17:15 〜 18:45
[SVC25-P03] Preliminary textural studies of pumice and scoria from the Plinian eruption of Raung volcano, East Java, Indonesia
キーワード：Raung volcano, Pumice fall, Scoria fall, Vesicle number density, Microlite number density
Raung volcano in Indonesia exhibits a wide variety of bulk-rock magma compositions, ranging from basaltic to dacitic. Its dacitic and andesitic magma composition has caused the VEI 4-5 Plinian eruption of pumice fall (Rjp1 unit; 0.7–1.3 km3) and scoria fall (Rjp2; 0.3 km3 and Rjp3 unit; 0.1–0.2 km3), respectively. However, the pre- and syn-eruptive processes that caused the Plinian eruptions from both magma compositions remain unclear. To answer this issue, we provide a preliminary investigation to characterize the unique textural properties between Raung pumice and scoria.
White and yellow pumice were selected from Rjp1 pumice fall, while dark grey scoria clasts were collected from Rjp2 and Rjp3 scoria fall. Volcanic glass compositions of each juvenile component were acquired from EPMA. Thermobarometer estimation and geochemical modelling using MELTS software were conducted to investigate the pre-eruptive magma chamber condition. Textural studies were conducted using digitized BSE images from SEM. Vesicles and crystals are quantified to determine vesicularity, crystallinity, vesicle size distribution (VSD), vesicle number density (VND), and microlite number density (MND).
Rjp1 pumice shows the most evolved glass composition, with white pumice having slightly higher SiO2 (~64 wt%) than yellow pumice (~63 wt%). Rjp2 scoria has an intermediate composition (~57 wt% SiO2), whereas Rjp3 scoria has the least evolved composition (~54 wt% SiO2). Higher SiO2 content corresponds to a larger eruptive volume. Thermobarometer and MELTS modelling suggest that Rjp1 magma is stored at the shallower chamber than Rjp3 magma. Rjp2 resulted from mixing between Rjp1 and Rjp3, and settled at the bottom part of the Rjp1 chamber.
Rjp1 pumice is characterized by high bulk-vesicularity and low pheno-crystallinity. In contrast, Rjp2 and Rjp3 scoria are characterized by low bulk-vesicularity and high pheno-crystallinity. Vesicle size distribution marks the boundary between the large vesicle (pheno-vesicle) and the small vesicle (matrix-vesicle) in 0.08 mm. Calculation of vesicle number density was conducted separately for pheno-vesicle (PVND; >0.08 mm) and matrix-vesicle (MVND; <0.08 mm). MVND shows a positive correlation with MND, but is negatively correlated with PVND.
Rjp1 pumice corresponds to relatively moderate PVND (2.42 x 1011 m-3) and lowest MND (0.16 x 1013 m-3) of all samples. Rjp3 scoria corresponds to highest PVND (3.2 x 1011 m-3) and moderate MND (1.2 x 1013 m-3). Both units show a similar range in low MVND (0.73 x 1013 m-3 to 0.96 x 1013 m-3), even though they resulted from distinct magma compositions. On the other hand, Rjp2 scoria has the lowest PVND (1.4 x 1011 m-3) but highest MVND (2.97 x 1013 m-3) and MND (3.69 x 1013 m-3) than those of Rjp1 and Rjp3. We find the correlation between the presence of phenocryst to PVND, MVND, and MND. Rjp2 scoria has highest pheno-crystallinity, followed by Rjp1 pumice and Rjp3 scoria. Pheno-crystallinity is positively correlated with MVND and MND, but shows negative correlation with PVND.
Based on this preliminary study, we confirm that there is a distinct mechanism between Rjp1-Rjp2 and Rjp3 eruption. Fractional crystallization and crystal settling in the shallow Rjp1-Rjp2 magma chamber caused Rjp2 (bottom part) to have higher pheno-crystallinity than Rjp1 (top part). While phenobubble rise to the upper part of the magma chamber (via buoyancy), causing Rjp1 to have higher PVND than Rjp2. The high viscosity and high PVND nature of Rjp1 magma inhibit microlite and matrix-bubble nucleation, therefore resulting in lower MVND and MND compared to Rjp2. At the deeper system, the highest PVND in Rjp3 magma also inhibits matrix-bubble nucleation and results in low MVND. However, microlite nucleation took place in Rjp3 magma because of the least viscous condition. Our study provides evidence that pre-eruptive condition in magma chamber controls the syn-eruptive conduit processes in Raung volcano.
