Showing papers in "Bulletin of Volcanology in 1978"

Gas release rates from pyroclastic flows: a assessment of the role of fluidisation in their emplacement

Abstract: Estimates are presented for the rates of release of dissolved water from the particles in a pyroclastic flow by diffusion. Velocities of gas escaping from pyroclastic flows of different thicknesses are calculated. For quite small residual gas contents (0.2 to 0.8% H2O), gas velocities of 10 to over 100 cm/sec during the first 103 sec of release are estimated for flows of thickness 1 to 20 m, which experimental studies demonstrate are the velocities required to fluidise fine to medium ash. Flows with high residual gas contents or large volume (thick flows) are likely to be substantially fluidised by exsolving gas. 30% to over 60% of the particles in such flows are predicted to be fluidised. Fluidisation is thus believed to be an important mechanism in the flow and in determining the mobility of the large magnitude, prehistoric pyroclastic flows which formed extensive ignimbrite sheets. Small pyroclastic flows, however, of the magnitude observed in several historic eruptions are not believed to be fluidised, because of their low residual gas contents, small volume, and the substantial amount of cooling that occurs during their emplacement.

Characteristics of widespread pyroclastic deposits formed by the interaction of silicic magma and water

Abstract: We have recognized a type of pyroclastic deposit formed by the interaction of water and silicic magma during explosive eruptions. These deposits have a widespread dispersal, similar to plinian tephra, but the overall grain size is much tiner. Several deposits studied can be associated with caldera lakes or sea water and water/magma interaction is proposed to account for the fine grain size. Several examples have been studied, including the Oruanui Formation, N.Z., and the Askja 1875 deposit. Both show little downwind decrease in median diameter, a downwind decrease in sorting (σφ) (more evident in the Askja deposit) and coarse tail grading. The Askja example has base surge deposits near source and some Oruanui members show multiple thin beds near source; both are common features of phreatomagmatic deposits. Isopachs of the Askja deposit indicate a source under Lake Oskjuvatn in Askja Caldera and those of the Oruanui indicate a source under the NW part of Lake Taupo. In terms of dispersal area, volume and calculated eruption column heights, these deposits are similar to plinian. However, their extreme fragmentation due to magma/water interaction, superimposed on fragmentation imparted by carlier vesiculation, gives a much finer and more complex grain size distribution than plinian counterparts. The field of phreatomagmatic equivalents to plinian pumice deposits was unoccupied onWalker’s (1973) classification of explosive volcanic eruptions. Such deposits are the phreatomagmatic analogue of plinian deposits and the name « phreatoplinian » is proposed.

The campanian ignimbrite: a major prehistoric eruption in the Neapolitan area (Italy)

Abstract: A geological, chemical and petrographical study of the Campanian ignimbrite, a pyroclastic flow deposit erupted about 30,000 years ago on the Neapolitan area (Italy), is reported. The ignimbrite covered an area of at least 7,000 km2; it consists of a single flow unit, and the lateral variations in both pumice and lithic fragments indicate that the source was located in the Phlegraean Fields area.

S-wave shadows in the Krafla Caldera in NE-Iceland, evidence for a magma chamber in the crust

Abstract: During the present tectonic activity in the volcanic rift zone in NE-Iceland it has become apparent that the attenuation of seismic waves is highly variable in the central region of the Krafla volcano. Earthquakes associated with the inflation of the volcano have been used to delineate two regions of high attenuation of S-waves within the caldera. These areas are located near the center of inflation have horizontal dimensions of 1–2 km and are interpreted as the expression of a magma chamber. The top of the chamber is constrained by hypocentral locations and ray paths to be at about 3 km depth. Small pockets of magma may exist at shallower levels. The bottom of the chamber is not well constrained, but appears to be above 7 km depth. Generally S-waves propagate without any anomalous aftenuation through laver 3 (vp=0.5 km sec−1) across the volcanic rift zone in NE-Iceland. The rift zone therefore does not appear to be underlain by an estensive magma chamber at crustal levels. The Krafla magma chamber is a localized feature of the Krafla central volcano.

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

Abstract: 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.

The exceptional magnitude and intensity of the Toba eruption, sumatra: An example of the use of deep-sea tephra layers as a geological tool

Abstract: The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km2. The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 106 km2. We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km3 of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km3 respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 106 m3/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity.

