Water saturated phase relations of some andesites with application to the estimation of the initial temperature and water pressure at the time of eruption

Near-liquidus phase relations in one-atmosphere dry and water-saturated high pressure conditions were experimentally determined on products of three historic andesitic eruptions. Run conditions ranged from 900 to 1100°C, at pressures up to 1500 bars with fO₂ controlled close to the nickel-nickel oxide (NNO) buffer. In order to represent the compositions of the true liquid parts of the erupting magmas, groundmass portions were mechanically separated from the porphyritic andesites. Such groundmass materials should lie exactly on the liquidus field boundary between the phases precipitating from the magma just prior to eruption under the prevailing P-T condition.All the samples showed a crossing of the plagioclase and orthopyroxene liquidi in the pressure-temperature range from 1 to 800 bar and 950–1090°C. The crossing condition approximates the magmatic condition immediately prior to eruption. In the case of the 1970 eruption of Akita-komagatake, the crossing point is at 150 bar and 1090°C, matching closely the observed explosive gas pressure and the temperature. In both cases of the 1783 eruption of Asama and the 1914–1915 eruption of Sakurajima volcanoes, the crossing point shift from higher water pressure and lower temperature for the earlier erupting magmas to lower pressure and higher temperature for the later magmas. This regularity may be explained by a vertical gradient of the temperature and water content within the magma column prior to eruption.     

Die ausseralpine Taphrogenese im kaledonisch-variszisch konsolidierten sardischen Vorlande

Zusammenfassung Ein großer Graben teilt das sardische Vorland von Norden nach Süden, wobei nur der Osthorst (Gennargentu) vollständig, jedoch zersplittert, stehengeblieben ist. Vom Westhorst dagegen ragen nur einige Bruchstücke (Iglesiente, Nurra) vom Meer empor. Viele Seitengräben zweigen vom Hauptgraben ab, so daß die Verstellung der getroffenen Schollen ein asymmetrisches Bild der kratonischen Tektonik hervorgerufen hat.Die Grabenfüllung ist hauptsächlich durch Vulkanite erfolgt; die Sedimente spielen nur eine untergeordnete Rolle. Die Eruptionen beginnen mit dem Oligozän und enden mit dem Pleistozän. Eine Trennung in zwei Zyklen (vorhelvetisch bzw. nachtortonisch) gilt für Südsardinien nicht, weil hier submarine Explosionen (Diatremen) und Tuffiteinschaltungen auch während der mittelmiozänen Transgression vorgekommen sind.Geochemisch gehören die Vulkanite der Kalkalkalireihe, jedoch mit atlantischen und mediterranen Tendenzen. Wenn man eine lokale Aufschmelzung des unteren Sockels des sardischen Kratons annimmt, kann man eine palingenetische Abstammung der tertiären Liparite von variszischen Graniten verstehen.Die Bruchlinien bei der taphrogenetischen Deformation unseres alten Vorlandes lassen zwei dominierende Richtungen erkennen: Südost-Nordwest in Südsardinien (Campidano), Nordost-Südwest in Nordsardinien (Tirsograben und Logudoro). Im Rahmen der Strukturelemente des westlichen Mittelmeeres kann man auch eine horizontale Schollenverschiebung (eine Art Epirophorese im Kleinen!) für Sardinien annehmen, und zwar eine relative Bewegung des Iglesiente in Südost-Richtung, d.h. parallel der großen Abschwenkung des Balearenbogens.

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A great Graben cuts the Sardic Foreland. Its Eastern lip is preserved in a fractured state, the Western rim subsidized to a great extent. Side-grabens diverge from the rift asymmetrically. The Graben-fill consists mainly of igneous material of Oligocene to Recent age. Maindirections of taphrogenesis are NW (in the S-portion) and NE in Northern Sardinia. Locally lateral movements are suspected.

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Résumé Un grand graben divise l'avant-pays sarde du Nord au Sud; seul le horst oriental (Gennargentu) est resté tout à fait en place bien que fracturé. Du horst occidental seuls quelques blocs (Iglesiente, Nurra) émergent de la mer. De nombreux graben latéraux bifurquent à partir du graben principal, de sorte que le déplacement des blocs a entraîné une disposition asymétrique de la tectonique cratonique.Le remplissage du graben s'est fait essentiellement par des volcanites; les sédiments n'occupent qu'une place secondaire. Les éruptions débutent à l'Oligocène et se terminent au Pléistocène. Une division en 2 cycles (préhelvétique et posttortonique) ne s'applique pas à la Sardaigne du Sud, car ici des explosions sous-marines (Diatremes) et des intercalations de tuffites sont également intervenues durant la transgression du Miocène moyen.Du point de vue géochimique les volcanites appartiennent à la série calcoalcaline avec cependant une tendance atlantique et méditerranéenne. En admettant une fusion locale du socle inférieur du craton sarde, on comprend alors la dérivation palingénétique des liparites tertiaires à partir des granites varisques.Les lignes de fracture liées à la déformation taphrogénétique de notre ancien avant-pays montrent deux directions dominantes: SE-NW en Sardaigne méridionale (Campidano), NE-SW en Sardaigne septentrionale (graben du Tirso et de Logudoro). Dans le cadre des éléments structuraux de la Méditerranée occidentale on peut également admettre en Sardaigne un déplacement horizontal des compartiments (du genre épirophorèse en petit!), à savoir un déplacement relatif d'Iglesiente vers le sud-est, c'est-à-dire parallèlement au grand décalage de l'arc des Baléares.

