4:00 PM - 4:15 PM
[SGC32-08] A Study on the Volatiles of Monogenetic Volcanoes in the Eastern Region of Jeju Island, South Korea
Keywords:Volatile, Melt Inclusion, Korea, Jeju
Jeju volcano began erupting around 1.8 million years ago, with its most recent activity having occurred a thousand years ago (Kigam & JRI, 2020). Various studies regarding its magmatism has been proposed, including direct ascent from the depleted asthenospheric mantle (Tatsumi et al., 2005) or the impact of a subducted oceanic crust has been indicated by the δ18O values of the basaltic lava samples (J.I. Kim, 2023). However, there is currently no consensus on the volatiles for the volcanic activity in Jeju Island, yet. Volatile components such as H2O or CO2 are known to play a crucial role in the mantle melting process. However, determining the pre-eruption concentration of those elements in the magma from typical onshore lava samples is difficult, due to degassing volatiles during slow cooling rates of the lavas after the eruption. Scoria, which is a porous and glassy black basaltic fragment, undergoes rapid cooling during and after eruption, enabling the measurement of primary volatile concentrations in the magma from melt inclusions (hereafter as MIs) trapped in the phenocrysts. Once a melt was trapped and sealed within a closed system in the phenocryst, MI typically do not undergo degassing process unless subjected to delayed diffusion or breakage of the host mineral. Therefore, there is a high probability that retain the volatile composition of the melt at the time of entrapment.
The scoria sample collected from Darangswi (~7ka, JS06) scoria cone, located in the eastern region of Jeju Island and separated olivine phenocrysts, which is the earliest crystallizing crystal from the primary melt. Samples were subjected to analyses of whole-rock major element compositions, major element compositions of host olivine and MIs, and volatile element concentrations (H2O, CO2, F, S, Cl) of MIs. The main purpose of this study is to investigate the volatile contents of the primary melts and understand the petrogenesis of the volcanoes in the eastern region of Jeju.
Volcanic rocks of Jeju Island have been classified into three types, each having distinct parental magmas (Tatsumi et al., 2005): High-Alumina Alkali (HAA), Low-Alumina Alkali (LAA), Sub-Alkali (SA). In this study, the whole-rock compositions of samples JS06 is represented by LAA. On the other hand, when plotting the compositions of MIs on silica versus total alkali diagram, MIs from JS06 exhibit two types of MIs—HAA and LAA—within a single scoria unit. From the major elements and volatile of the MIs, it was possible to estimate that JS06 has at least six magma sources.
To evaluate the influence of the subducted oceanic plate, we compared the volatile contents and halogen concentrations of the primitive melts obtained from this study with those of Mid-Ocean Ridge Basalt (MORB) and data from various locations in Japan (Kuritani et al., 2021), using the same analytical methods as the previous studies. The results indicate that Jeju MIs exhibits higher in halogen concentrations, as well as H2O/F and Cl/F ratios compared to MORBs. The H2O/Cl and Cl/F ratios are rather similar to those of the Gotō Islands, which are ~200km to the east of Jeju Island and are suggested to be influenced by the subducted oceanic plate. Results of this study also suggest the geochemical impact of the oceanic plate on the volcanism at Jeju Island.
This study presents evidence that the Darangswi scoria cone, a monogenetic volcano in the late stage of Jeju Island’s volcanic activity, consists of two types of magmatic sources—High-Al Alkali, and Low-Al Alkali— based on major elements of melt inclusions. And we have also identified at least 6 distinct magma sources from volatile components. Additionally, a comparison with nearby Quaternary volcanoes in Jeju suggests the continued influence of the oceanic plate slab in the later stages of the island's formation.
The scoria sample collected from Darangswi (~7ka, JS06) scoria cone, located in the eastern region of Jeju Island and separated olivine phenocrysts, which is the earliest crystallizing crystal from the primary melt. Samples were subjected to analyses of whole-rock major element compositions, major element compositions of host olivine and MIs, and volatile element concentrations (H2O, CO2, F, S, Cl) of MIs. The main purpose of this study is to investigate the volatile contents of the primary melts and understand the petrogenesis of the volcanoes in the eastern region of Jeju.
Volcanic rocks of Jeju Island have been classified into three types, each having distinct parental magmas (Tatsumi et al., 2005): High-Alumina Alkali (HAA), Low-Alumina Alkali (LAA), Sub-Alkali (SA). In this study, the whole-rock compositions of samples JS06 is represented by LAA. On the other hand, when plotting the compositions of MIs on silica versus total alkali diagram, MIs from JS06 exhibit two types of MIs—HAA and LAA—within a single scoria unit. From the major elements and volatile of the MIs, it was possible to estimate that JS06 has at least six magma sources.
To evaluate the influence of the subducted oceanic plate, we compared the volatile contents and halogen concentrations of the primitive melts obtained from this study with those of Mid-Ocean Ridge Basalt (MORB) and data from various locations in Japan (Kuritani et al., 2021), using the same analytical methods as the previous studies. The results indicate that Jeju MIs exhibits higher in halogen concentrations, as well as H2O/F and Cl/F ratios compared to MORBs. The H2O/Cl and Cl/F ratios are rather similar to those of the Gotō Islands, which are ~200km to the east of Jeju Island and are suggested to be influenced by the subducted oceanic plate. Results of this study also suggest the geochemical impact of the oceanic plate on the volcanism at Jeju Island.
This study presents evidence that the Darangswi scoria cone, a monogenetic volcano in the late stage of Jeju Island’s volcanic activity, consists of two types of magmatic sources—High-Al Alkali, and Low-Al Alkali— based on major elements of melt inclusions. And we have also identified at least 6 distinct magma sources from volatile components. Additionally, a comparison with nearby Quaternary volcanoes in Jeju suggests the continued influence of the oceanic plate slab in the later stages of the island's formation.