Petrology of arc volcanic rocks and their origin by mantle diapirs

Calc-alkalic chemical trends characteristic of arc volcanic rocks mainly result from three mechanisms which act additively: (1) fractional crystallization involving separation of titanomagnetite; (2) selective concentration of plagioclase phenocrysts and selective depletion of titanomagnetite phenocryst compared with the actually fractionated proportion; and (3) mixing of magmas on continuous fractionation trends. The association of calc-alkalic and tholeiitic trends in a single composite volcano may not represent different fractional crystallization processes or different chemistries of primary magmas, but the calc-alkalic chemical trend can be considered as a mixing trend resulting from mixing of various magmas on associated tholeiitic chemical trends. Chemical variations of most arc volcanic rocks, including calc-alkalic ones, can accordingly be essentially accounted for by the low-pressure fractional crystallization of phenocrystic phases from primary basaltic magmas.Crystallization sequences of arc magmas which are strongly dependent on water content in magmas are deduced from the phenocryst assemblages. The crystallization sequence changes laterally across-arc, suggesting increasing water contents in magmas toward the back-arc side, as is also seen for other incompatible elements such as K and Rb. Systematic differences in the characteristic crystallization sequence are also observed among arcs, roughly correlating with the crustal thickness. Water content in magma, like other incompatible elements, tends to increase with increasing crustal thickness. The variation of incompatible elements including water roughly represents that of the degree of partial melting of the upper mantle, which is broadly controlled by the crustal thickness.The variation of water content indicates that arc magmas are not saturated with water during differentiation to late differentiates such as dacite or rhyolite. This strongly constrains the maximum water contents in primary basaltic magma, at most 2.5 wt.%. This value suggests that magma generation beneath arcs is dependent on dry solidus of peridotite. Diapiric uprise of the hot deeper mantle and associated adiabatic decompression would be necessary for mantle peridotite to attain the temperature as high as dry solidus. Diapirs that begin to rise from the subduction zone may stop at or near the crust-mantle boundary because of the surrounding density change, and their degree of partial melting is roughly controlled by their stopped depth assuming their similar temperature. Across-arc variation is also explained by the stopped depth of diapirs, but is not controlled by crustal thickness.     

Middle Miocene to Quaternary primary basalt and high magnesian andesite magmas of North Hokkaido, Japan: Source mantle characteristics and degrees of partial melting

Abstract   Middle Miocene to Quaternary primitive basalts and high magnesian andesite (HMA) in North Hokkaido resulted from three periods of intense volcanism; early-stage (12–10 Ma), middle-stage (9–7 Ma) and late-stage (3–0 Ma). Based on the chemical compositions of olivines and chromian spinels and bulk chemistry of the primitive rocks, we examined depths of segregation of the calculated primary magmas and the degrees of partial melting of the source mantle. In the context of asthenospheric mantle upwelling, petrological data from the present study can be accounted for by the secular change in the depth of magma segregation from the upwelled asthenospheric mantle, which is composed of fertile peridotite. Thus, the early-stage primary magmas were generated by higher degrees of partial melting of the shallower part of hot asthenospheric mantle, whereas the middle- and late-stage primary magmas resulted from lower degrees of partial melting of a deeper part of the asthenospheric mantle. The early-stage HMA magma was generated by partial melting of the remnant subcontinental lithospheric mantle composed of refractory peridotite. This melting might have resulted from an increased geothermal gradient caused by upwelling of hot asthenosphere.     