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Riassunto Una geoanticlinale emergente dal centro del Mediterraneo occidentale formava l'Avampaese Sardo durante il Mesozoico.Questa dorsale, già consolidata con le orogenesi caledonica ed ercinica, poi peneplanata e solo in parte invasa da mari epicontinentali, dopo aver subito ampie oscillazioni, è stata alla fine ridotta in frammenti da una tettonica disgiuntiva terziaria.I pilastri, residui dello sbloccamento, formano il caratteristico rilievo attuale di media montagna (tipo Gennargentu), isolato da depressioni a sviluppo ed orientamento ineguali, convergenti verso una zona a direzione meridiana. Questa è la fossa principale del Sistema («fossa tettonica sarda» per eccellenza, fra il Golfo dell'Asinara a N e quello di Cagliari a S).In questa grande fossa la pianura del Campidano, nella Sardegna meridionale, non è che la depressione attuale, risultante dallo spostamento dell'asse di maggior sprofondamento, tuttora in via di assetto.D'altra parte, parecchie «fosse laterali», minori, penetrando nell'antico massiccio, hanno staccato dallo stesso altrettante schegge. Lo sbandamento di questi frammenti dei pilastri tettonici conferisce alle fosse una asimmetria caratteristica, specialmente evidente dalla Valle del Tirso al Golfo dell'Asinara.Lo sprofondamento delle fosse tettoniche è stato accompagnato da un vulcanismo a intensità decrescente dall'Oligocene al Plistocene.La distinzione in due cicli (pre-elveziano, rispettivamente post-tortoniano) non è valida per la Sardegna meridionale, dove (Marmilla e Campidano) le manifestazioni vulcaniche (diatremi e intercalazioni di tufiti) sono continuate anche durante l'invasione marina del Miocene medio.Nella colmata delle fosse tettoniche prevalgono le vulcaniti sui sedimenti; tra le lave prevalgono quelle acide sulle basiche, talvolta in alternanza fra loro.Dalle discordanze tra le varie fasi eruttive e dalle intercalazioni lacustri fra le stesse risulta che entro le fosse tettoniche sono awenute dislocazioni secondarie per piegamento.Dal punto di vista geochimico le lave appartengono alla serie alcalicalcica con tendenze verso le serie alcaline (atlantica e mediterranea). Se si ammette peró una parziale fusione della base granitica del cratone formante l'Avampaese Sardo, il vulcanismo delle nostre fosse tettoniche potrebbe essere un riflesso di oscuri rapporti fra palingenesi e tafrogenesi.In fine, nel tentativo di inserire le dislocazioni disgiuntive, che hanno colpito durante il Terziario l'Avampaese Sardo, si potrà forse riconoscere anche una traslazione orizzontale di zolle (epiroforesi) ed in particolare uno spostamento in blocco dell'Iglesiente, al di qua della fossa tettonica del Campidano, in direzione concordante con la grande deviazione verso Sud-est dell'arco delle Isole Baleari.

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Uranium in spinel peridotite inclusions in basalts from Sardinia

The uranium distribution in spinel peridotite inclusions and their host basalt from Sardinia, Italy, was determined by fission-track mapping. Whole-rock U concentrations range from 14 to 55 ppb. Although the partitioning of U among major silicate phases of the inclusions — olivine, orthopyroxene and clinopyroxene — remains roughly constant, the U content in the minerals is highly variable, e.g. ranging from 27 ppb to 177 ppb in clinopyroxene. The U variation in the minerals shows no apparent correlation with their major element chemistry. Liquid which equilibrated with the assemblages of inclusions with high U content, had U concentrations higher than those found in basaltic rocks. It is suggested that the inclusions were contaminated with a phase strongly enriched in U and subsequently recrystallized. The available data show that spinel peridotite inclusions of basaltic rocks frequently have complex multistage evolution and thus cannot provide a representative picture of the upper mantle radioactivity.     

