Akagi Volcano Research Articles

A1-33 赤城火山の噴火履歴の再検討と定量化(防災と噴火史・年代,口頭発表)

A1-33 赤城火山の噴火履歴の再検討と定量化(防災と噴火史・年代,口頭発表)

A22 赤城火山噴出物の全岩主化学組成 : 榛名火山との比較

A22 赤城火山噴出物の全岩主化学組成 : 榛名火山との比較

P41 赤城山と栗駒山の歴史時代の噴火記録(ポスターセッション)

P41 赤城山と栗駒山の歴史時代の噴火記録(ポスターセッション)

Magma-discharge rate and geochemical evolution during the pumice-eruption stage of Akagi Volcano, NE Japan.

赤城火山，新期成層火山の軽石流堆積物と降下軽石堆積物の対比と噴出物のマグマ体積の計測を行い，マグマ噴出量時間階段図を作成した．火山体形成期（約22－15万年前）に続く軽石噴火期には，5.8万年前の赤城カルデラ形成噴火前に，マグマ噴出率の低下期が起きている．軽石の微量元素成分組成の特徴は，K2O量の高い珪長質マグマほど下部地殻部分溶融メルトの関与が大きいことを示しており，下部地殻へのマグマの大量貫入を契機にマグマ噴出率などの活動様式の変化が起きたことを示唆している．

Considerations on the Geomorphological and Geological Maps of Akagi Volcano

Considerations on the Geomorphological and Geological Maps of Akagi Volcano

赤城火山起源の後期更新世テフラの記載岩石学的特徴

群馬県桐生市黒保根町水沼の露頭にみられる主に赤城火山起源の後期更新世テフラについて記載岩石学的特徴の検討を行った．層厚の大きいテフラについては，降下ユニットごとに検討し記載した．その結果，Ag-Nm2，Ag-UP，Ag-KPには同一テフラ内でも異なる屈折率を示す降下ユニットが存在することが明らかになった．このような降下ユニットごとの屈折率特性の差異は今後のテフラの対比・同定に有効であると考えられる．またAg-MzP3，Ag-MzP2，Ag-Nm2，Ag-UP の斜方輝石・角閃石の屈折率は，一般的な赤城火山起源テフラの屈折率から外れた特徴的な値を示し，対比に重要と考えられる．

Hemoilmenite as a carrier of SRTRM in dacitic pumice from Akagi, Ontake and Sambe Volcanoes, Japan

Hemoilmenite crystals (0.7>x>0.4, x: ilmenite molecular fraction in ilmenite-hematite solid solution series) in dacite pumice in Japanese arc volcanoes are common. However, only a small percentage of those pumices show classic self-reversed thermo-remanent magnetization (SRTRM), such as that first found in a Haruna dacite pumice. The dacite pumice samples from Akagi Volcano in central Japan, about 110 km NW of Tokyo (N36°33′, E139°12′), however, acquired an intense SRTRM like that of the Haruna Volcano samples. Bitter patterns of the oriented hemoilmenite crystals from both the Haruna and Akagi volcanoes indicate that ordered ferrimagnetic and disordered weak-ferromagnetic phases are inter-grown in relatively comparable amounts, with an irregular texture. This pattern is consistent with rapid quenching of these pumices. We conclude that not only the chemical composition (TiO2-content or x) of the hemoilmenite, but also the cooling speed or thermal condition on cooling control SRTRM-acquisition in hemoilmenite of appropriate composition.

Geochemical Evolution of Akagi Volcano, NE Japan: Implications for Interaction Between Island-arc Magma and Lower Crust, and Generation of Isotopically Various Magmas

