Peridotite-CO2-H2O and the low-velocity zone

The properties of the seismic low-velocity zone are consistent with incipient melting of mantle peridotite. Vapor-absent melting of amphibole-peridotite has been used to model the low-velocity zone, but evidence that CO₂ exists in the upper mantle indicates that peridotite-CO₂-H₂O would be a better model. The divariant solidus surface for peridodite-CO₂-H₂O is traversed by a series of univariant lines marking the intersections of divariant subsolidus reactions involving dolomite or magnesite, amphibole, or phlogopite (other hydrous minerals are neglected in this treatment), or combinations of these. The vapor phase compositions are buffered to specific values, which limits the range of vapor compositions that can coexist with peridotite at various pressures. Below about 30 kbar, the vapor phase is buffered by the melting of amphibole-peridotite, with composition ranging from H₂O to high CO₂/H₂O. Above about 25 kbar, the vapor phase is buffered by the melting of dolomite-peridotite, with composition ranging from CO₂ to high H₂O/CO₂ at pressures above 30 kbar. The buffered curve for phlogopite-peridotite intersects the dolomite-peridotite curve, generating another line for phlogopite-dolomite-peridotite; the strong buffering capacity of dolomite forces the vapor on this line to high H₂O/CO₂. Near the buffered curve for the solidus of partly carbonated peridotite there is a temperature maximum on the peridotite-vapor solidus surface. On the CO₂ side of the maximum, above 26 kbar, CO₂/H₂O is greater in liquid than in vapor; on the H₂O side of this maximum, and at all pressures below 26 kbar, CO₂/H₂O is greater in vapor than in liquid. The suboccanic low-velocity zone is caused by incipient melting of amphibole-peridotite in the presence of vapor with high CO₂/H₂O, with generation of forsterite-normative liquid. The subcontinental low-velocity zone, where present, is probably caused by incipient melting of dolomite-peridotite, or phlogopite-dolomite-peridotite, either with H₂O-rich vapor or without vapor, with the generation of CO₂-rich, alkalic, SiO₂-poor liquid (larnite-normative) that in extreme conditions may be carbonatitic.     

A double partial melt zone in the mantle beneath mid-ocean ridges

For a lherzolite mantle with about 0.1 wt.-percent CO₂ or less, and a CO₂/H₂O mole ratio greater than about one, the mantle solidus curve in P-T space will have two important low-temperature regions, one centered at about 9 kbar (30 km depth) and another beginning at about 28 kbar (90 km depth). It is argued that the depth of generation of primary tholeiitic magmas beneath ridge crests is about 9 kbar, and that the geotherm changes from an adiabatic gradient at greater pressures to a strongly superadiabatic gradient at lesser pressures. Such a ridge geotherm would intersect the solidus at two separate depth intervals corresponding to the two low-temperature regions on the solidus. With increasing age and cooling of the lithosphere, the shallow partial melt zone would pinch out and the thickness of the deep partial melt zone would decrease. With increasing depth in a mature oceanic lithosphere, the rock types would consist of depleted harzburgite from directly beneath the crust to about 30 km depth, fertile spinel lherzolite from about 30 km to 50–60 km, and fertile garnet lherzolite from about 50–60 km to the top of the deep partial melt zone at about 90 km.     

Discussion of recent papers on carbonated peridotite, bearing on mantle metasomatism and magmatism: final comment

The experimental study of Olafsson and Eggler [1] is not inconsistent with the model system CaOMgOSiO₂CO₂, if an alternate topology for peridotite-CO₂H₂O is considered. Although peridotite-H₂OCO₂ solidi present barriers to ascent of partially-melted peridotite, solidi should be applied to melts with caution. Melts such as kimberlites plausibly can ascend rapidly through fractures without appreciable reaction with peridotitic wallrocks.     

