10:45 AM - 12:15 PM
[SVC29-P09] The pre-eruptive process of the Suoana eruption in Miyake-Jima volcano, Japan, investigated from gabbroic xenoliths
Keywords:Miyake-Jima volcano, Crystal mush, pre-eruptive process, gabbroic xenoliths
Suoana tephra is the volcanic deposit of the explosive eruption that occurred in the 7th century in Miyake-Jima volcano, Japan. This deposit includes interstitial glass-bearing gabbroic xenoliths which are thought to be fragments of the mushy magma reservoir. These gabbroic xenoliths are expected to have information about the pre-eruptive magmatic processes in the mushy reservoir beneath the volcano. In this study, using EPMA and FE-EPMA at Earthquake Research Institute, the University of Tokyo, textural observations and chemical analyses were performed for the constituent minerals and glasses of the gabbroic xenoliths in the Suoana tephra to clarify the pre-eruptive process and the magma plumbing system of the volcano.
The investigated gabbroic xenoliths were collected from the Unit 2 of the Suoana tephra. Based on mineral assemblage, the samples are classified into 3 groups; the anorthosites and troctolites composed of plagioclase and olivine (Group-A), the troctolite and olivine gabbros composed of plagioclase, olivine, and clinopyroxene (Group-B), and the gabbronorite composed of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides (Group-C). Interstitial melt is found in the samples of all groups; the melt in Group-A and B samples is almost crystallized whereas that in Group-C is glassy. Melt inclusions (MIs) are found in all the constituent minerals excluding Fe-Ti oxides.
An# [=100Ca/(Ca+Na)] in plagioclase is almost identical in Group-A (~93-97) and B samples (~89-97) but that in Group-C is lower (~72-89). Fo# of olivine [=100Mg/(Mg+Fe)] is higher for Group-A (~82-85) compared to Group-B (~75-81). Mg# of clinopyroxene [=100Mg/(Mg+Fe)] is higher for Group-B (~78-83) than Group-C (~73-78). MIs are basaltic in Group-A, basaltic to basaltic andesitic in Group-B, and andesitic to dacitic in Group-C, respectively. The compositional range of the interstitial glass is identical to that of MIs in Group-C. Compositions of constituent minerals and olivine-hosted melt inclusions are identical between Group-A and the olivine-bearing anorthite megacrysts, suggesting that the parental melt (PM) compositions of Group-A sample is the same as that of the anorthite megacrysts; the PM composition was already estimated (SiO2~49.0wt%, MgO~7.2wt%; Seino et al., 2021 VSJ fall meeting abstract). The compositions of the Group-B PMs are estimated from those of olivine-hosted MIs to be basaltic to basaltic andesitic (SiO2~50-52wt%, MgO~5.0-5.8wt%). The Group-B PMs are slightly differentiated compared to Group-A PM. The interstitial melt of Group-C has the most differentiated composition of the volcano (SiO2~62.1wt%, MgO~2.6wt%). The PM and interstitial melt compositions are consistent with the tholeiitic trend found for the magmas of Miyake-Jima volcano; this result as well as the mineral compositions suggests that the trend is formed by crystallization differentiation. The temperature and H2O content conditions are estimated to be ~1110ºC and ~4.9wt% for Group-A, ~1060-1090ºC and ~4.4-5.4wt% for Group-B, and ~1000ºC and ~4.7wt% for Group-C, respectively. The estimated melt H2O contents are almost identical between all samples and correspond to the H2O-saturation depth of ~8 ± 3 km, suggesting that the gabbros were derived from almost the same depth.
Based on the results, the following pre-eruptive magmatic processes of the Suoana eruption are proposed. Before the eruption, the Group-A PM was stored at ~8 km depth. Then, the magma was cooled and crystalized; the crystalized minerals formed the mushy cumulate layers at the bottom of the chamber in the order of Group-A, B, and C. The residual andesitic melt was accumulated at the upper part of the chamber. At the initial stage of the eruption, by the reinjection of the Group-A PM induced the ascent and eruption of the residual andesitic melt. Thereafter, the Group-A PM including the fragments of gabbros erupted by passing through the mush layers.
The investigated gabbroic xenoliths were collected from the Unit 2 of the Suoana tephra. Based on mineral assemblage, the samples are classified into 3 groups; the anorthosites and troctolites composed of plagioclase and olivine (Group-A), the troctolite and olivine gabbros composed of plagioclase, olivine, and clinopyroxene (Group-B), and the gabbronorite composed of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides (Group-C). Interstitial melt is found in the samples of all groups; the melt in Group-A and B samples is almost crystallized whereas that in Group-C is glassy. Melt inclusions (MIs) are found in all the constituent minerals excluding Fe-Ti oxides.
An# [=100Ca/(Ca+Na)] in plagioclase is almost identical in Group-A (~93-97) and B samples (~89-97) but that in Group-C is lower (~72-89). Fo# of olivine [=100Mg/(Mg+Fe)] is higher for Group-A (~82-85) compared to Group-B (~75-81). Mg# of clinopyroxene [=100Mg/(Mg+Fe)] is higher for Group-B (~78-83) than Group-C (~73-78). MIs are basaltic in Group-A, basaltic to basaltic andesitic in Group-B, and andesitic to dacitic in Group-C, respectively. The compositional range of the interstitial glass is identical to that of MIs in Group-C. Compositions of constituent minerals and olivine-hosted melt inclusions are identical between Group-A and the olivine-bearing anorthite megacrysts, suggesting that the parental melt (PM) compositions of Group-A sample is the same as that of the anorthite megacrysts; the PM composition was already estimated (SiO2~49.0wt%, MgO~7.2wt%; Seino et al., 2021 VSJ fall meeting abstract). The compositions of the Group-B PMs are estimated from those of olivine-hosted MIs to be basaltic to basaltic andesitic (SiO2~50-52wt%, MgO~5.0-5.8wt%). The Group-B PMs are slightly differentiated compared to Group-A PM. The interstitial melt of Group-C has the most differentiated composition of the volcano (SiO2~62.1wt%, MgO~2.6wt%). The PM and interstitial melt compositions are consistent with the tholeiitic trend found for the magmas of Miyake-Jima volcano; this result as well as the mineral compositions suggests that the trend is formed by crystallization differentiation. The temperature and H2O content conditions are estimated to be ~1110ºC and ~4.9wt% for Group-A, ~1060-1090ºC and ~4.4-5.4wt% for Group-B, and ~1000ºC and ~4.7wt% for Group-C, respectively. The estimated melt H2O contents are almost identical between all samples and correspond to the H2O-saturation depth of ~8 ± 3 km, suggesting that the gabbros were derived from almost the same depth.
Based on the results, the following pre-eruptive magmatic processes of the Suoana eruption are proposed. Before the eruption, the Group-A PM was stored at ~8 km depth. Then, the magma was cooled and crystalized; the crystalized minerals formed the mushy cumulate layers at the bottom of the chamber in the order of Group-A, B, and C. The residual andesitic melt was accumulated at the upper part of the chamber. At the initial stage of the eruption, by the reinjection of the Group-A PM induced the ascent and eruption of the residual andesitic melt. Thereafter, the Group-A PM including the fragments of gabbros erupted by passing through the mush layers.