Rifting episode in north iceland in 1874–1875 and the eruptions of askja and sveinagja

Abstract: In 1874 and 1875 the fissure swarm of Askja central volcano was activated during a major rifting episode. This rifting resulted in a fissure eruption of 0.3 km3 basaltic magma in Sveinagja graben, 50 to 70 km north of Askja and subsequent caldera collapse forming the Oskjuvatn caldera within the main Askja caldera. Five weeks after initial collapse, an explosive mixed magma eruption took place in Askja. On the basis of matching chemistry, synchronous activity and parallels with other rifted central volcanoes, the events in Askja and its lissure swarm are attributed to rise of basaltic magma into a high-level reservoir in the central volcano, subsequent rifting of the reservoir and lateral flow magma within the fissure swarm to emerge in the Sveinagja eruption. This lateral draining of the Askja reservoir is the most plausible cause for caldera collpse.

Boninite as a possible calc-alkalic primary magma

Abstract: 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 PH2O. 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.

Stromboli and its 1975 eruption

Abstract: On November 4, 1975 in the evening, an eruption took place at Mt. Stromboli. On the following day lava flowed on the Sciara del Fuoco downward to the sea, accompanied by an intense explosive activity at the crater plane. Direct observations on the volcanic activity were carried out since November 6 while a seismic survey was made from Nov. 7 to 12. The total volume of the lava outpoured during this period of activity that lasted 21 days, was estimated to be about 104 m3. This paper reports the results of direct observations, and of the petrological, radioactive disequilibria and seismic activity studies performed for this eruption. The eruption was preceded by an insignificant change of seismic activity, which was monitored by a seismic station located about 2 km East of the crater. A shallow seismicity was strietly related to crater explosions accompanying the eruptive phenomenon. Radioactive disequilibria showed a lack of disequilibrium between228Ra and232Th explainable in terms of a fast rising of magma in the conduit. Chemical analyses of lava samples and deep seismic sounding data indicate a correspondence between the depth (10–15 km) at which crystallization pressure of phenocrysts occurs and a low velocity laver.

Geotectonic settings of miocene-quaternary volcanism in and around the eastern Tyrrhenian sea border (Italy) as deduced from major element geochemistry

Abstract: The volcanic rocks along the south-eastern and eastern Tyrrhenian border of Italy have been classified as converging plate margin magma types by using discriminant functions based on major element patterns. These rocks belong to calc-alkaline and shoshonitic associations (shoshonites, and predominant leucitic rocks) ranging in age from Upper Miocene to, mostly, Quaternary.

Andesites in island arcs and continental margins: relationship to crustal evolution

Abstract: Andesites of both island arc and continental margin environments contain petrologic evidence of mixing of mantle and crustal melts. Andesitic volcanism appears to involve addition of mantle-derived basaltic magma to the crust and fractionation of preexisting crustal material. Changes in andesitic volcanism with increasingly continental character of the crust reflect changes in a rhyolitic component derived from increasingly aged and fractionated crust. The initial stage in development of continental crust is partial melting of oceanic crust.

Origin of some calc-alkalic andesites in the Japanese Islands

Abstract: 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.

Volcanic rocks of the Witu Islands, Papua New Guinea: The origin of magmas above the deepest part of the New Britain Benioff zone

Abstract: The Witu Islands are Quaternary volcanoes that overlie the deepest (about 300–580 km) part of the New Britain Benioff zone. The islands are about 100 km south of the transcurrent-divergent plate boundary that crosses the Bismarck Sea, and they surmount the southeastern end of the Willaumez-Manus Rise. The rocks are olivine- and quartz-normative tholeiitic basalts, low- and high-SiO2 andesites, dacites, and rhyolites. Alkaline rocks that overlie the deep (greater than 300 km) parts of other Benioff zones have not been found in the Witu Islands. Compared to the Witu Islands rocks, those with similar SiO2 contents from New Britain volcanoes that overlie progressively shallower parts of the Benioff zone to the south, are, for example, generally poorer in Na+K, Ti, and P, and higher in Ca and Al. There are similar progressive changes in trace-element abundances, but Zr and Nb contents are distinctly richer in Witu Islands rocks.87Sr/86Sr values range between 0.70311 and 0.7038, which are typical for rocks from New Britain as a whole and from other island arcs in the southwest Pacific. Two143Nd/144Nd values of 0.512211 and 0.512271, taken together with the Sr isotopic results, define a source region equivalent to those for oceanic-island basalts; there is no evidence for sea-water contamination of the sources.