Origin of high-magnesian andesites in the Setouchi volcanic belt,southwest Japan,II. Melting phase relations at high pressures

Melting phase relations of an augite-olivine high-magnesian andesite and an augite-olivine basalt from the Miocene Setouchi volcanic belt in southwest Japan have been studied under water-saturated, water-undersaturated and under anhydrous conditions. Both the andesite and the basalt are characterized by low FeO*/MgO ratios (0.86 and 0.76 in weight, respectively) and qualify as primary magmas derived from the upper mantle.The andesite melt coexists with olivine, orthopyroxene and clinopyroxene at 15 kbar and 1030°C under water-saturated conditions, and at 10 kbar and 1070°C under water-undersaturated conditions (7 wt.% H₂O in the melt). The basalt-melt also coexists with the above three phases at 11 kbar and 1305°C under anhydrous conditions, and at 15 kbar and 1205°C in the presence of 4 wt.% water.Present studies indicate that high-magnesian andesite magmas may be produced even under water-undersaturated conditions by partial melting of mantle peridotite. It is suggested that two types of high-magnesian andesites in the Setouchi volcanic belt (augite-olivine and bronzite-olivine andesites) were produced by different degrees of partial melting; augite-olivine andesite magmas, whose mantle residual is lherzolite, were formed by lower degrees of partial melting than bronzite-olivine andesite magmas, which coexist with harzburgite. The basalt magmas, which were often extruded in close proximity to the high-magnesian andesite magmas, are not partial melting products of a mantle peridotite which had previously melted to yield high-magnesian andesite magmas.     

Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)

A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (<5 Ma?) of the Carnegie Ridge has permitted the progressive warming of the oceanic crust and its partial melting since ca. 1.5 Ma. Since then, the production of adakites in fore arc position has deeply transformed the magma genesis in the overall arc changing from ‘typical’ calc-alkaline magmatism induced by hydrous fluid metasomatism, to the space- and time-associated lithology adakite/high-Mg andesite/adakite-like andesite/high-Nb basalts characteristic of slab melt metasomatism.     

Boninite as a possible calc-alkalic primary magma

Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO₂ content of ? 55%. The relation of SiO₂ to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high P_H2_O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.     

Relations of cinder cones to the magmatic evolution of Mount Mazama,Crater Lake National Park,Oregon

Cinder cones at Crater Lake are composed of high-alumina basaltic to andesitic scoria and lavas. The Williams Crater Complex, a basaltic cinder cone with andesitic to dacitic lava flows, stands on the western edge of the caldera, against an andesite flow from Mount Mazama. Bombs erupted from Williams Crater contain cores of banded andesite and dacite, similar to those erupted during the climatic eruption of Mount Mazama.Major- and trace-element variations exhibit an increase in incompatible elements and a decrease in compatible elements, consistent with crystal fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, and magnetite. LREE patterns in the rocks are irregular; each successive basalt is enriched in LREE relative to the preceding andesite.Compositional variations in the magmas of the cinder cones suggest that three magmatic processes were involved, partial melting, fractional crystallization, and magma mixing. Partial melting of more than one source produced primary basaltic magma(s). Subsequent mixing and fractional crystallization produced the more differentiated basaltic to andesitic magmas.     

Melting experiments on a high-magnesian andesite

Melting experiments were conducted on a high-magnesian bronzite olivine andesite (Teraga-Ike andesite) which is considered to be a primary andesite. The high-magnesian andesite magma is in equilibrium with both olivine and orthopyroxene at about 15.5 kbar and 1080°C under H₂O-saturated conditions and at lower pressure and higher temperature under H₂O-undersaturated conditions. This suggests that high-magnesian andesites could be generated by the partial melting of upper mantle peridotite containing a small amount of H₂O.     

Adakitic magmas in the Ecuadorian Volcanic Front: Petrogenesis of the Iliniza Volcanic Complex (Ecuador)