Impact of road‐generated storm runoff on a small catchment response

The impact of road‐generated runoff on the hydrological response of a zero‐order basin was monitored for a sequence of 24 storm events. The study was conducted in a zero‐order basin (C1; 0·5ha) with an unpaved mountain road; an adjacent unroaded zero‐order basin (C2; 0·2 ha) with similar topography and lithology was used to evaluate the hydrological behaviour of the affected zero‐order basin prior to construction of the road. The impact of the road at the zero‐order basin scale was highly dependent on the antecedent soil‐moisture conditions, total storm precipitation, and to some extent rainfall intensity. At the beginning of the monitoring period, during dry antecedent conditions, road runoff contributed 50% of the total runoff and 70% of the peak flow from the affected catchment (C1). The response from the unroaded catchment was almost insignificant during dry antecedent conditions. As soil moisture increased, the road exerted less influence on the total runoff from the roaded catchment. For very wet conditions, the influence of road‐generated runoff on total outflow from the roaded catchment diminished to only 5·4%. Both catchments, roaded and unroaded, produced equivalent amount of outflow during very wet antecedent conditions on a unit area basis. The lag time between the rainfall and runoff peaks observed in the unroaded catchment during the monitoring period ranged from 0 to 4 h depending on the amount of precipitation and antecedent conditions, owing mainly to much slower subsurface flow pathways in the unroaded zero‐order basin. In contrast, the lag time in the roaded zero‐order basin was virtually nil during all storms. Copyright © 2009 John Wiley & Sons, Ltd.     

Differences in abundance and distribution of Alexandrium cysts in Sendai Bay,northern Japan,before and after the tsunami caused by the Great East Japan Earthquake

The tsunami caused by the Great East Japan Earthquake on 11 March 2011 greatly influenced the coastal benthic environment on the northern Pacific coast of Japan. We used the direct count method to investigate changes in the abundance and distribution of Alexandrium (Alexandrium tamarense and Alexandrium catenella) cysts before and after the tsunami in Sendai Bay. Densities of Alexandrium cysts in sediments collected in summer 2011 ranged from 0 to 8,190 cysts cm^?3. In the western part of the bay, the density increased greatly after the tsunami, the highest density being approximately 10 times the density recorded in 2005. Molecular identification of single cysts with multiplex polymerase chain reaction (PCR) showed that Alexandrium tamarense dominated the cyst population in the southwestern part of the bay in 2011. Furthermore, accumulation of cysts on the surface sediment after disturbance of the sediment was confirmed by a laboratory experiment. The main factor causing the drastic changes in abundance and distribution of Alexandrium cysts after the earthquake was considered to be vertical and horizontal redistribution of the cysts in sediments after the tsunami.     

Primary melt from Sannome-gata volcano,NE Japan arc: constraints on generation conditions of rear-arc magmas

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9–11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous ⁸⁷Sr/⁸⁶Sr ratios (0.70318–0.70320), whereas low-MgO scoriae (MgO < ~9 wt%) have higher ⁸⁷Sr/⁸⁶Sr ratios (>0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe²⁺)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160–1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5–0.6 wt% H₂O at 1,220–1,230 °C and at ~1.8 GPa, and that the H₂O content of the primary magma was 6–7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6–0.7 wt% H₂O at ~1,250 °C and at ~1.3 GPa.     

Growth of ringwoodite reaction rims from MgSiO3 perovskite and periclase at 22.5 GPa and 1,800 °C

The growth rate of ringwoodite reaction rims between MgSiO₃ perovskite and periclase was investigated at 22.5 GPa and 1,800 °C for 1–24 h using the Kawai-type high-pressure apparatus. The reaction was likely to proceed by a diffusion-controlled mechanism in which the dominant diffusion mechanism was grain-boundary diffusion. The reaction constant (the width of the ringwoodite reaction rim squared divided by time) determined from these experiments was between 1.3 × 10^?15 and 5.6 × 10^?15 m²/s. A Pt inert marker experiment indicated that the MgO component migrated faster than the SiO₂ component in ringwoodite. Thus, either Mg or O having the slower diffusion rate controlled the reaction. Because previous diffusion studies have shown that diffusion rates of O are slower than those of Mg, O would be a rate-controlling element for ringwoodite formation from MgSiO₃ perovskite and periclase. The growth rate appeared to be too fast to explain the observed topographic rise (~10 km) inside mantle plumes at the 660-km discontinuity.     

Relations between load and settlement of circular foundations on or in a dense sand expressed by a function of diameter and depth

When load acts on a circular foundation on or in a dense sand, average contact pressure on the lower surface of the foundation is q and settlement of the foundation is s. Diameter and depth of the foundation are B and D_f. When the sand, B and D_f are given, we can know the relation between q and s/B by, e.g. a loading test, i.e. the relation is determined by B and D_f for the sand. Using the results of numerical analyses, we express a relation between q and s/B up to s=0.1B by functions of a single variable which is a linear combination of B and D_f. Consequently when two foundations have different B's and different D_f's but have the same value of the variable, the relations are the same. Then we examine whether the functions can express the results of eleven tests of model foundations of wide range of B and/or D_f. In all the tests, the relations are expressed with sufficient accuracy. Copyright © 2005 John Wiley & Sons, Ltd.