著者

石橋 秀巳 岩橋 くるみ 安田 敦 諏訪 由起子 長崎 志保 外西 奈津美

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.66, no.2, pp.119-129, 2021-06-30 (Released:2021-07-27)

参考文献数

51

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Recent studies proposed empirical equations describing the relations between amphibole single-phase chemistry and the pressure-temperature-compositional conditions of the coexisting melt. These methods are called amphibole single-phase thermometer, barometer, and melt-chemometer, and have been used in the previous ten years to investigate magma reservoir processes of subduction-related volcanoes. Here, the three methods are briefly introduced with their reliabilities. Then, we review the applications of these methods to clarify magma reservoir processes, chiefly using as examples three volcanoes of Kyushu, i.e. the Tsurumi-dake, Aso and Unzen volcanoes. The pressure-temperature-SiO2 content conditions of the melts estimated from amphiboles enable us to determine physicochemical conditions of the end-member melts of magma mixing, even for cases in which the mixed melt is perfectly homogenized and/or the end-member melt is chemically similar to the mixed melt. We could further identify a phenocryst mineral-melt disequilibrium in a magma, which is usually difficult to recognize from petrography and is a potential factor of misinterpretation for magma reservoir processes, based on the results. Furthermore, the estimated pressures constrain the depth conditions of magma plumbing systems, which can be cross-checked by the results of geophysical observations. These results demonstrate the usefulness of the methods for investigating magma reservoir processes.

著者

杉山 芙美子 長谷中 利昭 安田 敦 外西 奈津美 森 康

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-03-10

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阿蘇-4巨大噴火(89 ka)と阿蘇-3噴火(123 ka)の間には,何枚かのテフラ(A, B, C, D, , , M, N, α,β,,,η)が記載されている(小野ほか,1977).このうちAso-ABCDテフラは最上位に位置し,一連の噴火の産物だと考えられている.長橋ほか(2007)はAso-ABCDテフラ年代を97.7 kaと見積もっている.テフラの等層厚線から給源は阿蘇カルデラ内・中央火口丘群の南側に推定され(小野ほか,1977),噴出物の体積は3.5km3と見積もられている(町田・新井,1992).阿蘇-4火砕噴火の直前には大峰火砕丘の噴火とそれに伴う高遊原溶岩の流出が起こった.溶岩とそれを覆う阿蘇-4テフラの間に土壌を挟まないので,大きな時間間隙は考えられない.噴出物の体積は2 km3である.阿蘇-4後の中央火口丘群の活動中,最大の珪長質マグマの噴火は3万年前の草千里ヶ浜火山の軽石噴出で,体積は1.4km3(宮縁ほか,2003)に過ぎないので,阿蘇-4噴火前にある程度の大きさの噴火が続いたことがわかる.阿蘇カルデラ東方約20 kmの大分県竹田市荻町野鹿の露頭で阿蘇-4火砕流堆積物直下に層厚3mの降下軽石層と降下火山灰層の互層として露出するAso-ABCDテフラの軽石および火山灰を採集し,岩石記載および全岩XRF分析,鉱物のEPMA分析および鉱物中のメルト包有物のFT-IR分析をした.斑晶鉱物組合せは斜長石,斜方輝石,単斜輝石,マグネタイトで,阿蘇-4に普通にみられる普通角閃石は含まれなかった.軽石の全岩化学組成(SiO2= 63-66 wt.%)はKaneko et al. (2007, 2015) の阿蘇-4ではなく阿蘇-3組成トレンド上にプロットするものが多かった.斜長石や輝石に含まれるメルト包有物組成はSiO2=70-72 wt.%に集中し,Aso-3の組成とほぼ一致した.斑晶鉱物のコアの組成は,斜長石An40-64,斜方輝石Mg# =70-74,単斜輝石Mg#= 74-81であった.ホストの鉱物組成がAn40-61,斜方輝石Mg#=73-76,単斜輝石Mg#=76-79に対応するメルト包有物の含水量は1.0-4.8 wt.%と見積もられた.ホスト単斜輝石組成と含水量から見積もられる温度は860-950℃,圧力は1.1-2.7 kbar(Putirka,2008)であった.マグマ溜りの深度は地表から約3-9 km下となり,この深度は須藤ほか(2006)の草千里マグマ溜り深度6kmと一致する. 古川ほか(2006)は阿蘇-3,阿蘇-4間のテフラ組成,推定温度,推定含水量,推定酸素分圧が漸次変化していくことを示したが,本研究ではAso-ABCD組成はAso-3の組成に近いことが分かった.阿蘇-4噴火の9千年前にはAso-4組成のマグマ溜りはまだ存在していなかったか,あるいはAso-ABCDとは独立して相互作用なく成長していたことが考えられる.

