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Spatial Evaluation of Java Arc Volcanics Geochemistry

阿蘇中央火口丘群北西部において完新世に活動した火山のスコリアを採取し斑晶メルト包有物を分析した結果,阿蘇火山における今までの報告値よりも高いS含有量をもつマグマが見つかった.また石基ガラスとメルト包有物,斑晶鉱物組成の比較から浅部マグマ溜まりへの高Sマグマの供給が示唆された.

阿蘇4火砕噴火の前駆噴火である大峰火山噴火について,鉱物中のメルト包有物の化学組成を測定した.得られた組成を阿蘇4火砕流堆積物のサブユニットの軽石中の鉱物に含まれるメルト包有物組成と比較し,阿蘇4噴火直前のマグマ供給系について考察を行った.大峰スコリアのメルト包有物と阿蘇4メルト包有物とのトレンドの違いから,大峰火山の噴火が阿蘇4巨大マグマ溜まりとは異なった浅部マグマ供給系で独立して分化したことがわかった.

研究課題「阿蘇山の噴火活動・マグマ水蒸気爆発を理解する」 2017年2月27日(月)-28(火), 於: 熊本大学教育学部(本館3-A講義室), 研究代表者: 横尾 亮彦

カルデラ形成噴火に至るマグマ供給システムの進化過程は,カルデラ形成噴火の発生メカニズムの解明や長期噴火予測のため重要な研究対象である（小林・他,2010）.姶良カルデラ火山の10万年前から現在までに活動した火山噴火物の化学組成の変化を調べたところ,3つのマグマ活動サイクルが認められたので,これを報告する.最初の2つのサイクルはマフィックなマグマの活動から始まり,最後に珪長質なマグマを大量に噴出する巨大噴火で終わった.現在の桜島火山の活動は,マグマの組成が第1,第2サイクルとは明瞭に異なっているので,第3サイクルに入っていると考えられる.

西日本火山研究の一環として火山岩の化学分析値のデータベース作りを目指している.現在利用できる地球化学データベースのレビューを行い,既存のデータベースがどの程度,利用できるか調べてみた.次に今後の研究にはどのようなデータベースが必要かを考えてみた.プロジェクトの目的によって必要とされるデータベースの内容は異なってくるが,将来他のプロジェクトでも利用することを考えれば,火山岩の化学分析値だけでなく,地質学,記載岩石学などの情報も必要になってくる.近年の野外調査や顕微鏡視察ではふつうに画像データが取り扱われており,画像データを組み込んだデータベース作りが必要である.

The Azas Plateau is a part of the East-Tuva Highland (West Sayan ridge, the south of Krasnoyarsk Region, Siberia). It is the westernmost lava field of the Baikal Rift System, covering about 2000km^2. A volume of volcanics is estimated as high as 600km^3. The volcanics are represented by massive lava flows and thick hyaloclastite units. Their composition corresponds to alkaline basalts and basanites. The most intensive volcanic activity started about 2 Ma from basaltic fissure eruptions producing voluminous lava flows. The second stage voluminous eruptions formed the upper lava unit on the surface of moderately eroded lower lava flows. During the next stages, the eruptions became centralized so that several volcanic centers were formed along certain lineaments marking deep-seated fissures. The peculiarity of the volcanic activity is that at least 7 volcanoes were formed during extensive glaciation covering the entire area of plateau. Moreover, the subglacial eruptions occurred about 0.7 and 0.1 Ma ago, whereas there was no sign of glaciation about 0.29 Ma ago, when Yurdawa shield volcano erupted. The subglacial eruptions formed table mountains (tuyas) with resembling structure of thick lower hyaloclastite units and armoring upper lava flows. The composition of lavas is rather uniform with slight variations in acidity and alkalinity of volcanics during eruptions. The most voluminous eruptions produced trachybasalts, smaller volcanoes are composed of tephrites/basanites. The basalts have typical for silica undersaturated rocks normalized REE patterns with regular enrichment in LREE and depletion in HREE. REE patterns, in accordance with eruption volumes, show lower degrees of partial melting for tephritic volcanoes (La/Yb=12-17) and higher degrees for trachybasalts (La/Yb=5-9). Correlation between La/Yb and Mg# indicates the formation of basalts by chemically identical source with one definite exception (Yurdawa volcano). The trace element compositions plotted on chondrite-normalized spidergram have OIB-like patterns., 論文, Article