Characteristics of tephra from cinder cone, Lassen volcanic National Park, California

Abstract: Cinder Cone, an undissected, 200 m high Holocene cone in Lassen Volcanic National Park, California, is mantled by basaltic blocks and bombs, including abundant large spherical accretionary bombs Types of pyroclasts, ranging from light brown sideromelane droplets to blocky, crystalline tachylite fragments, appear to reflect the vent history; when the vent was clear, an abundance of lava was erupted at higher temperature and lower viscosity, producing predominantly rapidly chilled sideromelane droplets When the vent was blocked by pooling of lava or by slumping of talus from crater walls, intermittent Strombolian eruptions ejected more viscous, crystalline to tachylitic fragments and comminuted talus Such activity has been observed at Mt Etna, Italy and Heimaey, Iceland Avalanching of debris into the crater and down outer slopes, one of the main processes in cinder cone formation, isalso responsible for thevarieties of pyroclast types formed during Strombolian eruptions

Major element frequency distribution of the Japanese Quaternary volcanic rocks

Abstract: Published chemical analyses of the Japanese Quaternary volcanic rocks are being collected to build a comprehensive data file. About 1600 selected major element analyses which include all 13 components of the bulk chemistry of the lavas and pyroclastics are considered here. Care was taken to correct misprints in the published analyses through correspondence with the original authors or analysts. Analyses are weighted against the volume of the volcano on which rock the analyses are made. Total volume of the volcanic material thus treated in this paper amounts to 3900 km3 or about 80% of the total Quaternary volcanic materials. The volume-weighted histograms for the most components show a unimodal profile when the large-scale caldera-forming pyroclastic flow and fall deposits are disregarded. SiO2 histogram shows a broad primary mode around 57 %. For many components, the large-scale pyroclastic deposits form a secondary mode at the positions corresponding to a felsic composition. This strongly suggests that the mode of generation and ascent of such calderaforming felsic magma is different from that of the ordinary cone- and dome-building basalt-andesite-dacite series of magmas. Number-of-analyses histograms for the same set of combinations reveals that number frequencies show quite similar profiles as those for the volume-weighted histograms for the Japanese Quaternary volcanics. The profiles of the histograms for the Northeast Japan arc is very much similar as those of the whole Japanese islands while those for Izu-Mariana and Southwest Japan arcs are different. The differences are due to the different proportions of the rocks belonging to tholeiitic, calc-alkalic and alkali rock series in each arc. The whole Japan histograms may be considered as a good representative of the chemical characteristics of the average Japanese Quaternary (less than the last half million years) volcanic rocks.

Evolution of the tertiary volcanic rocks in the Izu-Mariana arc

Abstract: Petrological evolution of the Tertiary island arc in the Izu-Mariana region has been accompanied by the development of three different volcanic suites: 1) oceanridge basalt now exposed as the metamorphic basement on Yap; 2) island-arc tholeiites of Eocene to early Oligocene age characterized by low contents of incompatible elements at all levels of silica enrichment; and 3) calc-alkalic rocks of late Oligocene to early Miocene age showing higher contents of silica and incompatible elements. All these three suites have primitive, undifferentiated basalts or andesites (boninites) characterized by high Mg/Fe, Cr, and Ni, suggesting that they have been derived from an upper mantle peridotite at relatively high temperatures. The earliest volcanism appears to have occurred at a spreading ridge. Later, as subduction proceeded, the island-arc tholeiite magma may have been produced by the introduction of a smaller amount of water into the locus of fusion from the subducted oceanic crust. An increasingly larger amount of water introduced into the same region could have led to the development of the more siliceous, calc-alkalic magma, as represented typically by the boninite.

Evolution comparée de deux séries alcalines du Pacifique Central: Rôle de la fugacité d’oxygène et de la pression d’eau

Abstract: L’etude mineralogique et geochimique de deux series alcalines (iles de Rapa et de Nuku-Hiva, Polynesie francaise) a permis de mettre en evidence des differences notables d’une serie a l’autre, dans l’enrichissement en fer des silicates ferromagnesiens, le comportement de Ti, V, Zn et les correlations inter-elements; differences qui sont liees a la separation, precoce a Nuku-Hiva et tardive a Rapa, de la magnetite. Le role variable joue par cette phase est une consequence des differences de fugacite d’oxvgene constatees entre les deux series; differences probablement elles-memes en rapport avec celles des pressions d’eau. Il est demontre que dans chacune des deux iles la cristallisation fractionnee rend compte des variations observees et que la sursaturation en silice de certains liquides residuels est la consequence de la separation de mica.

Remanent magnetism of poorly sorted deposits from the Minoan eruption of Santorini

Abstract: Thick, poorly sorted ash deposits produced during the Minoan cruption have differing palacomagnetic histories. Included clasts from deposits produced in the second phase of the eruption have random NRM and cleaned directions and were emplaced as cold mudflows. Clasts from ignimbrite flow units erupted in the third phase have significant directions and their palacomagnetic pole is consistent with today’s geomagnetic pole. These deposits acquired a TRM on deposition and were emplaced as hot pumice flows.