Experimental evidence for the role of accessory phases in magma genesis

Recent experimental studies have established petrogenetic models based on melting processes involving major phases. The possible residual character of trace-element-enriched accessory phases is not considered for temperatures well above the solidus in these models. In contrast, geochemists, applying trace element data to independently test the experimentally-based models, have concluded that residual (or fractionating) accessory phases may have an essential role in controlling the trace element (especially REE) distributions in magmas.Some recent experimental work provides data on the stability of potentially significant accessories such as sphene, rutile, apatite, zoisite and mica in basaltic compositions at elevated P and T. Sphene is stable to 1000°C with 60% melting of a hydrous tholeiite at 15 kbar. At higher pressure, rutile is the only Ti-rich accessory phase, and is present to at least 1000°C and high degrees of melting. Published REE data on sphene and rutile suggest that these phases may be important in controlling REE distribution in some magmas. For example, island are high-Mg, low-Ca-Ti tholeiites with low REE abundances and U-shaped patterns (Hickey and Frey, 1979) may reflect the role of sphene. In addition to rutile, similar close-packed Ti-rich accessory phases such as priderite, perovskite, crichtonite and loveringite may occur in mantle-derived magmas. These phases readily accommodate the REE but their possible role needs experimental confirmation.Apatite is recorded in hawaiite (1.16% P₂O_s) with 2% H₂O added at 5–6 kbar and 1050°C within 30°C of the liquidus, but at present no other experimental data are available on its high P, T stability, although thermodynamic calculations indicate that F may increase its stability markedly. Apatite is well known in high-pressure inclusions and as a phenocryst phase in rocks of the alkaline and calc-alkaline series.Ilmenite is known as a near-liquidus phase in some mafic magmas at 5–10 kbar, but its stability decreases to near-solidus at 25–30 kbar. Zoisite occurs in hydrous mafic compositions at mantle pressures, but it is confined to temperatures < 780°C. Finally, mica has a wide temperature range of stability at mantle pressures, especially in potassic magmas, and phlogopitic mica is stable to 1040°C at 20–25 kbar in a hydrous, K-rich “tholeiite” (1.6% K₂O).     

Mineral chemistry of a Cenozoic igneous complex,the Urumieh–Dokhtar magmatic belt,Iran: Petrological implications for the plutonic rocks

The Niyasar plutonic complex, one of the Cenozoic magmatic assemblages in the Urumieh‐Dokhtar magmatic belt, was the subject of detailed petrographic and mineralogical investigations. The Niyasar magmatic complex is composed of Eocene to Oligocene mafic rocks and Miocene granitoids. Eleven samples, representing the major rock units in the Niyasar magmatic complex and contact aureole were chosen for mineral chemical studies and for estimation of the pressure, temperature, and oxygen fugacity conditions of mineral crystallization during emplacement of various magmatic bodies. The analyzed samples are composed of varying proportions of quartz, plagioclase, K‐feldspar, hornblende, biotite, titanite, magnetite, apatite, zircon, garnet, and clinopyroxene. Application of the Al‐in‐hornblende barometer indicates pressures of around 0.2 to 0.4 kbar for the Eocene–Oligocene mafic bodies and around 0.5 to 1.7 kbar for the Miocene granitoids. Hornblende‐plagioclase thermometry yields relatively low temperatures (661–780 °C), which probably reflect late stage re‐equilibration of these minerals. The assemblage titanite–magnetite–quartz as well as hornblende composition were used to constrain the oxygen fugacity and H₂O content during the crystallization of the parent magmas in the Miocene plutons. The results show that the Miocene granitoids crystallized from magmas with relatively high oxygen fugacity and high H₂O content (~5 wt% H₂O). The Miocene granitoids show similar range of oxygen fugacity, H₂O contents and mineral chemical compositions, which indicate a common source for their magmas. Although the crystallization pressures of the Miocene plutons discriminate various categories of plutonic bodies emplaced at depths of about 5.7–6.5 km (Marfioun pluton), about 4.2 km (Ghalhar pluton) and 1.9–2.3 km (Poudalg pluton), they were later uplifted to the same level by vertical displacement of faults. The emplacement depths of the Niyasar plutons suggest that the central part of the Urumieh‐Dokhtar magmatic belt has experienced an uplift rate of ca. 0.25–0.4 mm/yr from the Miocene onwards.     

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.     

