Hiroaki Sato

Nickel content of basaltic magmas: identification of primary magmas and a measure of the degree of olivine fractionation

Abstract Available NiO analyses of olivine in peridotites of probable mantle origin are consistent in giving values around 0.40 weight per cent. Assuming that basaltic magma forming from the mantle was in equilibrium with such peridotitic olivine, the NiO content of primary basaltic magmas is estimated to be about 0.030–0.050 weight per cent. The fractionation behaviour of nickel in basaltic magma due to the crystallization of olivine has been calculated using constant NiMg and FeMg exchange partition coefficients between olivine and magma. It is shown that the NiO content of both magma and olivine decreases by 50 per cent after fractional crystallization of 6–12 per cent of olivine. The nickel distribution in some basaltic rocks and olivines is examined in the light of these results, and it is suggested that basaltic magmas, such as some of the ocean-floor basalt and the Hawaiian tholeiite and alkali basalts, represent primary magmas from mantle peridotites.

Diffusion coronas around quartz xenocrysts in andesite and basalt from Tertiary volcanic region in northeastern Shikoku, Japan

Coronas around quartz xenocrysts in andesite and basalt from Tertiary volcanics in northeastern Shikoku, Japan, have been described. The coronas are composed mainly of Ca-rich clinopyroxene and glass. Compositional profiles across the corona glass show monotonous variation of major elements except for alkalis. Preliminary experiment on the reaction between basaltic melt and quartz has shown that alkalis diffused against their concentration gradients. This particular feature of alkali enrichment in corona glass is explained by a diffusion model, in which non-ideality of alkalis in silicate melt is assumed. Preferred crystallization of Ca-rich clinopyroxene in coronas of orthopyroxene andesite is discussed using a chemical potential diagram in the system SiO2-CaO-RO (RO=MgO+FeO), and it is suggested that higher (Na+K)/Al ratio of the corona glass, which increases the effective CaO concentration and thus increases the μCaO/μRO, is responsible for the preferred crystallization of Ca-rich clinopyroxene.

Groundmass pargasite in the 1991–1995 dacite of Unzen volcano: phase stability experiments and volcanological implications

Abstract Pargasite commonly occurs in the dacitic groundmass of the 1991–1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the dacite. The 1991–1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe–Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen dacite to be 880±30°C, suggesting that the groundmass pargasite crystallized at >70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991–1995 dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.

Endogenous growth of dacite dome at Unzen volcano (Japan), 1993–1994

A dacite dome at Unzen volcano grew mainly exogenously when it was small and the effusion rate was high, but endogenously when the dome became large and the effusion rate declined. The endogenous dome that has grown since late 1993 shows a shape classified as “Pelean,” whereas the earlier stage had a “low lava dome” shape. The carapace of the endogenous dome moved like the crust of a basaltic lava pillow, although the dome is several hundred times larger than such pillows. The surface carapace was carried from the inside of the dome where it had been produced, and it thickened as it cooled. The crater floor was strongly deformed by the advancing endogenous dome. The movement and crater floor deformation can be compared to that of a tractor tread moving on unconsolidated ground.

Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products

Abstract Plagioclase, the most abundant phenocryst at Mount Pinatubo, displays varying textures and compositions within the 1991 eruption products. In June 7-14 dome-forming andesite, plagioclase phenocrysts show prominent rims with higher MgO (0.04-0.06 wt%), Fe2O3T (0.6-0.8 wt%), and K2O at given An than the interiors. The compositions of the rims are identical to those of microlites, which are abundant in the groundmass glass. White dacitic pumice, the most voluminous product of the June 15 eruption, contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites. The interiors of phenocrysts from the dacitic pumice and the dome-forming andesite are remarkably similar in terms of textures and compositions. They show oscillatory zoning (mostly An35-60), low MgO (<0.02 wt%) and Fe2O3T (0.10-0.30 wt%), and similar K2O at given An. This similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt. The oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids. A portion of the felsic magma (~800 °C) mixed with a mafic melt (~ 1000-1100 °C) to become an andesitic magma that extruded to form the June 7-14 dome. All plagioclase phenocrysts in the andesite were derived from the felsic magma. The mixing caused de stabilization of phenocrysts, forming sieve textures, dusty zones, and partial resorption. Extrusion of the mixed magma resulted in overgrowths on once-resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass. In the unmixed, remaining portion of the felsic magma, some plagioclase underwent partial resorption but did not develop overgrowths. This lack of overgrowths and the absence of microlites in the ground- mass glass of the June 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption, supporting the linkage between the two.

Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay, North Xinjiang

Field observation, petrological and geochemical characteristics of plagiogranite from Kuerti ophiolite indicate a similar origin to those in shearing zones. It is derived from partial melting of amphibolite that is developed from gabbro within the ocean layer 3 shear zone by the low-angle shearing deformation during the oceanic crust migrating process. Zircon SHRIMP age of 372±19 Ma for the plagioganite from Kuerti ophiolite indicates that this ophiolite formed in the Devonian period and it also represented the time of extension of the Kuerti backarc basin that is relevant to the northwards subduction of the Paleo-Asian oceanic crust. Therefore, the northwards subduction of the Paleo-Asian Ocean beneath the Siberian Plate began in the early stage of the Late Paleozoic era.

Relationship between crater size and ejecta volume of recent magmatic and phreato‐magmatic eruptions: Implications for energy partitioning

Relationships between crater diameter (D in meters) and ejecta volume (V in cubic meters) of recent magmatic and phreato-magmatic eruptions are expressed as D=0.11 V0.42, and D=0.97 V0.36, respectively. Crater diameters of phreato-magmatic eruptions are ca. 2–5 times larger than those of magmatic eruptions of similar ejecta volume, suggesting that magma-water interaction in phreato-magmatic eruptions generated mechanical energies of crater formation 1–2 orders of magnitude larger than equivalent-volume magmatic eruptions. The conversion ratio of thermal energy to kinetic energy in phreato-magmatic eruptions is estimated to be 0.7–10 percent, which is in accord with the presently available data of laboratory experiments on melt-water interaction [Wohletz and McQueen, 1984].

Textural difference between pahoehoe and aa lavas of Izu-Oshima volcano, Japan — an experimental study on population density of plagioclase

The groundmass textures of 1778 A.D. pahoehoe and 1986 A.D. aa lavas of Izu-Oshima volcano, Japan, differ in population density of plagioclase by about two orders of magnitude. The pahoehoe lavas are coarser grained and have a population density of lo’.’ cme3, while the aa lavas are finer grained and have a population density of 109.3 cmA3. The groundmasses of these texturally different lavas, however, have nearly the identical chemical compositions. One-atmosphere melting/crystallization experiments on the lavas showed that a 20°C difference in initial-melting temperature near the liquidus temperature can cause about five orders of magnitude difference in the population density of plagioclase after annealing at about 100°C below the liquidus temperature. Yet, more than two orders of magnitude difference in cooling rate of the experiments only bring about less than one order of magnitude difference in population density. The large effect of the initial-melting temperature on the population density of plagioclase is interpreted to reflect nucleation induced by the transformation of polymerized clusters in the melt into crystal nuclei by a reduction in the critical size of the nuclei; the initial size distribution of clusters in the melt largely affects the population density of plagioclase. During natural eruptive processes, degassing of magmas produces strongly undercooled conditions, and it is proposed that a slight difference in the degree of undercooling of magmas before final degassing and eruption may have caused the large difference in the population density of plagioclase of the pahoehoe and aa lavas.