Major and trace element, and Sr, Nd and Pb isotopic compositions lower-crustal assimilation; magma mixing were determined for whole-rock samples from the ‘isotopically anomalous’ Akagi volcano in the volcanic front of the NE Japan arc. Sr and Nd isotopic compositions of phenocrysts were also analyzed together with their major and trace element compositions. Compared with the other volcanoes from the volcanic front, the INTRODUCTION whole-rock isotope compositions of Akagi show highly enriched Island-arc volcanism has been considered to result from characteristics; 87 Sr/ 86 Sr = 0·7060–0·7088, Nd =− 0·40 partial melting of the wedge mantle induced by addition to −8·6, and 208 Pb/ 204 Pb = 38·4–38·8. The rare earth of slab-derived fluid-rich materials in subduction zones. element (REE) patterns are characterized by heavy REE (HREE) This general model is supported by many geochemical depletions with U-shaped patterns from middle REE (MREE) to studies employing major and trace element compositions, HREE, suggesting that amphibole fractionation was induced by a and radiogenic isotope systematics of arc volcanic ejecta reaction between clinopyroxene and H2O-rich magma in the lower (e.g. Nakamura et al., 1985; Woodhead & Fraser, 1985; crust. The integrated isotope and trace elements systematics, and Tatsumi et al., 1986; Ishikawa & Nakamura, 1994; Miller tectonic structure beneath Akagi volcano, suggest that lower-crustal et al., 1994; Ryan et al., 1995; Shibata & Nakamura, assimilation by the H2O-rich primary magma could have been 1997). These previous works mainly focused on the source affected by the double subduction of Philippine Sea and Pacific characteristics in relation to the evolution of the mantle oceanic plates. This double subduction could have supplied larger wedge in subduction zones. The magmatic evolution of amounts of water to the magma source region in the wedge mantle individual volcanoes, after partial melting of the wedge than in the case of a single subduction zone. Significant differences mantle, and in shallow magma chambers has received in isotopic compositions are observed between phenocrysts and the less attention. Isotope and trace element geochemistry coexisting melts. Such isotopic disequilibrium may have resulted has typically been restricted to basaltic volcanic rocks, from magma mixing between an isotopically depleted aphyric and not the felsic rocks, to avoid the complexities of processes an enriched porphyritic magma in a shallow magma chamber. such as crystal fractionation, magma mixing and crustal The geochemical characteristics of these end-member magmas were assimilation. It is, however, essential to characterize the retained in the lower crust, despite differing extents of lower-crustal shallower magmatic processes along with the source characteristics. Magmas erupted at the Earth’s surface assimilation by the H2O-rich magmas.

Multistage Magma Mixing Revealed in Phenocryst Zoning of the Yunokuchi Pumice, Akagi Volcano, Japan

Detailed study of the texture and chemical zoning of phenocrysts in a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, Japan, permits reconstruction of the subvolcanic magma chamber. Hypersthene phenocrysts are complexly zoned; some have spongy cellular textures whereas others exhibit simple reverse zoning. Augite phenocrysts have texturally similar cores but Al and Ti vs mg-number zoning patterns are correlatable with hypersthene cores. Most plagioclase phenocrysts have a uniform calcic core enclosed by a normally zoned rim; others have variable An contents and mottled textures. These characteristics and melt (glass) compositions suggest a complex, multistage mixing of lot-T (940–960°C) highly silicic and high-T (980–1060°C) less silicic magmas. Stratigraphic variations of the phenocryst types show that the earlier erupted magma inherited phenocrysts from more high-T magma components. We propose the presence of an inhomogeneous mushy chamber filled with a highly silicic magma which was recharged periodically by less silicic magmas. The large viscosity contrast and density balance of the new magma relative to the mush controlled the efficiency of mixing and the position of emplacement of the high-T magma in the mush. Tapping the chamber first expelled the fluidal mixed magma. Followed by progressive erosion of the mush as the eruption continued.

Multistage Magma Mixing Revealed in Phenocryst Zoning of the Yunokuchi Pumice Akagi Volcano, Japan

Detailed study of the texture and chemical zoning of phenocrysts in the compositional and thermal histories at the sites a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, where they grew. Only recently have such studies been Japan, permits reconstruction of the subvolcanic magma chamber. undertaken focusing on magma chambers as the sites Hypersthene phenocrysts are complexly zoned; some have spongy of phenocryst formation (Tait, 1988; de Silva, 1989; cellular textures whereas others exhibit simple reverse zoning. Augite Umino & Iwano, 1992; Fodor et al., 1993; Nakada et phenocrysts have texturally similar cores but Al and Ti vs mgal., 1994). The magma chambers depicted by these number zoning patterns are correlatable with hypersthene cores. studies are mainly composed of phenocryst-poor magma Most plagioclase phenocrysts have a uniform calcic core enclosed surrounded by a comparatively thin crystallizing boundby a normally zoned rim; others have variable An contents and ary layer. Phenocrysts are interpreted to have crysmottled textures. These characteristics and melt (glass) compositions tallized either within the main body of magma, or in suggest a complex, multistage mixing of low-T (940–960°C) situ within the boundary layer. highly silicic and high-T (980–1060°C) less silicic magmas. Combination of geophysical and petrological data, Stratigraphic variations of the phenocryst types show that the earlier together with fluid dynamical simulations of magmatic erupted magma inherited phenocrysts from more high-T magma processes beneath mid-oceanic ridges has shown that a components. We propose the presence of an inhomogeneous mushy mush of crystals and interstitial melt (25–55 vol. % chamber filled with a highly silicic magma which was recharged crystals) probably makes up the majority of the magma periodically by less silicic magmas. The large viscosity contrast and chamber and only a small volume exists as a meltdensity balance of the new magma relative to the mush controlled enriched lens (e.g. Sinton & Detrick, 1992; Nicolas et al., the efficiency of mixing and the position of emplacement of the high1993; Barth et al., 1994). Likewise, it is highly probable T magma in the mush. Tapping the chamber first expelled the that magma chambers beneath island-arc volcanoes fluidal mixed magma, followed by progressive erosion of the mush largely consist of a mush of crystals and interstitial melt. as the eruption continued. Recent studies on arc volcanic products have presented evidence for the existence of such ‘mushy’ magma chambers. These include an estimation of very high crystallinity