Basalt-andesite-rhyolite-H2O: Crystallization intervals with excess H2O and H2O-undersaturated liquidus surfaces to 35 kolbras,with implications for magma genesis

Three rocks representing the calc-alkaline rock series gabbro-tonalite-granite or basalt-andesite-rhyolite were reacted with varying percentages of water in sealed capsules between 600 and 1300°C and pressures to 36 kbars, corresponding to depths of more than 120 km within the earth. For each rock we present complete P-T diagrams with excess water, and the water-undersaturated liquids surface projected from P-T-X_H2O space mapped with contours for constant H₂O contents and with the fields for near-liquidus minerals. All changes in liquidus and solidus slopes can be correlated with changes in mineralogy from less dense to more dense, or with expansion of crystallization fields, without appeal to changes in molar volume of H₂O in liquid and vapor phases. The results indicate that tholeiites and andesites of the calc-alkaline series with compositions similar to the rocks studied are not primary magmas from mantle peridotite at depths greater than about 50 km. Primary andesitic magmas from shallower levels would require very high water contents and we do not believe such magmas could normally reach the surface. The liquids results are consistent with the derivation of andesites with little dissolved water as primary magmas from subducted ocean crust (quartz eclogite), but multi-stage models are preferred. Temperatures required for the generation of andesites by fusion of continental crust are higher than considered reasonable. The evidence precludes the generation of primary rhyolites or granites from the mantle of subducted oceanic crust at mantle depths. Primary rhyolite or granite magmas with moderate water contents (saturated or undersaturated) can be generated in the crust at reasonable temperatures, and could reach near-surface levels before vesiculation. Water-undersaturated granite liquid with residual crustal minerals could constitute plutonic magmas of intermediate composition.     

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.     

Melting of hydrous upper mantle and possible generation of andesitic magma: An approach from synthetic systems

Phase equilibria in a portion of the system forsterite-plagioclase (An₅₀Ab₅₀ by weight)-silica-H₂O have been determined at 15 kbar pressure under H₂O-saturated conditions. The composition of the liquid pertinent to the piercing point forsterite + enstatite solid solution + amphibole + liquid + vapor is similar to that of calc-alkaline andesite. The electron microprobe analysis of the glass coexisting with the above three crystalline phases is very close to that of the piercing point determined by phase assemblage observations; however, the glass near (< 8 μm) forsterite crystals is significantly depleted in the normative forsterite component. With the addition of 10 wt.% KAlSi₃O₈, the composition of this piercing point becomes even closer to the compositions of calc-alkaline andesites. It is also shown that the liquid coexisting with forsterite and enstatite solid solution remains silica-rich (60–62 wt.%) over a wide (～ 100°C) temperature range. The present experimental studies support the view that liquids similar in composition to calc-alkaline andesites can be generated by direct partial melting of hydrous upper mantle at least at or near 15 kbar.     

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.     

Apatite and phosphorus in mantle source regions: An experimental study of apatite/melt equilibria at pressures to 25 kbar

The solubility of fluorapatite in a wide variety of basic magmatic liquids was experimentally determined over a range of upper mantle P-T conditions (8–25 kbar, 1275–1350°C). Fluorapatite is stable over the entire range of conditions investigated, but its solubility in melts is variable, depending negatively on SiO₂ content of the melt and positively upon temperature, with relatively little sensitivity to pressure above 8 kbar. At upper mantle pressures and a temperature of 1250°C, molten basalt (50% SiO₂) will dissolve 3–4 wt.% P₂O₅ before saturation in apatite is reached. For a magma 100°C cooler or containing 10 wt.% more SiO₂, apatite saturation occurs at less than 2 wt.% dissolved P₂O₅. The observed high solubility of apatite in basic magmas at their normal near-liquidus temperatures virtually precludes the occurrence of residual apatite in mantle source regions. If relatively low-temperature melting conditions prevail (e.g., 1100°C), as might be possible in H₂O-bearing regions of the upper mantle, apatite could remain in the residue, but only in amounts too small to have significant effects on the rare earth patterns of the liquids.Because of the high solubility of apatite in basic magmas, phosphorus can be confidently treated as an incompatible element in peridotite melting models. Such models, in combination with observed characteristics of basic lavas, indicate that the upper mantle contains ～200 ppm of phosphorus, much less than the chondritic abundance of ～900 ppm.     

Magma chambers beneath the Klyuchevskoy volcanic group (Kamchatka)

A 3D velocity model of the Earth’s crust beneath the Klyuchevskoy volcanic group has been constructed using the seismic tomography method. Anomalies of the velocity parameters related to the zones of magma supply to active volcanoes have been distinguished. Petrological data on the composition, temperature, and pressure of generation and crystallization of parental melts of Klyuchevskoy volcano magnesian basalts have been obtained. The parental melt corresponds to picrite (MgO = 13–14 wt %) with an ultimate saturation of SiO₂ (49–50 wt %), a high H₂O content (2.2–2.9%), and incompatible elements (Sr, Rb, Ba). This melt is formed at pressures of 15–20 kbar and temperatures of 1280–1320°C. Its further crystallization proceeds in intermediate magma chambers at two discrete pressure levels (i.e., greater than 6, and 1–2 kbar). The results of the petrological studies are in good agreement with the seismotomographic model.     

