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.     

Lateral variation of basalt magma type across continental margins and Island Arcs

Quaternary basalt magmas in the Circum-Pacific belt and island arcs and also in Indonesia change continuously from less alkalic and more siliceous type (tholeiite) on the oceanic side to more alkalic and less siliceous type (alkali olivine basalt) on the continental side. In the northeastern part of the Japanese Islands and in Kamchatka, zones of tholeiite, high-alumina basalt, and alkali olivine basalt are arranged parallel to the Pacific coast in the order just named, whereas in the southwestern part of the Japanese Islands, the Aleutian Islands, northwestern United States, New Zealand, and Indonesia, zones of high-alumina basalt and alkali olivine basalt are arranged parallel to the coast. In the Izu-Mariana, Kurile, South Sandwich and Tonga Islands, where deep oceans are present on both sides of the island arcs, only a zone of tholeiite is represented. Thus the lateral variation of magma type is characteristic of the transitional zone between the oceanic and continental structures. Because the variation is continuous, the physico-chemical process attending basalt magma production should also change continuously from the oceanic to continental mantle. Suggested explanations for the lateral variation assuming a homogeneous mantle are: 1) Close correspondence between the variations of depth of earthquake foci in the mantle and of basalt magma type in the Japanese Islands indicates that different magmas are produced at different depths where the earthquakes are generated by stress release: tholeiite at depths around 100 km, high-alumina basalt at depths around 200 km, and alkali olivine basalt at depths greater than 250 km. 2) Primary olivine tholeiite magma is produced at a uniform level of the mantle (100–150 km), and on the oceanic side of the continental margin, it leaves the source region immediately after its production and forms magma reservoirs at shallow depths, perhaps in the crust, where it undergoes fractionation to produce SiO₂-oversaturated tholeiite magma, whereas on the continental side, the primary magma forms reservoirs near the source region and stays there long enough to be fractionated to produce alkali olivine basalt magma, and in the intermediate zone, the primary magma forms reservoirs at intermediate depths where it is fractionated to produce high-alumina basalt magma.     

Geochemistry and tectonic setting of Tertiary volcanism in the Güvem area, Anatolia, Turkey

Detailed field mapping in the Güvem area in the Galatia province of NW Central Anatolia, Turkey, combined with K–Ar dating, has established the existence of two discrete Miocene volcanic phases, separated by a major unconformity. The magmas were erupted in a post-collisional tectonic setting and it is possible that the younger phase could be geodynamically linked to the onset of transtensional tectonics along the North Anatolian Fault zone. The Early Miocene phase (18–20 Ma; Burdigalian) is the most voluminous, comprising of over 1500 m of potassium-rich intermediate-acid magmas. In contrast, the Late Miocene volcanic phase (ca. 10 Ma; Tortonian) comprises a single 70-m-thick flow unit of alkali basalt. The major and trace element and Sr–Nd isotope compositions of the volcanics suggest that the Late Miocene basalts and the parental mafic magmas to the Early Miocene series were derived from different mantle sources. Despite showing some similarities to high-K calc-alkaline magma series from active continental margins, the Early Miocene volcanics are clearly alkaline with higher abundances of high field strength elements (Zr, Nb, Ti, Y). Crustal contamination appears to have enhanced the effects of crystal fractionation in the petrogensis of this series and some of the most silica-rich magmas may be crustal melts. The mantle source of the most primitive mafic magmas is considered to have been an asthenospheric mantle wedge modified by crustally-derived fluids rising from a Late Cretaceous–Early Tertiary Tethyan subduction zone dipping northwards beneath the Galatia province. The Late Miocene basalts, whilst still alkaline, have a Sr–Nd isotope composition indicating partial melting of a more depleted mantle source component, which most likely represents the average composition of the asthenosphere beneath the region.     

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.     

Chemistry of volcanic rocks and seismicity of the earth’s Mantle in the Island arcs

Volcanic belts in island ares are associated in space with epicenter belts of mantle earthquakes. Primary magma of volcanoes on the oceanic side of an island are is rich in silica and poor in alkalies. In contrast, it is poor in silica and rich in alkalies on the continental side. Foci of the mantle carthquakes are close to a plane that dips away from the oceanic side toward the continental side. The spatial correlation between nature of primary magmas and depth of earthquake foci suggests a common origin. Moreover, the hypothesis that the generation of magma and the occurrence of mantle earthquakes are genetically related and that the primary magmas in the island ares are generated at levels about 150 to 250 km deep. is supported by the relationship among dillerent island ares, that is, primary magma of volcanoes fronting on oceanic trenches becomes more silicious from one are to another as the seismicity of mantle earthquake zones becomes greatre.     