The Iliniza Volcanic Complex (IVC) is a poorly known volcanic complex located 60 km SSW of Quito in the Western Cordillera of Ecuador. It comprises twin peaks, North Iliniza and South Iliniza, and two satellite domes, Pilongo and Tishigcuchi. The study of the IVC was undertaken in order to better constrain the role of adakitic magmas in the Ecuadorian arc evolution. The presence of volcanic rocks with an adakitic imprint or even pristine adakites in the Ecuadorian volcanic arc is known since the late 1990s. Adakitic magmas are produced by the partial melting of a basaltic source leaving a garnet rich residue. This process can be related to the melting of an overthickened crust or a subducting oceanic crust. For the last case a special geodynamic context is required, like the subduction of a young lithosphere or when the subduction angle is not very steep; both cases are possible in Ecuador. The products of the IVC, made up of medium-K basaltic andesites, andesites and dacites, have been divided in different geochemical series whose origin requires various interactions between the different magma sources involved in this subduction zone. North Iliniza is a classic calc-alkaline series that we interpret as resulting from the partial melting of the mantle wedge. For South Iliniza, a simple evolution with fractional crystallization of amphibole, plagioclase, clinopyroxene, magnetite, apatite and zircon from a parental magma, being itself the product of the mixing of 36% adakitic and 64% calc-alkaline magma, has been quantified. For the Santa Rosa rhyolites, a slab melting origin with little mantle interactions during the ascent of magmas has been established. The Pilongo series magma is the product of a moderate to high degree (26%) of partial melting of the subducting oceanic crust, which reached the surface without interaction with the mantle wedge. The Tishigcuchi series shows two stages of evolution: (1) metasomatism of the mantle wedge peridotite by slab melts, and (2) partial melting (10%) of this metasomatized source. Therefore, the relative ages of the edifices show a geochemical evolution from calc-alkaline to adakitic magmas, as is observed for several volcanoes of the Ecuadorian arc.     

Three-dimensional Mapping of Magma Source and Transport Regions from Seismic Data: The Mantle Wedge beneath Northeastern Japan

—We investigate the distribution of partial melt in island arc using the seismic velocity structure of the mantle wedge beneath northeastern Japan. The comparison of the seismic tomography with laboratory velocity data on a partially-molten mantle rock yields estimates of melting zones in three dimensions. We employ experimental data on the degree of partial melt in hydrous peridotite to give constraints on the melt fraction and temperature. Melting and magma-rich zones derived from the velocity structure coincide with observed low Q zones. The results of the three-dimensional mapping indicate that the source of magma in island arc is diapir-like melting patches localized within the low velocity zones of the mantle wedge. Extensive volcanic activity along the volcanic front is due to the presence of vast magma-rich zones just beneath the Moho. Those melting zones in the uppermost mantle may, in turn, cause melting of lower crustal materials and produce felsic magma. Melt appears to stay at and beneath the Moho, where crystallization fractionation may proceed. Melt exists at greater depths in the back-arc region, which may correlate with across-arc variations of chemical compositions of the volcanic rocks observed in northeastern Japan. We suggest that magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce mid-crustal S-wave reflectors.     

Water and magma generation at subduction zones

The basaltic ocean crust, metasomatized and metamorphosed during and after generation at the ocean ridge, contains H₂O stored in minerals and pore fluid. Phase equilibrium data establish the conditions for dehydration, and the conditions for melting of amphibole-gabbro or amphibole-quartz-eclogite, or for quartz-eclogite or mantle peridotite if aqueous fluids are available. But there is no concensus about the temperature distribution through the subducted crust, or within the overlying mantle wedge. Therefore, a variety of magmatic models can be derived from the experimental data. According to some calculations, endothermic dehydration reactions in the depth interval 75–125 km cool the oceanic crust to such an extent that it cannot be a major source of magmas; instead, concentrated aqueous fluids released from the crust generate magmas in the overlying peridotite. However, according to most existing thermal models, if temperatures in ocean crust are cool enough to prohibit melting of amphibolite, then temperatures in the mantle above the main sources of expelled fluids are too low for hydrous melting. The ocean crust appears to be effectively dehydrated by 100–125 km depth. Dense hydrous magnesian silicates are not likely candidates for deeper H₂O transport. The extent to which H₂O can be fixed in metasomatic phlogopite in crust or mantle is a significant but undetermined factor. Experimental data on minerals and liquid compositions do not support the concept of primary magmas for andesites and associated lavas from mantle or subducted crust. Complex, multi-stage processes appear to be more likely, which is consistent with recent interpretations of geochemical data.     