Sr, Nd and Pb Isotope Systematics of Akagi Volcano, Northeast Japan. Implications for Interaction between Island Arc Magma and Lower Crust.

Isotopic compositions of Sr, Nd and Pb and REE concentrations have been determined for samples from“isotopically anomalous”Akagi volcano locating on the volcanic front of Northeast Japan Arc. The Sr isotopic compositions show a large variation with highly enriched isotope character (87Sr/86Sr=0.7060 to 0.7088) compared to those from other volcanoes on the front. The Nd isotopic compositions, ranging from eNd=-0.40 to -8.6, have a negative correlation with the Sr isotopic compositions. The Pb isotopic data along with Sr and Nd isotope systematics clearly indicate that the isotopic variations of Akagi volcano were caused by a two-component mixing between an end member isotopically similar to the primary magma on the volcanic front of Northeast Japan Arc and a lower crustal component. Chondrite-normalized REE patterns of Akagi samples show U-shaped, HREE-depleted pattern with positive Eu anomaly. Such REE features may have been developed by fractionation of amphibole formed by the reaction between a fluid-rich magma and clinopyroxene in granulitic lower crust. The fluid-rich magma could be originated from a highly metasomatized mantle wedge caused by the dehydration of oceanic slabs of the Pacific plate and the overlapping Philippine Sea Plate. Such a unique tectonic setting could result in higher water supply to the source region of magma than normal circumstances and thereby generating fluid-rich magma which could enhance assimilation of lower crust beneath Akagi volcano.

北関東那須野原周辺に分布する指標テフラ層

The purpose of this paper is to present the stratigraphy, distributions and characteristic properties of tephra layers around Nasuno Plain in North Kanto, central Japan. This tephrostratigraphical study is carried out as a basic work for the applicative tephra study which will contribute to volcanology and Quaternary study of this region in late Quaternary.43 tephra layers deposited in the latter half of middle Pleistocene are identified. The tephra layers derived from nearby volcanoes located in North Kanto are as follows; 12 tephra layers from Nasu Volcano, four tephra layers from Takahara Volcano, 17 tephra layers from Nikko Volcanic Group and one tephra layer from Akagi Volcano. Two widespread tephra layers named APm-U and APm-L (300-350 ka) derived from Momisawadake Volcano in the Hida Mountains are identified. However, the sources of seven tephra layers remain unknown. Ages of these tephra layers are estimated to be younger than 400 ka on the basis of thickness of volcanic soil and ages of several dated tephra layers.On the other hand, 32 tephra layers formed in late Pleistocene are described. They include the following five well-known widespread tephra layers, that is, AT (22-25 ka), DKP (48 ka), Ng P, Aso-4 (86-90 ka), On-Pm I (80-95 ka). They are derived from distant volcanoes locating in central and southwest Japan. At the same time, 25 local tephra layers from Akagi, Nasu Volcanoes and Nikko Volcanic Group are recognized.