Isotope geochemistry (Sr–Nd–Pb) and petrogenesis of leucite-bearing volcanic rocks from “Colli Albani” volcano, Roman Magmatic Province, Central Italy: inferences on volcano evolution and magma genesis

The “Colli Albani” composite volcano is made up of strongly silica-undersaturated leucite-bearing rocks. Magmas were erupted during three main periods, but a complex plumbing system dominated by regional tectonics channelled magmas into different reservoirs. The most alkali-rich magmas, restricted to the caldera-forming period (pre-caldera), are extremely enriched in incompatible trace elements and display more radiogenic Sr (⁸⁷Sr/⁸⁶Sr?=?0.71057–0.71067), with slightly less radiogenic Pb with respect to those of the post-caldera period. Post-caldera volcanic activity was concentrated in three different volcanic environments: external to the caldera, along the caldera edge and within the caldera. The post-caldera magmas produced melilite- to leucitite-bearing, plagioclase-free leucitites. In contrast to the pre-caldera lavas, they are characterised by lower incompatible trace element abundances and less radiogenic Sr (⁸⁷Sr/⁸⁶Sr?=?0.71006–0.71039). Magmas evolved through crystal fractionation plus minor crustal assimilation in a large magma chamber during the pre-caldera period. The multiple caldera collapses dissected and partially obliterated the early magma chamber. During the post-caldera stage, magmas were channelled through several pathways and multiple shallow-level magma reservoirs were established. A lithospheric mantle wedge previously depleted in the basaltic component and subsequently enriched by metasomatic slab-derived component is suggested as the mantle source of Colli Albani parental magmas. Two different parental magmas are recognised for the pre- and post-caldera stages. The differences may be related to the interplay between smaller degrees of melting for the pre-caldera magmas and more carbonate-rich recycled subducted lithologies in the post-caldera magmas.     

Source depletion and extent of melting in the Tongan sub-arc mantle

The fluid immobile High Field Strength Elements (HFSE) Nb and Ta can be used to distinguish between the effects of variable extents of melting and prior source depletion of the Tongan sub-arc mantle. Melting of spinel lherzolite beneath the Lau Basin back-arc spreading centres has the ability to fractionate Nb from Ta due to the greater compatibility of the latter in clinopyroxene. The identified spatial variation in plate velocities and separation of melt extraction zones, combined with extremely depleted lavas make Tonga an ideal setting in which to test models for arc melt generation and the role of back-arc magmatism.We present new data acquired by laser ablation-ICPMS of fused sample glasses produced without the use of a melt fluxing agent. The results show an arc trend towards strongly sub-chondritic Nb/Ta (< 17) with values as low as 7.2. Melting models show that large degree melts of depleted MORB mantle fail to reproduce the observed Nb/Ta. Alternatively, incorporation of residual back-arc mantle that has undergone less than 1% melting into the sub-arc melting regime reproduces arc values. However, the extent of partial melting required to produce the composition of the Lau Basin back-arc basalts averages 7%. This apparent discrepancy can be explained if only the lowermost 4 km of the residua from the mantle melt column beneath the back-arc is added to the source of arc magmas. We have identified that the degree of arc/back-arc coupling displayed in the rock record provides an index of the depth of hydrous melting beneath the arc. In this case, this would imply a depth of ~ 75 km for generation of arc magmas, indicating that hydrous melting in the mantle wedge is triggered by the breakdown of hydrous phases in the subducting slab.     

Back-arc basin basalt systematics

The Mariana, east Scotia, Lau, and Manus back-arc basins (BABs) have spreading rates that vary from slow (<50 mm/yr) to fast (>100 mm/yr) and extension axes located from 10 to 400 km behind their island arcs. Axial lava compositions from these BABs indicate melting of mid-ocean ridge basalt (MORB)-like sources in proportion to the amount added of previously depleted, water-rich, arc-like components. The arc-like end-members are characterized by low Na, Ti and Fe, and by high H₂O and Ba/La; the MORB-like end-members have the opposite traits. Comparisons between basins show that the least hydrous compositions follow global MORB systematics and an inverse correlation between Na8 and Fe8. This is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures (Lau/Manus hotter than Mariana/Scotia). This interpretation accords with numerical model predictions that faster subduction-induced advection will maintain a hotter mantle wedge. The primary compositional trends within each BAB (a positive correlation between Fe8, Na8 and Ti8, and their inverse correlation with H₂O(8) and Ba/La) are controlled by variations in water content, melt extraction, and enrichments imposed by slab and mantle wedge processes. Systematic axial depth (as a proxy for crustal production) variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate. Hydrous fluxing enhances decompression melting, allowing depleted mantle sources just behind the island arc to melt extensively, producing shallow spreading axes. Flow of enriched mantle components around the ends of slabs may augment this process in transform-bounded back-arcs such as the east Scotia Basin. The re-circulation (by mantle wedge corner flow) to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the East Lau Spreading Center, Manus Southern Rifts, and Mariana Trough and the very depleted lavas of east Scotia segments E8/E9. The crust becomes mid-ocean ridge (MOR)-like where the spreading axes, further away from the island arc and subducted slab, entrain dominantly fertile mantle.     

