. Fuji (Suijin lava) was collected and cut into 2cm-sized cubes for the starting materials. The cubic sample was placed in the alumina crucible and heated in the 1-atm furnace at the temperatures of 1150-1180 ℃ for 15-90 minutes. After heating, the sample was quenched in water. First, the shapes of the retrieved run samples were described. Then they were processed into polished thin sections for microscopic observation. Using a transmissive scanner, whole-range optical photographs were taken for the thin section samples. In addition, the samples heated for 45 min at 1160 ℃ and 1150 ℃ were selected and their groundmass textures were observed using FE-EPMA. Image analyses using GIMP (version 2.10.24) and ImageJ (version 1.53c) were carried out for the optical photographs and the backscatter electron (BSE) images to quantify the volume fractions of phenocrysts and the groundmass minerals, respectively.
All run samples showed vitreous luster, indicating they contain abundant glasses. No deformation was observed for the samples heated at 1150 ℃. In contrast, the samples heated at >=1160 ℃ flowed within 45 min and the onset time of deformation decreased as the temperature increased. The temperature dependence of the onset time is attributed to the increase in the melting rate of minerals with increasing temperature. The results indicate that the liquid-solid transition (or the drastic change of viscosity) occurs within a narrow temperature range of 1155 ± 5 ℃.
Volume fractions of phenocrysts and groundmass minerals were quantified for the samples heated for 45 min. Phenocryst contents were constant at ~37.5 vol% for all of the samples, indicating that the melting of phenocryst is not the cause of the liquid-solid transition. In contrast, the abundance of groundmass minerals decreased as the temperature increased (~2.5 vol% at 1160 ℃ and ~9.6 vol% at 1150 ℃, respectively, on the phenocryst-free basis). The viscosities were estimated to be ~104 Pa s and ~106 Pa s for the lava samples heated at 1160 ℃ and 1150 ℃, respectively. The results indicate that the small change (~7 vol% on a phenocryst-free basis) of the volume fraction of groundmass minerals induced the drastic change (~2 order of magnitude) of lava viscosity, resulting in the observed liquid-solid transition. Present results constrain the critical volume fraction of suspended crystals (phenocryst + groundmass minerals) at which the liquid-solid transition occurs to be 40-47 vol%.
Crystallization simulation was carried out using the rhyolite-MELTS program for the melt having the composition of groundmass in the studied lava. The result indicates that for phenocryst-free lava, ~45 ℃ cooling is required to achieve the crystal content to be the critical value of the liquid-solid transition. In contrast, for the case the lava contains ~37.5 vol% of phenocrysts, the crystal content achieves the critical value within only ~5 ℃ cooling. Therefore, the lava emplacement process is strongly affected by phenocryst content, and the effective suppression of cooling is required for phenocryst-rich lava to flow long distances.
The phenocryst-rich basaltic lava from Mt. Fuji (Suijin lava) was collected and cut into 2cm-sized cubes for the starting materials. The cubic sample was placed in the alumina crucible and heated in the 1-atm furnace at the temperatures of 1150-1180 ℃ for 15-90 minutes. After heating, the sample was quenched in water. First, the shapes of the retrieved run samples were described. Then they were processed into polished thin sections for microscopic observation. Using a transmissive scanner, whole-range optical photographs were taken for the thin section samples. In addition, the samples heated for 45 min at 1160 ℃ and 1150 ℃ were selected and their groundmass textures were observed using FE-EPMA. Image analyses using GIMP (version 2.10.24) and ImageJ (version 1.53c) were carried out for the optical photographs and the backscatter electron (BSE) images to quantify the volume fractions of phenocrysts and the groundmass minerals, respectively.
All run samples showed vitreous luster, indicating they contain abundant glasses. No deformation was observed for the samples heated at 1150 ℃. In contrast, the samples heated at >=1160 ℃ flowed within 45 min and the onset time of deformation decreased as the temperature increased. The temperature dependence of the onset time is attributed to the increase in the melting rate of minerals with increasing temperature. The results indicate that the liquid-solid transition (or the drastic change of viscosity) occurs within a narrow temperature range of 1155 ± 5 ℃.
Volume fractions of phenocrysts and groundmass minerals were quantified for the samples heated for 45 min. Phenocryst contents were constant at ~37.5 vol% for all of the samples, indicating that the melting of phenocryst is not the cause of the liquid-solid transition. In contrast, the abundance of groundmass minerals decreased as the temperature increased (~2.5 vol% at 1160 ℃ and ~9.6 vol% at 1150 ℃, respectively, on the phenocryst-free basis). The viscosities were estimated to be ~104 Pa s and ~106 Pa s for the lava samples heated at 1160 ℃ and 1150 ℃, respectively. The results indicate that the small change (~7 vol% on a phenocryst-free basis) of the volume fraction of groundmass minerals induced the drastic change (~2 order of magnitude) of lava viscosity, resulting in the observed liquid-solid transition. Present results constrain the critical volume fraction of suspended crystals (phenocryst + groundmass minerals) at which the liquid-solid transition occurs to be 40-47 vol%.
Crystallization simulation was carried out using the rhyolite-MELTS program for the melt having the composition of groundmass in the studied lava. The result indicates that for phenocryst-free lava, ~45 ℃ cooling is required to achieve the crystal content to be the critical value of the liquid-solid transition. In contrast, for the case the lava contains ~37.5 vol% of phenocrysts, the crystal content achieves the critical value within only ~5 ℃ cooling. Therefore, the lava emplacement process is strongly affected by phenocryst content, and the effective suppression of cooling is required for phenocryst-rich lava to flow long distances.