Geochemistry of calc-alkaline volcanic rocks from southeastern Iran (kouh-e-shahsavaran)

The Upper Tertiary to Quaternary volcanic complex of Kouh-e-Shahsavaran in southeastern Iran is composed of calc-alkaline rocks of island are type (high-alumina basalts, basic andesites, andesites and dacites) even though it was emplaced on the continental basement. The volcanic rocks of the complex are genetically related and were probably derived by low-pressure fractional crystallization of high-Al basalts. The anomalously high content of Sr in some rocks probably reflects an accumulation of plagioclase. The trace element data are consistent with the origin of the parental magma by partial melting of an “enriched” upper mantle peridotite.     

Aleutian magnesian andesites: Melts from subducted Pacific ocean crust

Several diagnostic chemical characteristics of an uncommon Aleutian magma type support a proposed origin that involves a small amount of partial melting of subducted Pacific ocean crust (basalt) consisting mainly of garnet and clinopyroxene (eclogite or garnet websterite). Among the characteristics are high La/Yb ratios and Sr contents and low ratios of radiogenic to non-radiogenic Sr and Pb. The major element composition of the andesites resembles that of hydrous melts in equilibrium with peridotite: a low ratio of total Fe to Mg is distinctive. These disparate observations can be reconciled if large ion lithophile (LIL)- element-rich hydrous melt from the subducted oceanic crust equilibrates with olivine and orthopyroxene in overlying LIL-element-depleted mantle and then erupts without interacting with the island are crust. The compositional dissimilarity of the magnesian andesites and most other andesites from the Aleutian island arc precludes application of this model to island are magmatism in general.     

Nishinoshima volcano in the Ogasawara Arc: New continent from the ocean?

Nishinoshima, a submarine volcano in the Ogasawara Arc, approximately 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Olivine‐bearing phenocryst‐poor andesites found in older submarine lavas from the flanks of the volcano have been used to develop a model for the genesis of andesitic lavas from Nishinoshima. In this model, primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. Thus, it only operates beneath Nishinoshima and submarine volcanoes in the Ogasawara Arc and other oceanic arcs, where the crust is thin. The primary magma compositions have changed from basalt, produced at considerable depth, to andesite, produced beneath the existing thinner crust at this location in the arc. This reflects the thermal and mechanical evolution of the mantle wedge and the overlying lithosphere. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. Andesites produced by this shallow and hydrous melting of the mantle could accumulate through collisions of plates to generate continental crust.     

Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches

Rocks dredged from the forearc very close to the intersection of the Yap and Mariana trenches include a suite of highly depleted arc tholeiites, and several samples of transitional to slightly alkaline basalt. The tholeiites range from magnesian quartz tholeiites with 0.46–0.6% TiO₂, to andesites with up to 62% SiO₂ and 8.2% FeO^*. All show pronounced LREE depletion and have very low contents of Ba and Sr. They are postulated to have been produced by partial melting of upper mantle peridotite residual after MORB extraction, following influx of hydrous fluids from the subducted slab. While these fluids were responsible for small enrichments in Ba, K, Rb and Sr in melts generated, LREE were not involved in the metasomatism, and the strong LREE depletion probably reflects the unmodified, depleted source peridotite.

The second lava suite includes slightly Ne-normative, Ti-augite-bearing basalts with convex-upward REE patterns, showing slight LREE depletion ((La/Sm)_N = 0.76). The chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening. A KAr age on one sample(7.8 ± 1.3m.y.) is in good agreement with the initial opening of the Mariana Trough.

The tectonic significance of the dredged arc tholeiite suite is less obvious. A KAr age of10.8 ± 0.4 My on one andesite, and the occurrence of similar lavas in dredges from at least 300 km along the length of the Yap arc, suggest that subduction was occurring beneath the Yap arc in the Late Miocene, after overthrusting of the Yap greenschist allochthon, and while calc-alkaline arc magmatism was occurring further north on the West Mariana Ridge. We suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the Yap forearc following subduction beneath this area of young, hot Sorol Trough crust. These arc tholeiites represent a magma type transitional between more typical arc tholeiites (e.g. Tongan) and high-Mg andesites and boninites.     