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.     

Origin of some calc-alkalic andesites in the Japanese Islands

Origin of calc-alkalic andesite in the Japanese Islands is reviewed on the basis of the recent trace element data and new experimental results. It is suggested that calc-alkalic andesites in the Japanese Islands have at least four different origins; (1) fractional crystallization with separation of magnetite of high-alumina basalt magma, (2) partial melting of hydrous upper mantle peridotite (for magnesian andesite), (3) fractional crystallization with separation of olivine and/or orthopyroxene of magnesian andesite magma and (4) mixing of dacitic and basaltic magmas. Emphasis is placed on the possible generation of primary magnesian calc-alkalic andesite magmas by direct partial melting of the upper mantle peridotite under hydrous conditions at depths between 40 and 60 km.     

Geochemistry of the oldest MORB and OIB in the Isua Supracrustal Belt, southern West Greenland: Implications for the composition and temperature of early Archean upper mantle

Abstract Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate‐type stratigraphy and a duplex structure. Plate history from mid‐oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid‐oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO₂ and Na₂O contents. The Isua Cpx show slightly light (L)REE‐depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least‐altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al₂O₃, and depleted in TiO₂, FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE‐depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg? and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480°C, indicating that the Early Archean mantle was hotter by at most approximately 150°C than the modern mantle.     

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.     

Melilite-derived mineral inclusions in chromite from the Gaositai complex: Implications for an extensional tectonic setting in Early Permian at the north North China Craton

The Early Permian mafic-ultramafic concentrically zoned Gaositai intrusion at Chengde, on the northern margin of the North China Craton (NCC), is a cumulative complex emplaced along a giant fracture that penetrates deeply into the continental lithosphere. Melt inclusions are present in chromite crystals from the inner dunite and chromitite zones of the Gaositai complex. The melt inclusions have experienced post-trap crystallization and resulted in multiple mineral phases, including melilite, garnet, phlogopite, magnesite and apatite, which can indicate the liquidus minerals of the primitive magma. The characteristics of the melilite+melanite+clinopyxene assemblage indicate that the primary parental magma was highly undersaturated and derived from an alkali-rich mantle source. The crystallization of phlogopite, magnesite and apatite suggests a primary magma rich in K, H₂O and CO₂. When compared with experimental data, the primary magma of the Gaositai intrusion is concordant with a kamafugite magma originating from partial melting of enriched mantle with H₂O and CO₂ at pressures greater than 2.7 GPa. This magmatic process would have been related to extensional thinning of the continental lithosphere. The Gaositai primary magmas have high Nb/La ratios, which are similar to those of ocean island basalts, but different from arc-related magmas. This suggests that the northern margin of the NCC was not an active continental margin of the Paleo-Asian Ocean subduction zone during the Early Permian: an extensional tectonic setting during the emplacement of the Gaositai intrusion is more likely.     

Azores mantle blob: Rare-earth evidence

Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted.The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources.The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.     

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.     

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO₂), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO₂). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt     

The Ploskie Sopki volcanic massif: Geology,petrochemistry, mineralogy,and petrogenesis (Klyuchevskoi Volcanic Cluster,Kamchatka)

This paper is concerned with the geological history and petrology of a major polygenic volcanic edifice dating back to Upper Pleistocene to Holocene time. This long-lived volcanic center is remarkable in that it combines basaltic and trachybasaltic magmas which are found in basaltic andesite and trachybasaltic–trachyandesite series. The inference is that the coexisting parent magmas are genetically independent and are generated at different sources at depth in an upper mantle volume. The associated volcanic rocks have diverse compositions, stemming from a multi-stage spatio–temporal crystallization differentiation of the magmas and mixing of these in intermediate chambers.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

Seismicity pattern: an indicator of source region of volcanism at convergent plate margins