Trace element distribution in the cainozoic lavas of Nevado Coropuna and Andagua Valley, Central Andes of Southern Peru

Abstract: Major and minor elements have been determined on 26 samples of andesitie to rhyolitic lavas from Nevado Coropuna and Andagua valley in Southern Peru. Nevado Coropuna dating back since late Miocene is the highest stratovolcano of Peru. It is located at 150 km NW of Arequipa and at 110 km E of the Pacific coast. Andagua valley is situated at about 30 km E of Coropuna. The magmatic activity there, as shown by the presence of several cones, is more recent than that of Corpouna and is related to the tectonic graben characterizing this valley. The geological position of the valley is very important because it is near the transverse line separating the zone of rather flat subduction of the Nazca plate from another one dipping more steeply to the SE.

Petrographic evidence of magma mixing in Shirouma-Oike volcano, Japan

Abstract: Detailed petrographic analysis of calcalkaline volcanic rocks of Shirouma-Oike volcano, Japan, reveals that the complex phenocryst assemblage (Ol+Cpx+Opx+Hb+Bt+Qz+Pl+Mt+Hm) in the younger group volcanic rocks can be divided into two groups, a high temperature group (Ol+Cpx±An-rich Pl) and a low temperature group (Op+Hb+Bt+Qz±Ab-rich Pl+Mt+Hm). Compositional zonation of the phenocrystic minerals, normal zoning in olivine and clinopyroxene, and reverse zoning in orthopyroxene and plagioclase, indicate that these two groups of phenocrysts precipitated from two different magmas which mixed before the eruption. The low temperature magma is a stagnant magma in a shallow magma chamber, to which high temperature basaltic magma is intermittently supplied. Magma mixing is also indicated in olivine-bearing two pyroxene andesite of the older group volcanic rocks, by the coexistence of normally zoned Mg-rich clinopyroxene phenocrysts and reversely zoned Fe-rich clinopyroxene phenocrysts, and by reverse zoning in orthopyroxene phenocrysts. It is concluded that magma mixing is an important process responsible for the generation of the disequilibrium features in calc-alkaline volcanic rocks.

The role of magma vapors in volcanic tremors and rapid eruptions

Abstract: A fracture dynamics model in which an igneous intrusion of magma within a crack occurs is used to describe the psysical processes of magma transport. A symbiotic relationship exists between the crack and the fluid. The crack tip cannot accelerate faster than the fluid within it can flow in the channel provided by the crack, and the speed of the fluid is limited by its own viscosity. A volatile phase at the tip of the crack at lithostatic pressures will allow the crack to accelerate to high speeds, since the viscosity of a volatile is small. It is proposed that periods of quiescence in volcanic activities may not in fact be quiet, but only periods where the crack velocity (and therefore the magma transport rate) is slow. At rapid crack velocities, where there is sufficient kinetic energy for the generation of abundant acoustic radiation, the crack generates a detectable seismic signal. From this point of view, seismic methods always underestimate the size of cracks. The analyses here apply fracture dynamics to the Kilauea cruption of 1963.

Prediction and characteristics of the 1975 eruption of Tolbachik volcano, Kamchatka

Abstract: According to a long-term prediction. Tolbachik volcano was expected to erupt with a 0.7 probability, some time in the period 1964–1978. An eruption of Tolbachik commenced at 21.45 GMT on July 5, 1975. It took place on the southwestern Hank of the volcano at an altitude of 880 m a.s.l. about 18 km from the central crater. An earthquake swarm preceded it. The place and time of eruption were predicted three days belore it began on the basis of epicenter locations and characteristics of recorded seismic activity. During the period July 5–28 gases and incandescent magma were continuously ejected to a height of 2,000 m above ground level. Ash clouds rose to a height of 6 to 8 km, with a train of ash extending over a horizontal distance of 300 km. The velocity of jets from the crater was about 200 m/see. During the first days of the eruption the quantity of materials erupted and the eruption power amounted to 1.25 · 105 kg/see and 2.1 · 1011W, respectively. The vertical growth of the scoria cone was consistent with the lawH=2.153√vt, where the time and height are expressed in seconds and meters, respectively. The mouth of the volcano conduit was estimated to be 12 m in diameter. Lava began to erupt at 22h23m on July 28. During the period July 5–31 about 3 · 1011 kg of magmatic material, consisting of ash, scoria and lava, was erupted onto the earth’s surface. The energy released over the period of eruption accounted for 5 · 1017 J.