第14回宇宙科学シンポジウム (2014年1月9日-10日. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS)相模原キャンパス), 相模原市, 神奈川県, 14ｔh Space Science Symposium (January 9-10, 2014. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS)Sagamihara Campus), Sagamihara, Kanagawa Japan, 著者人数: 24名, 資料番号: SA6000058202, レポート番号: P2-170

論文, Article

Spinel Iherzolite, dunite, wehrlite, and clinopyroxenite xenoliths in the Pleistocene alkaline basalts from Wangqing, Jilin Province, northeastern China were examined to provide chemical and mineralogical data of the continental mantle beneath northeastern China. Spinel Iherzolite seems to represent chemical composition of the continental upper mantle beneath northeastern China, and no distinctive chemical difference can be recognized between oceanic and continental mantle. The recorded temperature in the spinel Iherzolites ranges from approximately 1100℃ to lower than 800℃. The investigated spinel Iherzolites should represent the continental mantle materials of variable depths beneath northeastern China. In contrast, the dunite and clinopyroxenite retain rather low equilibrium temperature: it ranges from 850℃ to 750℃. The geothermal gradient beneath northeastern China is similar to that beneath the oceanic region. Although their host alkaline basalts are highly enriched in K_2O, the Na_2O contents of them are not so considerable. Neither significant chemical discrepancy nor essential inconsistency of geothermal gradient has been emphasized between the continental mantle material investigated in this paper and oceanic mantle material reported elsewhere. It should be significant, however, to point out that the studied mantle Iherzolites are highly magnesian and depleted compared with the Hawaiian Iherzolites., 論文, Article

Geochemical investigation has been carried out for the volcanic rocks from Ulreung island, Korea. The volcanic rock suite is characterized by highly alkalic nature, and some of them contain more than 6wt.% K_2O and Na_2O. The igneous activity may not be related to Pacific plate subduction, or the Japan Sea spreading. It will be hardly possible to explain how to concentrate the abundant incompatible elements in the mantle source in a limited short period beneath Ulreung island. Most of the chemical features of the volcanic rocks are consistent with simple fractional crystallization. It should be pointed out, however, that significant compositional gap was recognized only in K_2O content, but not in the other LILEs. The difference between K_2O and the other LILEs will be hardly explained if fractional crystallization differentiation plays a major role. The volcanic rocks from the island are either highly enriched in K_2O or not so enriched in K_2O. No intermediately K_2O-enriched sample was found. This is contrasted with gradual increase in Na_2O content. This chemical features will require complicated mechanisms., 論文, Article

Websterite and hornblendite inclusions in the alkaline basalts from Kuandian, Liaoning province, northeastern China were investigated to clarify the physico-chemical conditions of the continental mantle beneath northeastern China. Compressional stress seems prevailing in the mantle beneath Kuandian. The websterites were produced in the uppermost mantle at approximately 1000℃. The geothermal gradient in northeastern China is rather steep, and may resemble to the oceanic geothermal gradient. The continental mantle beneath northeastern China may have considerable potential power to generate magma and to promote igneous activity., 論文, Article

第43回月・惑星シンポジウム (2010年8月4-6日. 宇宙航空研究開発機構宇宙科学研究所), 相模原市, 神奈川県, 43rd ISAS Lunar and Planetary Symposium (August 4-6, 2010. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan, 著者人数: 18人, Number of authors: 18, 資料番号: AA0065116029