Volcanological and petrological evolution of Marsili Seamount (southern Tyrrhenian Sea)

A swath bathymetric survey was conducted on Marsili Volcano, the biggest seamount in the Tyrrhenian Sea. It stands 3000 m above the surrounding oceanic crust of the 3500 m-deep Marsili back-arc basin and is axially located within the basin. The seamount has an elongated shape and presents distinctive morphology, with narrow (<1000 m) ridges, made up of several elongated cones, on the summit zone and extensive cone fields on its lower flanks. A dredging campaign carried out at water depths varying between 3400 and 600 m indicates that most of Marsili Seamount is composed of medium-K calc-alkaline basalts. Evolved high-K andesites were only recovered from the small cones on the summit axis zone. Petrological and geochemical characteristics of the least differentiated basalts reveal that at least two varieties of magmas have been erupted on the Marsili Volcano. Group 1 basalts have plagioclase and olivine as dominant phases and show lower Al, Ca, K, Ba, Rb and Sr, and higher Fe, Na, Ti and Zr with respect to a second type of basaltic magma. Group 2 basalts reveal the presence of clinopyroxene as an additional phenocryst phase. In addition, the two basaltic magmas have different original pre-eruptive H₂O content (group 1, H₂O-poor and group 2, H₂O-rich). Moreover, comparison of the compositional trends and mineralogical compositions obtained from MELTS [Ghiorso, M.S., Sack, R.O., Contrib. Mineral. Petrol. 119 (1995) 197–212] fractional crystallization calculations reveal that the evolved andesites can only exclusively be derived from a low-pressure (0.3 kbar) fractionation of magmas compositionally similar to the least evolved group 2 basalts. Finally, we suggest that the high vesicularity of the basalts sampled at relatively great depths (>2400 m) on the edifice is governed by H₂O and, probably, CO₂ exsolution and is not a feature indicative of shallow water depth eruption.     

Petrological models of the oceanic lithosphere: Geophysical and geochemical tests

Petrological models of the oceanic lithosphere are tested to satisfy geophysical and geochemical constraints within the framework of plate tectonics. Quartz eclogite, olivine eclogite, peridotite and dunite are considered as the material of the lithosphere. The temperature at the base of the lithosphere is assumed to be the solidus temperature. This temperature, the thermal conductivity, and the heat flow and topography changes with age are used as the geophysical constraints. The compressional wave velocity-depth profile is used to select preferred models. Among geophysically successful models, high-temperature models are preferred to wet low-temperature models, because the low-temperature models have difficulties in explaining the mechanism of generation of oceanic basalt magmas. A preferred model is a two-layer model 70 km thick consisting of peridotite at the upper lithosphere and olivine eclogite at the lower lithosphere bounded at the base by the dry solidus.     

Phase equilibria and metamorphic evolution of garnet‐mica‐plagioclase gneisses from the Qiliping terrane in the western Dabie Orogen,central China