The results of detailed investigation into the geometry of distribution of earthquakes around and below the volcanoes Korovin, Cleveland, Makushin, Yake-Dake, Oshima, Lewotobi, Fuego, Sangay, Nisyros and Montagne Pelée at convergent plate margins are presented. The ISC hypocentral determinations for the period 1964-1999, based on data of global seismic network and relocated by Engdahl, van der Hilst and Buland, have been used.The aim of this study has been to contribute to the solution of the problem of location of source regions of primary magma for calc-alkaline volcanoes spatially and genetically related to the process of subduction. Several specific features of seismicity pattern were revealed in this context. (i) A clear occurrence of the intermediate-depth aseismic gap (IDAG) in the Wadati-Benioff zone (WBZ) below all investigated active volcanoes. We interpret this part of the subducted slab, which does not contain any teleseismically recorded earthquake with magnitude greater than 4.0, as a partially melted domain of oceanic lithosphere and as a possible source of primary magma for calc-alkaline volcanoes. (ii) A set of earthquakes in the shape of a seismically active column (SAC) seems to exists in the continental wedge below volcanoes Korovin, Makushin and Sangay. The seismically active columns probably reach from the Earth surface down to the aseismic gap in the Wadati-Benioff zone. This points to the possibility that the upper mantle overlying the subducted slab does not contain large melted domains, displays an intense fracturing and is not likely to represent the site of magma generation. (iii) In the continental wedge below the volcanoes Cleveland, Fuego, Nisyros, Yake-Dake, Oshima and Lewotobi, shallow seismicity occurs down to the depth of 50 km. The domain without any earthquakes between the shallow seismically active column and the aseismic gap in the Wadati-Benioff zone in the depth range of 50-100 km does not exclude the melting of the mantle also above the slab. (iv) Any earthquake does not exist in the lithospheric wedge below the volcano Montagne Pelée. The source of primary magma could be located in the subducted slab as well as in the overlying mantle wedge. (v) Frequent aftershock sequences accompanying stronger earthquakes in the seismically active columns indicate high fracturing of the wedge below active volcanoes. (vi) The elongated shape of clusters of epicentres of earthquakes of seismically active columns, as well as stable parameters of the available fault plane solutions, seem to reflect the existence of dominant deeply rooted fracture zones below volcanoes. These facts also favour the location of primary magma in the subducting slab rather than in the overlying wedge.We suppose that melts advancing from the slab toward the Earth surface may trigger the observed earthquakes in the continental wedge that is critically pre-stressed by the process of subduction. However, for definitive conclusions it will be necessary to explain the occurrence of earthquake clusters below some volcanoes and the lack of seismicity below others, taking into account the uncertainty of focal depth determination from global seismological data in some regions.     

143Nd/144Nd,87Sr/86Sr and trace element constraints on the petrogenesis of Aleutian island arc magmas

¹⁴³Nd/¹⁴⁴Nd,⁸⁷Sr/⁸⁶Sr and trace element results are reported for volcanic and plutonic rocks of the Aleutian island arc. The Nd and Sr isotopic compositions plot within the mantle array with ε_Nd values of from 6.5 to 9.1 and⁸⁷Sr/⁸⁶Sr ratios of from 0.70289 to 0.70342. Basalts have mildly enriched light REE abundances but essentially unfractionated heavy REE abundances, while andesites exhibit a greater degree of light to heavy REE fractionation. Both the basalts and andesites have significant large ion lithophile element to light rare earth element (LILE/LREE) enrichments. Variations in the isotopic compositions of Nd and Sr are not related to the spatial distribution of volcanoes in the arc, nor are they related to temporal differences. ε_Nd and⁸⁷Sr/⁸⁶Sr do not correlate with major element compositions but do, however, correlate with certain LILE/LREE ratios (e.g. Ba_N/La_N). Plutonic rocks have isotropic and trace element characteristics identical to some of the volcanic rocks. Rocks that make up the tholeiitic, calc-alkaline and alkaline series in the Aleutians do not come from isotopically distinct sources, but do exhibit some differing LILE characteristics.Given these elemental and isotopic constraints it is shown that the Aleutian arc magmas could not have been derived directly from homogeneous MORB-type mantle, or fresh or altered MORB subducted beneath the arc. Mixtures of partially altered MORB with deep-sea sediment can in principle account for the isotopic characteristics and most of the observed LILE/LREE enrichments. However, some samples have exceedingly high LILE/LREE enrichments which cannot be accounted for by sediment contamination alone. For these samples a more complex scenario is considered whereby dehydration and partial melting of the subducted slab, containing less than 8% sediment, produces a LILE-enriched (relative to REE) metasomatic fluid which interacts with the overlying depleted mantle wedge. The isotopic and LILE characteristics of the mantle are extremely sensitive to metasomatism by small percentages of added fluid, whereas major elements are not substantially effected, Major element compositions of Aleutian magmas are dominantly controlled by the partial melting of this mantle and subsequent crystal fractionation; whereas isotopic and LILE characteristics are determined by localized mantle heterogeneities.