5:15 PM - 7:15 PM
[SVC34-P13] Magma processes for Hachinohe caldera-forming eruption products at Towada volcano
Keywords:Towada volcano, Hachinohe eruption, pumice, magma processes
Towada volcano is an active volcano located in the northern part of the Northeast Japan Arc. The Towada caldera was formed by several large-scale eruptions including the Hachinohe eruption (eruptive episode L) at 15 ka (Hayakawa 1985; Kudo et al., 2019). Whole-rock SiO2 contents of the episode-L products range 66.6-71.9 wt.%, and the phenocryst assemblage is plagioclase, orthopyroxene, clinopyroxene, amphibole, and Fe-Ti oxides (Kudo et al., 2019). Hunter & Blake (1995) conducted chemical analyses of products from some eruptive episodes and discussed the magma generation and evolution processes. Nakatani et al. (2022) conducted high-pressure crystallization experiments and estimated the magma temperature and pressure conditions for the two caldera-forming eruptions, eruptive episodes L and N.
With an aim of understanding the duration and process of the magma generation for the caldera-forming eruptions at Towada, Ito et al. (2024) conducted whole-rock XRF analyses and mineral compositional analyses for representative episode-L samples. In this presentation, we report the results of the additional mineral compositional analyses by EPMA, whole-rock trace element analyses by ICP-MS, and whole-rock Sr-Nd-Pb isotopic analyses by MC-ICP-MS, and discuss the magmatic processes responsible for forming the whole-rock compositional variations of the episode-L products.
The whole-rock SiO2 compositional variation of the studied rocks is 66.0-73.5 wt.%, which is slightly wider than that reported by the previous study. The whole-rock major and trace element variations are essentially linear in any SiO2 variation diagrams. The primitive mantle-normalized trace element concentration pattern shows typical features of island-arc magmas, and negative Eu anomaly is observed. The Sr, Nd, and Pb isotopic ratios are constant within analytical uncertainties, irrespective of the SiO2 content.
Phenocryst assemblage of the studied samples is plagioclase, orthopyroxene, clinopyroxene, Fe-Ti oxides, and rare olivine. Amphibole is also observed in samples with higher SiO2 content. The plagioclase phenocrysts with the core compositions of An#50-90 are commonly present throughout the products and no systematic variation with the whole rock SiO2 contents is observed. The Mg#’s of amphibole, orthopyroxene and clinopyroxene phenocrysts do not show systematic variation with respect to the whole-rock SiO2 content. Some amphibole phenocrysts coexist with An#50-51 plagioclase phenocrysts. Crystal aggregates consisting of orthopyroxene, clinopyroxene, and plagioclase phenocrysts with An#56-87 are present. The crystallization temperature and pressure conditions for amphibole and two-pyroxenes, estimated by Ridolfi (2021) and Putirka (2008), respectively, are 825-885℃ and 110-170 MPa for amphibole and 870-940℃ and 500-820 MPa for two-pyroxenes.
Nakatani et al. (2022) estimated the magma accumulation pressure for the episode-L magmas to have been 150-170 MPa, which is similar to the estimated pressure condition of 110-170 MPa for amphibole in this study. Therefore, the amphibole and low An# plagioclase phenocrysts are suggested to have originated from the shallow-level large magma reservoir located at <170 MPa. On the other hand, the crystallization pressure for the two-pyroxenes is much higher than 150-170 MPa, suggesting that the two-pyroxene and coexisting An#56-87 plagioclase phenocrysts were derived from a deeper magma reservoir (500-820 MPa). This observation, along with the presence of reversely-zoned plagioclase phenocrysts, suggests that the whole-rock compositional variation of the episode-L products could not have been produced by fractional crystallization in the shallow-level magma reservoir, and magma mixing should have been involved. However, considering that the whole-rock Sr-Nd-Pb isotopic ratios of the products are essentially homogeneous, the compositionally-variable episode-L magmas originated from a common single magma. Therefore, a plausible scenario is that fractional crystallization from a single parental magma proceeded in the deeper magma reservoir, from which the magmas with variable compositions intermittently ascent to inject into the shallow-level magma reservoir, in which magma mixing occurred.
With an aim of understanding the duration and process of the magma generation for the caldera-forming eruptions at Towada, Ito et al. (2024) conducted whole-rock XRF analyses and mineral compositional analyses for representative episode-L samples. In this presentation, we report the results of the additional mineral compositional analyses by EPMA, whole-rock trace element analyses by ICP-MS, and whole-rock Sr-Nd-Pb isotopic analyses by MC-ICP-MS, and discuss the magmatic processes responsible for forming the whole-rock compositional variations of the episode-L products.
The whole-rock SiO2 compositional variation of the studied rocks is 66.0-73.5 wt.%, which is slightly wider than that reported by the previous study. The whole-rock major and trace element variations are essentially linear in any SiO2 variation diagrams. The primitive mantle-normalized trace element concentration pattern shows typical features of island-arc magmas, and negative Eu anomaly is observed. The Sr, Nd, and Pb isotopic ratios are constant within analytical uncertainties, irrespective of the SiO2 content.
Phenocryst assemblage of the studied samples is plagioclase, orthopyroxene, clinopyroxene, Fe-Ti oxides, and rare olivine. Amphibole is also observed in samples with higher SiO2 content. The plagioclase phenocrysts with the core compositions of An#50-90 are commonly present throughout the products and no systematic variation with the whole rock SiO2 contents is observed. The Mg#’s of amphibole, orthopyroxene and clinopyroxene phenocrysts do not show systematic variation with respect to the whole-rock SiO2 content. Some amphibole phenocrysts coexist with An#50-51 plagioclase phenocrysts. Crystal aggregates consisting of orthopyroxene, clinopyroxene, and plagioclase phenocrysts with An#56-87 are present. The crystallization temperature and pressure conditions for amphibole and two-pyroxenes, estimated by Ridolfi (2021) and Putirka (2008), respectively, are 825-885℃ and 110-170 MPa for amphibole and 870-940℃ and 500-820 MPa for two-pyroxenes.
Nakatani et al. (2022) estimated the magma accumulation pressure for the episode-L magmas to have been 150-170 MPa, which is similar to the estimated pressure condition of 110-170 MPa for amphibole in this study. Therefore, the amphibole and low An# plagioclase phenocrysts are suggested to have originated from the shallow-level large magma reservoir located at <170 MPa. On the other hand, the crystallization pressure for the two-pyroxenes is much higher than 150-170 MPa, suggesting that the two-pyroxene and coexisting An#56-87 plagioclase phenocrysts were derived from a deeper magma reservoir (500-820 MPa). This observation, along with the presence of reversely-zoned plagioclase phenocrysts, suggests that the whole-rock compositional variation of the episode-L products could not have been produced by fractional crystallization in the shallow-level magma reservoir, and magma mixing should have been involved. However, considering that the whole-rock Sr-Nd-Pb isotopic ratios of the products are essentially homogeneous, the compositionally-variable episode-L magmas originated from a common single magma. Therefore, a plausible scenario is that fractional crystallization from a single parental magma proceeded in the deeper magma reservoir, from which the magmas with variable compositions intermittently ascent to inject into the shallow-level magma reservoir, in which magma mixing occurred.