Rare-earth abundances in indian kimberlite

Abundances of some rare-earth elements (REE), together with Hf, Ta, Th, and Ba have been determined by neutron activation analyses in twelve kimberlite samples from Peninsular India. The kimberlites have fractionated REE patterns with enrichment of light rare earths; La/Yb ratio varies from 39 to 166. A systematic relationship exists between the total rare-earth abundances and the petrochemistry of the rocks. The rare-earth abundances observed in the kimberlites are consistent with their derivation by partial melting of a hydrous garnet peridotite mantle and subsequent fractional crystallization of the melt.     

High-pressure inclusions in tholeiitic basalt and the range of lherzolite-bearing magmas in the Tasmanian volcanic province

Spinel-lherzolite xenoliths have been found in olivine tholeiite near Andover in the Tasmanian Tertiary volcanic province. They show a high-pressure mineralogy of predominant olivine (Mg₉₀), with aluminous enstatite (Mg₉₀) and lesser aluminous diopside and chrome-bearing spinel, and resemble lherzolite xenoliths commonly found in undersaturated lavas. Such xenoliths are unusual in tholeiitic basalts and the occurrence directly attests to a mantle origin for at least some tholeiitic magmas.The lherzolites are accompanied by doleritic and pyroxenitic xenoliths and by olivine, orthopyroxene, clinopyroxene and plagioclase xenocrysts. If near-liquidus phases are represented amongst the xenocrysts, then the magnesian number of the host basalt and its xenocryst assemblage provisionally suggest a magma derived by more than 15–20% partial melting of mantle peridotite, before commencing xenocryst crystallisation at pressures between 8–13 kbar.With this new record, lherzolite-bearing lavas in Tasmania now cover an extremely wide compositional range, extending from highly undersaturated olivine melilitite to olivine tholeiite. They also include a considerable number of fractionated alkaline rocks that are only sparsely reported in the literature as lherzolite hosts. This latter group contains representatives of a previously suggested but unestablished alkaline fractionation series based on olivine nephelinite, viz. calcic olivine nephelinite → sodic olivine nephelinite → potassi-sodic olivine nephelinite → mafic nepheline benmoreite → mafic phonolite.Lherzolite and megacryst-bearing lavas are relatively more abundant in peripheral parts to the main basalt sequences in Tasmania. This suggests that they developed in fringing zones of less intense mantle melting which enhanced stagnation and fractionation of magmas within the mantle before eruption. Calculated crustal thicknesses under these areas suggest that the magmas were generated at pressures exceeding 6–11 kbar, with the Andover tholeiitic magma exceeding 9 kbar.     

Anhydrous and water-saturated melting experiments of an olivine andesite from Mt Yakushi-Yama, northeastern Shikoku, Japan

Abstract Melting experiments have been carried out on an olivine andesite of Mt Yakushi-Yama from the Miocene Setouchi volcanic belt in northeastern Shikoku, Japan. This andesite has been characterized by a low ratio of FeO*/Mg° (= 0.78). Phase relations have been determined within the pressure range of 2.8 to 19.3 kbar at 1000-1300°C under anhydrous and water-saturated conditions. At pressures less than 8.8 kbar, olivine is a liquidus phase. Orthopyroxene appears on the liquidus at 9.3 kbar under the anhydrous conditions. The multiple saturation point rises up to 17.5 kbar for water-saturated experiments. The andesite melt coexists with olivine and orthopyroxene just below the liquidus at 8.8–9.3 kbar and 1230°C for dry conditions, and at 17.5 kbar and 1060°C under water-saturated conditions. These experimental results indicate that the Yakushi-Yama olivine andesite magma could coexist with a harzburgitic mantle at depths between about 30 and 60 km, and at temperatures between 1060 and 1230°C. Experimental data also suggest a possibility that a high magnesian andesite magma would be generated by a direct partial melting of the uppermost harzburgitic mantle under anhydrous conditions.     