テフロクロノロジーからみた赤城火山最近２０万年間の噴火史

Akagi volcano situated in the North Kanto district of central Japan is a Quaternary volcano. The eruptive history of this volcano during the last 200, 000 years is clarified by the tephrochronological study.The plinian pumice fall deposits derived from Akagi volcano are as follows in ascending order; the Moka Pumice (MoP), Akagi-Mizunuma Pumice-16. …-12, -10…-1 (MzP-16…-12, -10…-1), Namekawa Pumice-2, -1 (Nm-2, -1), Yunokuchi Pumice (UP) and Kanuma Pumice (KP) (Fig. 1). Stratigraphy, distributions and petrographic characteristics of these tephras are described (Figs. 5, 6, 8, 9 and Tables 1, 3).The MoP pumice fall deposit covering the most part of the eastern part of the North Kanto district, erupted in the penultimate glacial stage preceding the Last Interglacial Stage. Moreover, the stratigraphic relations of the MzP-10…-1, Nm-2, -1 and UP with the well dated widespread tephras, which are the K1P-7 (ca. 130 ka), DPm, On-Pm I (ca. 80 ka), K-Tz (ca. 75-80 ka), Aso-4 (ca.. 70 ka) and DKP (ca. 45-48 ka), are clarified (Fig. 7). These data can give the chronological framework for the eruptive history.The MzP series, Nm-2, Nm-1 and UP erupted during the stage called the younger stratovolcano (YS) of the Akagi volcano in previous work (Fig. 10). The total volume of the plinian pumice fall deposits from the MzP-10 to the UP amounts to 28 km3. This corresponds to the discharge rate of the pumice equivalent to 0.33 km3/1, 000 years and the frequency of the plinian eruption 0.15/1, 000 years.Before the formation of the central cone, it occurred the most eruptive episode of Akagi volcano. This is represented by the members : the KP pumice fall deposit (ca. 31-32 ka) and the Mizunuma lithic (chert lapilli) fall deposit (CLP). The volume of the KP deposit which amounts to 25 km3, is the largest volume of the plinian pumice fall deposits derived from Akagi volcano.

47B. Tephrochronological study of the Akagi volcano.(Abstracts of Papers Presented at the Fall Meeting of the Society)

47B. Tephrochronological study of the Akagi volcano.(Abstracts of Papers Presented at the Fall Meeting of the Society)

尾瀬地方に分布する第四紀火山岩の岩石記載および全岩化学組成

Compositions of Quaternary volcanic rocks from Oze area, central part of Honshu arc show trends inconsistent with the general trend of the arc. Analyzed volcanic samples include those from Pre-Nikura, Ooeyama, Chouzugatama volcanoes of shield volcano group, those from monogenetic volcano group, and those from Nikura volcano of stratovolcano group, younging in this order. They are mainly andesites but minor augite basalts are also found in Pre-Nikura volcano. Dacitic andesites of stages Nikurayama and Shiraoyama of Nikura volcano contain basic inclusions and show disequilibrium features such as quartz with pyroxene-jacket, plagioclase with dusty-zone and coexistence of magnesian olivine and quartz. Most samples from Nikura volcano and monogenetic volcano group plot in the field of highalmina basalt series in the alkali vs. silica diagram. Whereas, samples of stage Nikurayama of Nikura volcano and shield volcano group plot in the field of tholeiite series. Samples of all the groups show higher K2O/Na2O ratios than those of Nasu volcanic zone to the north, reflecting a regional character. Interestingly they are richer in K2O content than those from Niigata prefecture, to the side farther from the trench. When major element compositions of lavas are estimated at 60% silica for each volcano of central part of Honshu by linear regression, they show increase of K2O content and K2O/Na2O ratio from Akagi volcano to Oze-Hiuchigatake volcano. In contrast, the Na2O and CaO contents slightly decrease in the same direction.