White and yellow pumice were selected from Rjp1 pumice fall, while dark grey scoria clasts were collected from Rjp2 and Rjp3 scoria fall. Volcanic glass compositions of each juvenile component were acquired from EPMA. Thermobarometer estimation and geochemical modelling using MELTS software were conducted to investigate the pre-eruptive magma chamber condition. Textural studies were conducted using digitized BSE images from SEM. Vesicles and crystals are quantified to determine vesicularity, crystallinity, vesicle size distribution (VSD), vesicle number density (VND), and microlite number density (MND).
Rjp1 pumice shows the most evolved glass composition, with white pumice having slightly higher SiO2 (~64 wt%) than yellow pumice (~63 wt%). Rjp2 scoria has an intermediate composition (~57 wt% SiO2), whereas Rjp3 scoria has the least evolved composition (~54 wt% SiO2). Higher SiO2 content corresponds to a larger eruptive volume. Thermobarometer and MELTS modelling suggest that Rjp1 magma is stored at the shallower chamber than Rjp3 magma. Rjp2 resulted from mixing between Rjp1 and Rjp3, and settled at the bottom part of the Rjp1 chamber.
Rjp1 pumice is characterized by high bulk-vesicularity and low pheno-crystallinity. In contrast, Rjp2 and Rjp3 scoria are characterized by low bulk-vesicularity and high pheno-crystallinity. Vesicle size distribution marks the boundary between the large vesicle (pheno-vesicle) and the small vesicle (matrix-vesicle) in 0.08 mm. Calculation of vesicle number density was conducted separately for pheno-vesicle (PVND; >0.08 mm) and matrix-vesicle (MVND; <0.08 mm). MVND shows a positive correlation with MND, but is negatively correlated with PVND.
Rjp1 pumice corresponds to relatively moderate PVND (2.42 x 1011 m-3) and lowest MND (0.16 x 1013 m-3) of all samples. Rjp3 scoria corresponds to highest PVND (3.2 x 1011 m-3) and moderate MND (1.2 x 1013 m-3). Both units show a similar range in low MVND (0.73 x 1013 m-3 to 0.96 x 1013 m-3), even though they resulted from distinct magma compositions. On the other hand, Rjp2 scoria has the lowest PVND (1.4 x 1011 m-3) but highest MVND (2.97 x 1013 m-3) and MND (3.69 x 1013 m-3) than those of Rjp1 and Rjp3. We find the correlation between the presence of phenocryst to PVND, MVND, and MND. Rjp2 scoria has highest pheno-crystallinity, followed by Rjp1 pumice and Rjp3 scoria. Pheno-crystallinity is positively correlated with MVND and MND, but shows negative correlation with PVND.
Based on this preliminary study, we confirm that there is a distinct mechanism between Rjp1-Rjp2 and Rjp3 eruption. Fractional crystallization and crystal settling in the shallow Rjp1-Rjp2 magma chamber caused Rjp2 (bottom part) to have higher pheno-crystallinity than Rjp1 (top part). While phenobubble rise to the upper part of the magma chamber (via buoyancy), causing Rjp1 to have higher PVND than Rjp2. The high viscosity and high PVND nature of Rjp1 magma inhibit microlite and matrix-bubble nucleation, therefore resulting in lower MVND and MND compared to Rjp2. At the deeper system, the highest PVND in Rjp3 magma also inhibits matrix-bubble nucleation and results in low MVND. However, microlite nucleation took place in Rjp3 magma because of the least viscous condition. Our study provides evidence that pre-eruptive condition in magma chamber controls the syn-eruptive conduit processes in Raung volcano.