著者

安田 敦 馬場 章 藤井 敏嗣 外西 奈津美

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.64, no.2, pp.83-101, 2019-06-30 (Released:2019-07-06)

参考文献数

52

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Gabbroic inclusions newly found in the Yakeno lava flow, erupted about 1300 years ago on the northwestern flank of Fuji volcano, are divided into two groups; one consists of troctolite and the other consists of olivine gabbronorite and gabbronorite. Troctolite fragments are considered to originate from shallow dykes because of their mineral composition and high porosity. On the other hand, estimated equilibrium conditions of gabbronorite and olivine gabbronorite are 1020-1050°C and 250-380MPa, showing their deep origin; probably they had been caught in the Yakeno magma (i.e. the magma erupted as the Yakeno lava flow) at differentiated small magma bodies just above a substantial basaltic magma chamber of Fuji volcano. Pre-eruptive temperature and water content of the Yakeno magma are evaluated by olivine-liquid thermometer and plagioclase-liquid hygrometer to be 1090°C and 2.8wt%, respectively. So the viscosity of the Yakeno magma is calculated to be several tens Pascal second. Therefore the minimum ascent velocity required to lift the gabbroic fragments is ca.10m/h, implying that the Yakeno lava flow had ascent without significant pause en route to the surface.

著者

種田 凌也 石橋 秀巳 安田 敦 外西 奈津美

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2019年大会

巻号頁・発行日

2019-05-17

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Plagioclase-melt compositional relations sensitively depend on temperature, T, and water content, XW, of melt. Therefore, the relations are formulated as functions of T and XW and used as geothermohygrometer (e.g., Putirka, 2008). Plagioclase-melt equilibrium is postulated for applying the thermohygrometer models. However, validity of the assumption is unobvious for microlite crystallization driven by degassing and/or cooling during eruption. Can we apply the thermohygrometers to the plagioclase-melt pairs formed by dynamic crystallization? To examine the issue, the cooling crystallization experiments of the high-Al basaltic melt were carried out and textural and chemical analyses were performed the run samples.We used the high-Al basalt lava from Waianae, Hawaii Oahu as the starting material for the cooling crystallization experiments. This experiment was conducted using the 1atm fO2-controlled furnace at University of Hawaii at Manoa. After 3 hours pre-heating at 1180℃, ~30K higher than the liquidus, the samples were cooled at cooling rates, Rc, of 0.1, 0.3, 1, 3, and 10K/min, and then quenched in water at four target temperatures, Tq, of 1150, 1120, 1090, and 1060℃. We used FE-EPMA (JEOL-JXA-8530FPlus) and EPMA (JEOL8800R) at Earthquake Research Institute, University of Tokyo, for textural and chemical analyses of the run samples.As Rc and/or Tq increase, the abundance and size of plagioclase crystals decrease. As Rc increases, the shape of plagioclase changes from euhedral to dendritic. The Rc-dependent shape change is more obvious at lower Tq. Pyroxene crystallization was suppressed in the run samples coolied at higher Rc. In addition, melt boundary layers are observed around pyroxene in the samples cooled at 0.3-1K/min, quenched at 1090℃ and cooled at 0.3-3K/min, quenched at 1060℃, and around plagioclase in the samples cooled at >3K/min, quenched at <1120℃. Lower diffusivity of Al2O3 compared to those of FeO and MgO is responsible for lower threshold cooling rate of melt boundary layer formation around pyroxene.The maximum An# [=100Ca/(Ca + Na)] of plagioclase was almost the same among all run samples. We applied both plagioclase liquidus and plagioclase-melt An-partitioning thermometers of Putirka (2008) to the pairs of plagioclase rim-boundary layer melt (BLM) and plagioclase rim-far field melt (FFM) to estimate temperatures recorded in their phase compositions. At Tq=1150℃, the estimated temperatures well represent Tq for both BLM and FFM. However, at lower Tq, both of the thermometers estimate temperatures higher than Tq; difference between estimated temperature and Tq, ΔT, increases as decreasing Tq. The two thermometers estimate similar temperatures for each sample, and the differences between the two estimated temperatures do not depend on Rc. These results suggest that the plagioclase-melt partition coefficient of An component does not depend on the Rc. Therefore, the increase of ΔT at lower Tq is not due to the Rc-dependence of the partition coefficient. It is attributed to the kinetic delay of the crystal growth at lower temperature. On the other hand, the coincidence between calculated temperatures and Tq at 1150℃ suggests that kinetic delay of plagioclase crystal growth is small at near liquidus temperature.Present results show that plagioclase-melt thermometers can be applied regardless of the Rc at higher temperatures, while magmatic temperature tends to be overestimated due to kinetic delay of crystal growth at lower temperature.

著者

新正 裕尚 古川 邦之 折橋 裕二 外西 奈津美 和田 穣隆

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.124, no.7, pp.533-538, 2018-07-15 (Released:2018-08-18)

参考文献数

13

被引用文献数

6

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岐阜県南部に分布する可児層群の最下部を占める蜂屋層は第一瀬戸内累層群東部で最も古い地層であり,主に非海成の火砕岩類からなる.蜂屋層最下部の栃洞溶結凝灰岩部層の溶結凝灰岩から分離したジルコンのレーザーアブレーションICP-MSによるU-Pb年代測定を行ったところ,238U-206Pb年代の加重平均として22.38±0.17Ma(2σ)が得られた.この年代は蜂屋層の堆積開始時期を拘束する.分析を行った溶結凝灰岩試料の蛍光X線分析による全岩主成分・微量元素組成を併せて報告する.