Amphibole–melt disequilibrium in silicic melt of the Aso-4 caldera-forming eruption at Aso Volcano, SW Japan

Abstract

The most recent and largest caldera-forming eruption occurred at ~ 90 ka at Aso Volcano, SW Japan, and is known as the "Aso-4 eruption. " We performed chemical analyses of amphibole phenocrysts from Aso-4 pyroclasts collected from the initial and largest pyroclastic unit (4I-1) of the eruption to infer the composition-temperature-pressure conditions of the melt that crystallized amphibole phenocrysts. Each amphibole phenocryst is largely chemically homogeneous, but inter-grain chemical variation is observed. Geothermometry, geobarometry, and melt-SiO 2 relationships based on amphibole single-phase compositions reveal that most amphibole phenocrysts were in equilibrium with hydrous melt comprising ~ 63-69 wt% SiO 2 (SiO melt 2) at 910-950 °C, although several grains were crystallized from more mafic and higher-temperature melts (~ 57-60.5 wt% SiO 2 and 965-980 °C). The amphibole temperatures are comparable with those previously estimated from two-pyroxene geothermometry, but are much higher than temperatures previously estimated from Fe-Ti oxide geothermometry. The estimated SiO melt 2 contents are lower than that of the host melt in the 4I-1 pyroclasts. Chemical and thermal disequilibrium between the amphibole rims and the host melt, as well as intra-grain homogeneity and inter-grain heterogeneity of amphibole compositions, suggests that these amphiboles were incorporated into the host melt immediately prior to the caldera-forming eruption. Our results suggest that the amphibole phenocrysts, and perhaps some of the pyroxene and plagioclase phenocrysts, were derived from a chemically and thermally zoned crystal mush layer that had accumulated beneath the chamber of the host 4I-I melt. Amphibole geobarometry indicates a crystallization depth of ~ 13.9 ± 3.5 km, which is consistent with the present-day magma chamber depth beneath the volcano as inferred from geophysical observations. The results suggest that the depth of the post-caldera magma plumbing system is strongly influenced by a relic magma reservoir related to a previous caldera-forming eruption.

Preeruptive P‐T Conditions and H2O Concentration of the Aso‐4 Silicic End‐Member Magma Based on High‐Pressure Experiments

“…In this study, KJ5665 is peraluminous and MY5774 is metaluminous; therefore, the amphibole may be more stable in metaluminous MY5774 magma that is richer in Na 2 O. However, amphibole phenocryst is not equilibrated with silicic magma (this study and Ishibashi et al, ) and amphibole phenocryst was not found in MY5774 fiamme; we consider that amphibole did not crystallize in the Aso‐4 silicic end‐member magma. The absence of hornblende phenocrysts suggests that (1) the hornblende phenocrysts did not crystallize in the silicic end‐member magma; (2) the hornblende phenocrysts formed in a hidden mafic magma near an interface with the silicic end‐member magma, as is the case for hornblende within microclots in the Us‐b rhyolite associated with Usu volcano (Tomiya & Takahashi, ); and (3) the phenocrysts survived in the silicic end‐member magma due to a short enough residence time in the silicic magma to avoid decomposition of the phenocryst or the phenocrysts are nearly stable in the silicic magma.…”

“…In this study, KJ5665 is peraluminous and MY5774 is metaluminous; therefore, the amphibole may be more stable in metaluminous MY5774 magma that is richer in Na 2 O. However, amphibole phenocryst is not equilibrated with silicic magma (this study and Ishibashi et al, 2018) and amphibole phenocryst was not found in MY5774 . Solid, open, and double circles indicate assemblages containing orthopyroxene without biotite, biotite without orthopyroxene, and neither orthopyroxene nor biotite, respectively.…”

“…In summary, KJ5665 is homogeneous, exhibits no evidence of magma mixing, and is suitable for high‐P equilibria experiments. Ishibashi et al () indicate that hornblende phenocryst of Aso‐4 silicic magma is not equilibrated with surrounding melt because of the morphology and difference of estimated temperatures. We ignored hornblende phenocryst from the phase matching because hornblende phenocryst in Aso‐4 is not equilibrium with the melt, and because the amount of the hornblende content is minimal.…”

Amphibole perspective to unravel the rhyolite as a potential fertile source for the Yonaguni Knoll IV hydrothermal system in the southwestern Okinawa Trough

“…The types of amphibole that may exist in rhyolite mainly include crystalline amphibole crystallized from the host melt, xenocrystic amphibole entrained by the host melt (e.g., Ishibashi, Suwa, Miyoshi, Yasuda, & Hokanishi, ; Kiss et al, ; Nagasaki et al, ), and altered amphibole. Thus, distinguishing among the different types of amphibole is a prerequisite for using the compositions of amphibole to reveal magma evolution or mineralization.…”

Special issue “Advancement of our knowledge on Aso volcano: current activity and background”

“…This suggests degassing of magma in the chamber associated with magma convection within a conduit. Ishibashi et al (2018) investigated chemical compositions of amphibole phenocrysts in pyroclasts collected from the initial and largest pyroclastic unit(4I-1) of the Aso-4 caldera-forming eruption occurred at ~ 90 ka to clarify their crystallization conditions and pre-eruptive magmatic processes. The results suggest the following: (1) most of the amphibole phenocrysts coexisted with silicate melt with 66-72 wt% SiO 2 and temperatures of 910-950 °C, whereas some amphiboles crystallized from more mafic and higher-T melt; (2) the amphibole phenocrysts are in thermal and chemical disequilibrium with the host 4I-1 melt, indicating that they were incorporated into the melt immediately prior to eruption; and (3) amphibole phenocrysts crystallized at a depth of ~ 13.9±3.5 km, which coincides with the depth of the present low-velocity zone beneath the volcano, implying that the depth of the post-caldera magma plumbing system is strongly influenced by a relic collapsed magma reservoir related to the most recent caldera-forming eruption.…”