The extensive gneisses in the high‐pressure and ultrahigh‐pressure metamorphic terrane in the Dabie‐Sulu orogen usually show no evidence of eclogite‐facies metamorphism. The garnet‐mica‐plagioclase gneisses from the Qiliping region in the western Dabie Orogen, comprise garnet, phengite, biotite, plagioclase, quartz, rutile, ilmenite, chlorite, epidote, and hornblende. The garnet porphyroblasts, with inclusions of quartz, epidote, and rutile, exhibit slight compositional zonations, from core to mantle with an increase in pyrope and a decrease in spessartine, and from mantle to rim with a decrease in pyrope and grossular and an increase in spessartine. The high‐Si phengite indicates that the gneisses may be subjected to a high‐pressure metamorphism. By the P–T pseudosections calculated in a system NCKMnFMASHTO (Na₂O‐CaO‐K₂O‐MnO‐FeO‐MgO‐Al₂O₃‐SiO₂‐H₂O‐TiO₂‐O) for two representative samples, the metamorphic P–T path, reconstructed by the compositionally zoned garnet, shows that the prograde metamorphism is characterized by a temperature increase with a slight pressure increase from the conditions of 17.6 ± 1.5 kbar at 496 ± 15°C to the peak‐pressure ones of 21.8 ± 1.5–22.7 ± 1.5 kbar at 555 ± 15–561 ± 15°C; the early retrograde stage is dominated by decompression with a temperature increase to the maximum of 608 ± 15–611 ± 18°C at 10.3 ± 1.5–11.0 ± 1.5 kbar; and the late retrograde one is predominated by pressure and temperature decreases. The mineral assemblages in the prograde metamorphism are predicted to contain garnet, glaucophane, jadeite, lawsonite, phengite, quartz, rutile, and/or chlorite, which is different from those observed at present. Such high‐pressure metamorphism can partly be reconstructed by the P–T pseudosection in combination with the high‐Si phengite and garnet compositions in the core and mantle. This provides an important constraint on the subduction and exhumation of the terrane during the continent–continent collision between the Yangtze and Sino‐Korean cratons.     

Dehydration of clastic sediments in subduction zones: Theoretical study using thermodynamic data of minerals

Pseudosections for two sediments and one basalt calculated in the system K₂O–Na₂O–CaO–MgO–FeO–Fe₂O₃–Al₂O₃–TiO₂–SiO₂–H₂O for the P–T range 10 to 35 kbar, 300 to 900°C give useful insights into the amount of H₂O released from oceanic crust in subduction zones. In cold subduction zones (20 kbar–300°C to 35 kbar–500°C) hydrous minerals storing 3 to 4 wt% H₂O are still present in metasediments at depths of 120 km. In the same environment, metabasite releases 1 wt% H₂O in the depth range 100 to 120 km, but 4.5 wt% H₂O is transported to greater depths. In hot subduction zones (300°C hotter than the cold subduction zone at 100 km depth), dehydration events of metasediments in the depth range 50 to 80 km correspond to the breakdown of chlorite and paragonite. In the calculations no further water is released at greater depths because the modal content of phengite, the only hydrous mineral phase at these depths, remains almost constant. For the same P–T path, metabasite shows continuous dehydration between 40 and 80 km releasing almost 3 wt% H₂O. At 120 km depth less than 0.4 wt% of H₂O remains. In an average modern subduction zone (～6°C/km) most dehydration of sediments occurs at depths of 70 to 100 km and that of basalts at depths of 80 to 120 km. Only 1.3 wt% H₂O in metasediments and 1.6 wt% H₂O in metabasalt has the potential to be subducted to depths greater than 120 km. The dehydration behavior of sediments concurs with the generally held idea that subduction zone fluids are most effectively transported to great depths by cold subduction. In hot subduction zones, such as those characteristic of early Earth, most H₂O carried by oceanic crust is liberated at depths less than 120 km and, thus, would not contribute to island‐arc magmatism.     

Phase equilibria modeling in igneous petrology: use of COMAGMAT model for simulating fractionation of ferro-basaltic magmas and the genesis of high-alumina basalt

A new version of COMAGMAT-3.5 model designed for computer simulations of equilibrium and fractional crystallization of basaltic magmas at low to high pressures is presented. The most important modifications of COMAGMAT include an ability to calculate more accurately the crystallization of magnetite and ilmenite, allowing the user to study numerically the effect of oxygen fugacity on basalt magma fractionation trends. Methodological principles of the use of COMAGMAT were discussed based on its thermodynamical and empirical basis, including specific details of the model calibration. Using COMAGMAT-3.5 a set of phase equilibria calculations (called Geochemical Thermometry) has been conducted for six cumulative rocks from the Marginal Border Series of the Skaergaard intrusion. As a result, initial magma temperature (1165±10°C) and trapped melt composition proposed to be parental magma to the Skaergaard intrusion were determined. Computer simulations of perfect fractionation of this composition as well as another proposed parent produced petrochemical trends opposite to those followed from natural observations. This is interpreted as evidence for an initial Skaergaard magma containing a large amount of olivine and plagioclase crystals (about 40–45%), so that the proposed and calculated parents are related through the melt trapped in the crystal–liquid mixture. This promotes the conclusion that the Skaergaard magma fractionation process was intermediate between equilibrium and fractional crystallization. In this case the classic Wager's trend should be considered an exception rather than a rule for the differentiation of ferro-basaltic magmas. A polybaric version of COMAGMAT has been applied for the genetic interpretation of a volcanic suite from the Klyuchevskoi volcano, Kamchatka, Russia. To identify petrological processes responsible for the observed suite ranging from high-magnesia to high-alumina basalts, we used the model to simulate the Klyuchevskoi suite assuming isobaric crystallization of a parental HMB magma at a variety of pressures and a separate set of simulations assuming fractionation during continuous magma ascent from a depth of 60 km. These results indicate that the Klyuchevskoi trend can be produced by 40% fractionation of Ol–Aug–Sp±Opx assemblages during ascent of the parental HMB magma over the pressure range 19–7 kbar with the rate of decompression being 0.33 kbar/% crystallized (at 1350–1110°C), with 2 wt.% of H₂O in the initial melt and 3 wt.% of H₂O in the resultant high-Al basalt.     