Nature of the magma conduit under the east rift zone of Kilauea Volcano, Hawaii

Abstract: From a combination of results of gravity, magnetic and seismic refraction surveys, the dike complex under the east rift zone of Kilauea Volcano in Hawaii was found to extend for 110 km from the summit area of the volcano to a point 60 km at sea beyond the eastern tip of the island. Near the summit the complex is 20 km wide, and at about 40 km distance from the summit, the complex narrows to 12 km wide. The main body of the dike complex is 2.3 km deep, but some parts are as shallow as 1 km. From extrapolation of temperature data of a deep well and from analysis of magnetic data, it was inferred that temperature of the dike complex is above the Curic point of 540°C. The internal part of the complex can approach the melting point of 1060°C. The dike complex was formed by numerous excursions of magma from the holding reservoir under the volcano summit. The theory of forceful intrusion of magma into rift zones accounts for the magma excursions and migration of the passageways. Gravity and seismic velocity data indicate that density of the material left in the dike complex is 3.1 g/cm3. In the light of recent density determinations of Hawaiian rocks under high pressure and temperature, it is concluded that during Hawaiian volcanic activity, less dense components of the parent magma crupt through surface vents while the more dense components remain trapped below. Samples of the dense material from the dike complex are required before we can have a complete picture of the parent magma of Hawaiian volcanoes. The dike complex is the source of thermal energy for a commercial quality geothermal reservoir that was found by drilling.

Magma characterization of the Mexican volcanic belt (Mexico)

Abstract: Over 330 bulk chemical analyses of rocks (Oligocene or Miocene to Recent) from the Mexican Volcanic Belt (MVB) have been compiled. These analyses all together represent a calc-alkaline sequence of continental margin type. The North-Eastern part of the Belt seems to overlap with another volcanic province, namely. Eastern Cordillera in which magmas are mostly of alkaline sequence. A model involving fractional crystallization as the principal mechanism is proposed for magma genesis in the MVB.

Water and magma generation at subduction zones

Abstract: 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.

Banda arc of eastern indonesia: Petrology and geochemistry of the volcanic rocks

Abstract: A detailed petrological study of the Banda Arc volcanism, documented by extensive microprobe whole-rock and mineral analysis, shows that the apparent geographical eastwards continuity of the Sunda Arc conceals a major geochemical discontinuity adjacent to the southern end of the Weber Deep beneath Serua. The alkali contents and Sr isotope ratios suggest that Nila, Teun and Damar form one volcanic group distinct from Banda and Manuk, and Serua is in the border discontinuity, reflected in its widely fluctuating Fe/Mg partitioning between mineral phases and its variable Sr isotope ratios.

Ar and Sr isotopes of mantle-derived rocks from the Japanese Islands

Abstract: 40Ar-30Ar and Sr isotope analyses have been made for ultramatic nodules in basaltic rocks from Itinome-gata and the Oki-Dōgo Island, northeast and southwest of Japan respectively, in order to examine the state of the upper mantle under the Japanese Islands.

Basic magma generation and aggregation

Abstract: Basic magma generation in the mantle at the present stage of earth history probably begins most commonly in metamorphosed garnet peridotite at those points where the four major phases meet; the solidus defines the spatial limits of the region of melting at the site of origin. On the basis of the forsterite-diopside-pyrope system and the melting relations of natural garnet peridotite at high pressures, the melting is invariant-like up to about 30% liquid. If the melt is fractionally removed, melting temporarily ceases after this limit is reached, terminating the production of liquid of invariant-like composition. Because one phase is eventually consumed at the invariant-like point, melting might be resumed at a higher temperature, generating a different liquid of more basic composition at the invariant-like point governing the assemblage of remaining phases. The garnet peridotite becomes permeable to melt almost immediately after the melting process begins, as has been demonstrated by the large increase in measured electrical conductivity. A large volume of relatively homogeneous liquid can, therefore, be extracted as it is produced.

Dynamics of magma in a volcanic conduit — Special emphasis on viscosity of magma with bubbles

Abstract: A possible dynamic process for magma flow in a volcanic conduit is briefly described. In many of the governing equations, viscosity of magma is involved, and hence, the effective viscosity of magma with small concentration of bubbles was calculated under the assumption of small Reynolds number. The result is $$\eta _\ell = \eta _u (1 + \Phi ),$$ where ηo is the viscosity of a liquid and ϕ is the volume concentration of bubbles. Thus, the effective viscosity increases with nucleation of gas bubbles in magma. This result reduces the effect of a thermal feedback evele which is postulated as a possible thermodynamical process in viscous magma in a volcanic conduit.