17:15 〜 18:45
[SVC25-P05] Pre-eruptive conditions and eruption mechanism of giant pumice from Toba Caldera: Insight from geochemistry and textural data
キーワード：Giant Pumice, Textural, Geochemistry, Subaqueous eruption , Magma, Toba Caldera
The Toba Caldera experienced several caldera-forming eruptions, the largest of which was the Youngest Toba Tuff (YTT) eruption at 74 ka. Post-YTT activity continued with the formation of several lava domes around the caldera, namely Sipisu-pisu, Samosir, Pardepur, Singgalang, and Tuk-tuk. In addition, post-YTT activity also recorded the presence of giant pumice deposits. Giant pumices (GP) are defined as meter-scale vesicular blocks, which form by the eruption of silicic magma in water. However, until now there has been no detailed study on the origin of GP in the Toba Caldera. This study aims to characterize the GP based on its mineralogy, texture and chemistry to understand its pre-eruptive conditions and eruption mechanism.
In this study, the textural analysis was performed on selected thin sections using backscattered scanning electron images. The major and trace element compositions of phenocrysts and matrix-glass were obtained by electron-probe micro analyzer and laser ablation inductively coupled plasma mass spectrometer, respectively.
The GP mineralogy consists of plagioclase, orthopyroxene, amphibole and oxide. It is noteworthy that GP shares some similarities with YtD, a fourth type of distinct YTT 74 ka pumice population (Bunga Naen et al. 2023), namely: 1) lower silica content (74.2 to 77.2 wt. %) of matrix-glass compositions compared to other YTT pumice types, 2) moderate to high Ba (665-1112 ppm), high Sr (93-128 ppm), and low Y (20-29 ppm) contents of matrix-glass compositions, 3) low FeO/MgO ratio and MnO content in amphibole and orthopyroxene, 4) absence of quartz and sanidine, 5) plagioclase shows many hollow textures and intermediate anorthite content (~An50).
Pressure estimated from orthopyroxene-melt barometry show values of 79–134 MPa, equivalent to 3–5 km depth. The similarities in chemical features and in estimated depth to YtD magma indicate that GP magma is likely a remnant of YtD magma that was not erupted during the YTT 74 ka caldera-forming eruption. We suggest that GP is a subaqueous eruption product that occurred after water filling in the caldera following the caldera collapse event of the YTT 74 ka eruption. The vesicularity (crystal-free) of GP ranges from 42 to 70 %. The inner part (core) of GP has higher vesicularity compared to the outer part (rim), characterized by abundant large vesicles (0.1 – 1.4 mm), which may indicate the occurrence of pheno-bubbles in magma chamber as well as post-fragmentation bubble growth. The Matrix Vesicle Number Density (MVND) values of the GP range from 1014 to 1015 m-3 and the estimated decompression rate from these values is ~107 – 108 Pa/s. Such high decompression rates suggest explosive eruption. However, when the eruptions occurred under water, the fragmentation will not perfectly occur, resulting in giant size of pumice.
Reference
Bunga Naen et al. (2023) JVGR
In this study, the textural analysis was performed on selected thin sections using backscattered scanning electron images. The major and trace element compositions of phenocrysts and matrix-glass were obtained by electron-probe micro analyzer and laser ablation inductively coupled plasma mass spectrometer, respectively.
The GP mineralogy consists of plagioclase, orthopyroxene, amphibole and oxide. It is noteworthy that GP shares some similarities with YtD, a fourth type of distinct YTT 74 ka pumice population (Bunga Naen et al. 2023), namely: 1) lower silica content (74.2 to 77.2 wt. %) of matrix-glass compositions compared to other YTT pumice types, 2) moderate to high Ba (665-1112 ppm), high Sr (93-128 ppm), and low Y (20-29 ppm) contents of matrix-glass compositions, 3) low FeO/MgO ratio and MnO content in amphibole and orthopyroxene, 4) absence of quartz and sanidine, 5) plagioclase shows many hollow textures and intermediate anorthite content (~An50).
Pressure estimated from orthopyroxene-melt barometry show values of 79–134 MPa, equivalent to 3–5 km depth. The similarities in chemical features and in estimated depth to YtD magma indicate that GP magma is likely a remnant of YtD magma that was not erupted during the YTT 74 ka caldera-forming eruption. We suggest that GP is a subaqueous eruption product that occurred after water filling in the caldera following the caldera collapse event of the YTT 74 ka eruption. The vesicularity (crystal-free) of GP ranges from 42 to 70 %. The inner part (core) of GP has higher vesicularity compared to the outer part (rim), characterized by abundant large vesicles (0.1 – 1.4 mm), which may indicate the occurrence of pheno-bubbles in magma chamber as well as post-fragmentation bubble growth. The Matrix Vesicle Number Density (MVND) values of the GP range from 1014 to 1015 m-3 and the estimated decompression rate from these values is ~107 – 108 Pa/s. Such high decompression rates suggest explosive eruption. However, when the eruptions occurred under water, the fragmentation will not perfectly occur, resulting in giant size of pumice.
Reference
Bunga Naen et al. (2023) JVGR