The before-eruption water content of some high-alumina magmas

An analysis by difference technique yields estimates of H₂O in basaltic and andesitic glasses, which are sufficiently accurate (± 1.4 percent absolute) to be useful. Glass inclusions trapped in large olivine crystals from tephra-rich eruptions have 1 to 5 percent H₂O. The highest H₂O contents are found in basaltic inclusions in magnesium rich olivines from Mount Shasta, California. Andesitic inclusions have less H₂O. It seems probable that tephra-rich high-alumina magmas evolve in a vapor saturated environment at fairly shallow depths (few kilometers). This depth appears to be less for Medicine Lake Highlands than for Mount Shasta. Vapor saturation probably inhibits the rise of magma, thus the initial vapor content of a magma may govern its stagnation level. Volatile-rich parental magmas like Mount Shasta basalt probably tend to stagnate at deeper levels, crystallize early amphibole and produce comparatively calcic differentiates.     

Lateral variations of H2O contents in quaternary magmas of Northeastern Japan

Volcanoes of the East Japan volcanic arc are divided into two groups on the basis of their phenocryst assemblages; volcanoes with lavas or pyroclastic rocks containing quartz phenocrysts and no hornblende phenocrysts (type A), and those with rocks containing hornblende phenocrysts and no quartz phenocrysts (type B). Type A volcanoes occur only in the narrow region along the volcanic front, whereas type B volcanoes are distributed in the area closer to the Sea of Japan.Recent experimental studies on calc-alkaline andesite-dacite under H₂O-saturated and -undersaturated conditions indicate that the liquidus temperature (maximum thermal stability limit) of quartz decreases drastically with increasing H₂O content in magma, whereas the liquidus temperatures of hornblende and biotite are relatively constant with variations in the H₂O content and bulk chemical composition of the magma.It is suggested from the lateral variation of mafic phenocryst assemblages [1] and from the above result that the temperature of the parental magmas of these volcanoes increases, and their H₂O contents decrease, towards the volcanic front in the East Japan volcanic arc.Such lateral variations in the H₂O contents of magmas under the East Japan volcanic arc are in agreement with those of other incompatible elements (K, Rb, REE, etc.). If H₂O-undersaturated partial melting of upper mantle peridotite can be represented by the univariant line (olivine, Ca-rich clinopyroxene, orthopyroxene and liquid coexist) in the system H₂OMg₂SiO₄z.sbnd;CaMgSi₂O₆z.sbnd;SiO₂, the decrease of H₂O content in the magma suggests that the melting temperature of the peridotitic mantle may gradually increase, and so the degree of partial melting may increase, towards the volcanic front. The lateral variation of other incompatible elements can also be explained by this model.     

New data on magmatic H2O contents of pantellerites,with implications for petrogenesis and eruptive dynamics at Pantelleria

Infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at Pantelleria, Italy, show that the magmas contained moderate pre-eruptive H₂O contents, ranging from 1.4 to 2.1 wt.%. Melt H₂O concentrations increase linearly with incompatible elements, demonstrating that H₂O contents were not buffered significantly during fractionation by any crystalline or vapor phase. The relatively low H₂O contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt. Preeruptive H₂O concentrations do not correlate with the volume or explosivity of pantellerite eruptions; decompression history is critical in determining the style of pantellerite (and other) eruptions.