Mass balance and phase density constraints on early differentiation of chondritic mantle

A surprisingly simple and precise major element mass balance is consistent with derivation of average upper mantle peridotite from a partially molten chondritic Earth by subtraction of perovskite and addition of olivine. Majorite involvement is precluded unless some as yet unidentified components play a role. Perovskite subtraction during a primordial melting event is expected to occur by crystal fractionation at depth, while olivine addition is accomplished by a combination of buoyancy mechanisms: crystal flotation from a deep layer of melt buried by its own compressibility to the base of the solidifying upper mantle and subsequent solid state convection of this buoyant magnesian olivine upward. These processes are consistent with known density relations of crystals and liquid at very high pressure. Mass balance predicts that the residual magma body at depth after supplying olivine by flotation upward can be komatiitic. Distribution of originally C1 chrondritic bulk Earth material a few 100 m.y. after primordial differentiation is solid peridotite upper mantle, perovskite lower mantle, and a komatiitic liquid sandwich horizon.     

Magma plumbing system beneath Ontake Volcano, central Japan

Ontake Volcano in central Japan was last active from ~ 100–35 Ka. The eruptions contained rhyodacite pumice and lavas in the first stage (stage O1, > 33 km³), followed by eruptions of andesite lavas and pyroclastics (stages O2 and O3, > 16 km³). Modeling of major and incompatible elements with Sr isotope ratios suggests that the primary magma was a high-alumina basalt. One andesite magma type appears to have evolved from the basalt in a closed system magma chamber, in part by fractional crystallization, and its generation included crustal assimilation. The other andesite magma type is considered to have evolved in an open system magma chamber in which repeated input of primary magma occurred together with wall-rock assimilation and fractional crystallization. The rhyodacite is inferred to have evolved in a closed system magma chamber by fractional crystallization of the second type of andesite. These genetic relationships require that the magma chamber functioned alternately as an open and a closed system. Geobarometry indicates that there may have been multiple magma chambers, located in the upper crust for the rhyodacite, near the upper–lower crust interface for the andesite and in the mid-lower crust for the basalt. These chambers were stacked to form the magma plumbing system of Ontake. Incompatible element compositions of the basalt are considered to have changed during the eruptions, suggesting that two different plumbing systems for stage O1 magma and for stages O2, O3 magmas existed during the 65 Ka of activity. Evolutionary history of the systems implies that the primary magma was introduced into the magma plumbing system each for ~ 17 500 years and that the life span of a magma plumbing system was shorter than 40 Ka.     

Petrologic model of the crust and upper mantle of the Japanese Island arcs

Mafic and ultramafic xenoliths, in the Holocene calc-alkali andesite of Ichinomegata(1) crater in Oga peninsula and those in the Plio-Pleistocene alkali-olivine basalts of Oki-Dōgo island in the Japan Sea, have been studied in detail. Based on geothermometry and geobarometry, and relative abundance of the rock types of the xenoliths, petrologic models of the crust and upper mantle beneath these two areas were constructed. The crust and upper mantle beneath Ichinomegata crater are characterized by hydrous and relatively low temperature conditions. On the other hand, the crust and upper mantle beneath Oki-Dōgo island are characterized by nearly anhydrous and high temperature conditions, and presence of thick lavers of peridotite and pvroxenite cumulates in the uppermost mantle. The crust and upper mantle of the western part of the Northeast Honshū Arc can be considered as similar to those beneath Ichinomegata crater, because of the common occurrence of similar mafic xenoliths from many andesite volcanoes in this area. The crust and upper mantle of the northern part of the Southwest Honshū Arc, in the same way, can be regarded as similar to those beneath Oki-Dōgo island. Differences in amount of hydrous minerals of deep-seated rocks between the two areas can be interpreted as due to the presence of migrating water derived from the subducting Pacific plate in the Northeast Honshū Arc. Difference in slope of the geotherm may be due to the difference in temperature of the partial melt zones beneath these two areas. Bulk chemical compositions of the lower crustal materials of the Japanese island arcs, 85 mafic inclusions from 15 volcanoes, are listed, and it is concluded that they are cumulates or metamorphosed cumulates in the lower crust.