16:00 〜 16:15
[SVC25-08] Conditions of the ignimbrite formations of the Khangar volcano (Kamchatka peninsula)
キーワード：Ignimbrite, Fiamme, Water content
Ignimbrites are indicators of catastrophic volcanic processes. In this regard, the conditions of their formation attract the attention of many scientists. Here we present result of detailed mineralogy, melt and fluid inclusion study, along with water content determination in melt inclusions and groundmass glass of welded ignimbrites from caldera-forming deposit of Khangar volcano (Simonov et al., 2023). Khangar volcano is the southernmost volcano of the Sredinny Range of Kamchatka, which stretches N-S across western Kamchatka. It is the dominant feature within a larger volcano-tectonic depression composed of two parts: a stratovolcano with a 2-km-wide Holocene caldera (0.4 Ma), and a large lava dome.
It was found that the crystallization of most plagioclase and quartz phenocrysts from ignimbrites of the Khangar volcano occurred at temperatures of 840-960°C, under the pressure of 1.1 kbar in shallow crustal conditions at a depth of about 4 km.
The fiamme of ignimbrites consists of transparent groundmass glass and spherules consisting of dark glass with abundant crystals of amphibole and feldspars. The transparent glass represents melt quenched in the early stages of pyroclastic flow formation, while later portions of more degassed melt are represented by dark spherules. According to mineral thermometry, it was found that crystallization of transparent glass occurred at 770-840°C, while later darker spherules crystallized at 680-760°C. Thus, the temperature sequence of crystallization was established.
The water content in fiamme glasses and melt inclusions was determined by Raman spectroscopy, in accordance with a previously published method (Kotov et al., 2021). Estimated H2O contents in the MIs vary within 2.97-3.23 wt. %, in transparent groundmass glass of fiamme within 2.35-2.91 wt. % and late dark groundmass glass of fiamme contains 0.77-1.35 wt. % H2O. This reflects the sequence of degassing of the melt involved in the catastrophic eruption.
In addition, the syngenetic primary melt and fluid inclusions we studied in plagioclase and quartz phenocrysts from ignimbrites of the Khangar volcano indicate phase separation ("boiling") of melt with massive formation of CO2 microbubbles. During decompression, the release of volatile components leads to the formation of bubbles with low density, which can expand catastrophically, and a significant silica content ensures high viscosity of the melt, which increases the exclusivity of eruptions. Using cryometric methods, the density of CO2 in the gas phase of the fluid inclusion was found to vary between 0.076 and 0.078 g/cm3, and the estimated mole fraction of water in the fluid (XH2O) is about 0.84. This means that the fluid of the shallow-depth source before the catastrophic eruption was saturated predominantly with aqueous fluid.
References:
Kotov A.A., Smirnov S.Z., Plechov P Y., Persikov E.S., Chertkova N.V., Maksimovich I.A., Karmanov N.S., Buhtiyarov P.G. Method for determining water content in natural rhyolitic melts by Raman spectroscopy and electron microprobe analysis // Petrology, 2021, v. 29 (4), p. 386-403. DOI: 10.1134/S0869591121040044.
Simonov V.A., Kotlyarov A.V., Kotov A.A., Perepelov A.B., Karmanov N.S., Borovikov A.A. Conditions of the ignimbrite formations of the Khangar volcano (Kamchatka) // Geol. Geofiz., doi:10.15372/GiG2023197. Published online: 11/05/2023. (in Russ.)
It was found that the crystallization of most plagioclase and quartz phenocrysts from ignimbrites of the Khangar volcano occurred at temperatures of 840-960°C, under the pressure of 1.1 kbar in shallow crustal conditions at a depth of about 4 km.
The fiamme of ignimbrites consists of transparent groundmass glass and spherules consisting of dark glass with abundant crystals of amphibole and feldspars. The transparent glass represents melt quenched in the early stages of pyroclastic flow formation, while later portions of more degassed melt are represented by dark spherules. According to mineral thermometry, it was found that crystallization of transparent glass occurred at 770-840°C, while later darker spherules crystallized at 680-760°C. Thus, the temperature sequence of crystallization was established.
The water content in fiamme glasses and melt inclusions was determined by Raman spectroscopy, in accordance with a previously published method (Kotov et al., 2021). Estimated H2O contents in the MIs vary within 2.97-3.23 wt. %, in transparent groundmass glass of fiamme within 2.35-2.91 wt. % and late dark groundmass glass of fiamme contains 0.77-1.35 wt. % H2O. This reflects the sequence of degassing of the melt involved in the catastrophic eruption.
In addition, the syngenetic primary melt and fluid inclusions we studied in plagioclase and quartz phenocrysts from ignimbrites of the Khangar volcano indicate phase separation ("boiling") of melt with massive formation of CO2 microbubbles. During decompression, the release of volatile components leads to the formation of bubbles with low density, which can expand catastrophically, and a significant silica content ensures high viscosity of the melt, which increases the exclusivity of eruptions. Using cryometric methods, the density of CO2 in the gas phase of the fluid inclusion was found to vary between 0.076 and 0.078 g/cm3, and the estimated mole fraction of water in the fluid (XH2O) is about 0.84. This means that the fluid of the shallow-depth source before the catastrophic eruption was saturated predominantly with aqueous fluid.
References:
Kotov A.A., Smirnov S.Z., Plechov P Y., Persikov E.S., Chertkova N.V., Maksimovich I.A., Karmanov N.S., Buhtiyarov P.G. Method for determining water content in natural rhyolitic melts by Raman spectroscopy and electron microprobe analysis // Petrology, 2021, v. 29 (4), p. 386-403. DOI: 10.1134/S0869591121040044.
Simonov V.A., Kotlyarov A.V., Kotov A.A., Perepelov A.B., Karmanov N.S., Borovikov A.A. Conditions of the ignimbrite formations of the Khangar volcano (Kamchatka) // Geol. Geofiz., doi:10.15372/GiG2023197. Published online: 11/05/2023. (in Russ.)