火山灰土壌の腐植特性の空間分布の推移にみる環境変化 赤城火山南麓における事例

By a series of studies to clarify the humus characteristics of volcanic ash soils and soil forming environment, the influence of climate variation was recognized in the eastern foot area of Nantai Volcano, Nikko (Watanabe 1985, 1987). The purpose of this study is to obtain useful information for the reconstruction of global scaled environmental changes from spatial distribution of soil humus characteristics. Volcanic ash soils developed in the southern foot area of Akagi Volcano were examined as a case.The result is summarized as follows.1. Soils developed over southern foot area of Akagi Volcano show several types of soil profiles regulated by the thickness and rate of tephra deposits and geomorphic surfaces. On mountain slopes formed by pyroclastic flow, brownish volcanic ash bed are to be seen between surface and buried humus layer. Soils distributed on central cone lacustrine surface have thick and dark humus layer called “gleyed Kuroboku”. Soils with light humus layer develop on piedmont surface under the influence of artificial activity. The brownish volcanic ash bed mentioned above is difficult to distinguish from this layer.2. The chronological sequence of soil formation in the study area was considered on the basis of properties of soil parent materials and radiocarbon dates. Horizon I is the layer which continues to accept humus supply and humification under soil forming environment in the present. Fresh volcanic ash errupted from Asama and Haruna Volcano, about 200-1700 years ago are included in this layer. Horizon II correspondent to the brownish volcanic ash bed has accepted humus substances from the upper layer. Though the source volcano is unknown, this layer is assumed to be deposited about 4, 000 years ago. Horizon III, a buried humus layer intercepted from surface environment, started soil formation about 9, 000 years ago following to the erruption of Asama Itahana Yellowish Pumice (YP).3. The vertical distribution of humus characteristics (brightness of soil color, carbon content and humus composition) was classified into two types, which reflects the conditions of vegetation and groundwater level regulated by gemorphic surface.4. The spatial distribution patterns of humus characteristics such as carbon content and RF value of humic acid in each horizon show vertical zonality. The principal areas of humus accumulation in the age of horizon II and III formation were located in higher location than present, about 400m upward. Humic acid classified such as type A+, P+ +, have remarkable absorption spectra near 615, 570, 455nm which are regarded to be the features of soils formed under vegetations such as Fagus, Quercus mongolica and also under cool and wet condition in the study area. The appearance of these humic acid types in each horizon also tend to move its location upward by 300m in the age of horizon III formation.

Two overlapping plates subducting beneath central Japan as revealed by strontium isotope data

In central Japan, the Pacific plate subducts westward beneath the Eurasian plate and the Philippine Sea plate subducts northwestward into the mantle wedge between the Eurasian plate and the subducted Pacific slab. There, the Northeast Japan arc is joined to the Izu-Ogasawara arc. We determined 87Sr/ 86Sr ratios and Rb and Sr contents for 47 volcanic rock samples from 15 Quaternary volcanoes in central Japan and summarized the geographical distribution of the ratios. The general trend of slowly increasing 87Sr/ 86Sr ratio from the back-arc side toward the volcanic front in the Northeast Japan arc is broken by a marked high ratio (above 0.7060) centered around Akagi volcano located at the southernmost region of the arc. Elsewhere, the ratio along the volcanic front in this arc varies within the range 0.7038 to 0.7045. The marked high 87Sr/ 86Sr ratio is considered to be due to the addition of slab-derived components transported by the Philippine Sea plate to the magma-generating region in the mantle wedge beneath central Japan. Therefore, the geographical distribution of the high ratio may correspond to that of the Philippine Sea slab-derived components in the mantle wedge and we may draw the underground outline of the Philippine Sea plate. This outline implies that an aseismic portion of the Philippine Sea plate continues a few tens km ahead of the seismic one. A belt of low 87Sr/ 86Sr ratios from the Izu Peninsula northwestward along the northern end of the Izu-Ogasawara arc coincides with the zone where the subducting Philippine Sea plate is not observed seismologically, while it is detected seismologically on both sides of the belt.

Mantle contamination under Akagi Volcano, Japan, as inferred from combined Sr‐O isotope relationships

The Northeast Japan arc is associated with westward subduction of the Pacific plate beneath the Eurasian plate. At the southernmost region of the arc, the Philippine Sea plate subducts northward into the mantle wedge between the Eurasian plate and the Pacific slab. Akagi volcano located at this region yields volcanic rocks with combined Sr‐O isotope relationships which are explained in terms of the mixing of the mantle material from the mantle wedge and deep ocean and/or continent derived sediments conveyed with the subducting Philippine Sea plate. This implys that the Philippine Sea plate continues to the location just under Akagi volcano, although the seismic Philippine Sea plate does not reach just under the volcano.

Petrology of Akagi volcano(Abstracts of Papers Presented at the Spring Meeting of the Society, 1982)

Petrology of Akagi volcano(Abstracts of Papers Presented at the Spring Meeting of the Society, 1982)

北関東ローム層中の水銀濃度

The purpose of this investigation is to find the total mercury concentration in the weathered volcanic ash bed (so-called Kanto loam) in a non-polluted area and to compare the values with those of aqueous sediments and surface black soils which are more or less polluted. The 26 samples were collected systematically according to the columnar section of Kanto loam at Minasawa, on the southern foot of Akagi volcano. These samples were powdered, dried at room temperature and analyzed by an atomic absorption spectrometry with a reduction-vapor phase technique. Data show a considerable range from 0.077 ppm to 0.005 ppm. The average of the primary mercury concentration in northern Kanto loam is estimated to be of 0.033ppm.