Water contents and hydrogen isotopic ratios of rocks and minerals from the 1991 eruption of Unzen volcano, Japan

Abstract Water contents and hydrogen isotopic ratios were determined for blocks from pyroclastic flow deposits, and bread-crust bombs and blocks from the 1991 Vulcanian eruptions of Unzen volcano, Japan. Groundmass water contents and δD values of samples were calculated by subtracting the contribution of major hydrous minerals (hornblende and biotite) from the bulk rock analyses, and range from 0.1 to 0.5 wt.% and −83 to −49‰, respectively. The samples do not show a systematic H2O–δD relationship, although the block samples tend to have lower δD values than the bomb samples. The non-systematic H2O–δD relationship is likely a result of near surface, kinetically-controlled gas loss. High viscosity of this magma would hinder attainment of hydrogen isotopic equilibrium between exsolved vapor and melt in the final degassing stage. The near surface degassing, however, was accompanied by kinetic fractionation resulting in enrichment of deuterium in the final products as exemplified by bread-crust bombs with high H2O–low δD margins and low H2O–high δD cores. Relatively high δD values of the blocks and bombs as well as high temperature volcanic gas (−30 to −35‰) suggest a closed system degassing of an initial water-rich magma (H2O=6 wt.%) until its water content was reduced to 0.5 wt.%. The pre-eruptive δD value (−46‰) was estimated from the volcanic gas data and D/H analysis of hornblende phenocrysts coupled with assumed isotopic equilibration in the initial hydrous magma.

Olivine phenocrysts in some Cameroonian Basalts—implications for primary magma composition

SummaryThe chemical composition of olivine phenocrysts was determined for 13 basaltic samples taken from the central part of the Cameroon Volcanic line to estimate the compositions of the primary olivine phenocrysts and the primary magmas. The Mg/(Mg + Fe) ratios of the olivines attain 0.91, though many of the magnesian olivines are identified on a textural basis as xenocrysts from peridotite inclusions. Most magnesian olivine phenocrysts have Mg/(Mg + Fe) ratios of 0.87–0.88 and NiO contents of 0.32–0.35 wt %. The NiO versus Mg/(Mg + Fe) ratios of the olivines can be used to estimate the compositions of the primary phenocrysts in a range of Mg/(Mg + Fe) = 0.88–0.90. Assuming the Fe(3)/(total Fe) ratios of the magmas are 0.17–0.25, which is estimated from the Fe-Mg partitioning between plagioclase and groundmass, the compositions of the primary magmas were obtained from the Fe2+-Mg partition equilibrium with the primary olivine phenocrysts. The primary magmas of the Cameroonian basalts have 13.5 ± 4.0 wt MgO, which is similar to the compositions of basalts with highest MgO contents reported so far from the Cameroon volcanic line.ZusammenfassungDie chemische Zusammensetzung von Olivin Phenokristallen wurde in 13 Basalt-Proben, die vom Zentralteil .des Kamerun-Vulkangürtels stammen, bestimmt, um Hinweise auf die Zusammensetzung der primären Olivineinsprenglinge and des primären Magmas zu erlangen. Die Mg/(Mg + Fe) Verhältnisse der Olivine reichen bis 0.91, obwohl viefle dieser Mg-Olivine auf Grund textureller Kriterien als aus Peridotit-Einschlüssen stammende Xenokristalle zu identifizieren sind. Die Mg/(Mg + Fe) Verhältnsse der Hauptmasse der Olivin-Phenokristalle liegen im Bereich von 0.87 bis 0.88, die Ni0 Gehalte zwischen 0.32 und 0.35 Gew %. Die Ni0 gegen Mg/(Mg + Fe) Verhältnisse eignen sich zur Abschätzung der Zusammensetzung der primären Phenokristalle, solange die Mg/(Mg + Fe) Verhältnisse im Bereich von 0.88 bis 0.90 liegen. Die Zusammensetzung des Primärmagmas wurde mittels der Fe2+-Mg-Verteilung zwischen dem Magma und den sich mit dem Magma im Gleichgewicht befindlichen primären Olivinkristallen unter der Annahme eines Fe3+/Fe total Verhältnisses von 0.17–0.25, welches sich aus der Mg-Fe Verteilung zwischen Plagioklas und Grundmasse ergibt, bestimmt. Demnach ergibt sich für das primäre Magma der Kamerun Basalte ein MgO Gehalt von 13.5 + 4.0 Gew%. Dieser Wert ist mit der Zusammensetzung von Basalten mit höchsten MgO-Gehalten, wie sie bisher vom Kamerun-Vulkangürtel berichtet worden sind, vergleichbar.