月山火山の地質とＫ‐Ａｒ年代

Sugarloaf Mountain, central Arizona, USA: A small-scale example of Miocene basalt-rhyolite magma mixing to yield andesitic magmas

Abstract. Recent ground networks and satellite remote-sensing observations have provided useful data related to spatial and vertical distributions of mineral dust particles in the atmosphere. However, measurements of temporal variations and spatial distributions of mineral dust deposition fluxes are limited in terms of their duration, location, and processes of deposition. To ascertain temporal variations and spatial distributions of mineral dust deposition using wet and dry processes, weekly deposition samples were obtained at Sapporo, Toyama, Nagoya, Tottori, Fukuoka, and Cape Hedo (Okinawa) in Japan during October 2008–December 2010 using automatic wet and dry separating samplers. Mineral dust weights in water-insoluble residue were estimated from Fe contents measured using an X-ray fluorescence analyser. Wet and dry deposition fluxes of mineral dusts were both high in spring and low in summer, showing similar seasonal variations to frequency of aeolian dust events (Kosa) in Japan. For wet deposition, highest and lowest annual dust fluxes were found at Toyama (9.6 g m−2 yr−1) and at Cape Hedo (1.7 g m−2 yr−1) as average values in 2009 and 2010. Higher wet deposition fluxes were observed at Toyama and Tottori, where frequent precipitation (> 60% days per month) was observed during dusty seasons. For dry deposition among Toyama, Tottori, Fukuoka, and Cape Hedo, the highest and lowest annual dust fluxes were found respectively at Fukuoka (5.2 g m−2 yr−1) and at Cape Hedo (2.0 g m−2 yr−1) as average values in 2009 and 2010. The average ratio of wet and dry deposition fluxes was the highest at Toyama (3.3) and the lowest at Hedo (0.82), showing a larger contribution of the dry process at western sites, probably because of the distance from desert source regions and because of the effectiveness of the wet process in the dusty season. Size distributions of refractory dust particles were obtained using four-stage filtration: > 20, > 10, > 5, and > 1 μm diameter. Weight fractions of the sum of > 20 μm and 10–20 μm (giant fraction) were higher than 50% for most of the event samples. Irrespective of the deposition type, the giant dust fractions generally decreased with increasing distance from the source area, suggesting the selective depletion of larger giant particles during atmospheric transport. Based on temporal variations of PMc (2.5 < D < 10 μm), ground-based lidar, backward air trajectories, and vertical profiles of potential temperatures, transport processes of dust particles are discussed for events with high-deposition and low-deposition flux with high PMc. Low dry dust depositions with high PMc concentrations were observed under stronger (5 K km−1) stratification of potential temperature with thinner and lower (< 2 km) dust distributions because the PMc fraction of dust particles only survived after depletion of giant dust particles by rapid gravitational settling at the time they reach Japan. In contrast, transport through a thicker (> 2 km) dust layer with weak vertical gradient of potential temperature carry more giant dust particles to Japan. Because giant dust particles are an important mass fraction of dust accumulation, especially in the North Pacific, which is known as a high-nutrient, low-chlorophyll (HNLC) region, the transport height and fraction of giant dust particles are important factors for studying dust budgets in the atmosphere and their role in biogeochemical cycles.

The South Aegean active volcanic arc: relationships between magmatism and tectonics

The Denham Caldera on Raoul Volcano: dacitic volcanism in the Tonga–Kermadec arc

Dacitic rocks that crop out around the Zigana Mountain (Gumushane) in the eastern Pontide (NE Turkey), are mainly composed of quartz, plagioclase, sanidine, amphibole, muscovite, and biotite as the main minerals. Zircon and rutile are the accessory minerals. Pyrite, chalcopyrite, and covellite are the opaque components. On the basis of trace-element data, dacites have been classified as Dacite-I and Dacite-II. Dacite-I is tholeitic-transitional, whereas Dacite-II is transitional-calc-alkaline. The geochemical variation can be explained by the fractionation of the common mineral phases, such as plagioclase, hornblende, magnetite, and apatite. Dacites also show island-arc properties, with negative Nb, Sr, P, and Ti anomalies. The trace-element distrubitions of the dacitic rocks reflect the typical characteristics of rocks from the subduction-related tectonic setting, with enrichment of large-ion lithofile elements and light rare-earth element, but depletion in high-field-strength elements. The dacitic rocks are developed through plagioclase ± hornblende-controlled fractionation from the same parental magma that settled in two successive stages and are derived from an enriched source, probably by the mixing of slab-derived and lithospheric melts.

Venus, Earth's divergent twin?: Testing evolutionary models for Venus with the DAVINCI+ mission

Eruptive chronology and tectonic context of the late Pleistocene Tres Vírgenes volcanic complex, Baja California Sur (México)

Late Amazonian dike-fed distributed volcanism in the Tharsis volcanic province on Mars

The paper reports the isotope-geochronological and petrological-geochemical studies of the Pliocene moderately-acid volcanism of the Akhalkalaki Plateau in the central part of the Lesser Caucasus (Javakheti highland, Georgia). K-Ar dating showed that young dacitic lavas and pyroclastic rocks were formed in the Mid-Pliocene (3.28 ± 0.10 Ma) in relation with the explosive–effusive eruptions of small composite volcanic cones and formation of minor extrusive domes confined mainly to the eastern margin of the region. Isotope-geochronological data in the combination with results of structural drilling indicate that the considered short-term pulse of the volcanic activity occurred during a short gap between two phases of the Pliocene–Early Pleistocene mafic magmatism widespread within the Akhalkalaki plateau. The studied Pliocene dacites were erupted at the post-collisional stage of the evolution of the Lesser Caucasus, but bear petrological-geochemical affinity of adakitic series. They are characterized by the steady presence of amphibole phenocrysts, the elevated contents of Sr, Ba, LILE and the lowered contents of Y, Nb, Ta, and HREE, and have depleted Sr isotopic composition (87Sr/86Sr < 0.7045). Analysis of petrogenetic models earlier proposed to explain the generation of adakite-like magmas in the modern collision zones showed that the origin of the Pliocene dacitic lavas of the Akhlkalaki plateau is best described by the crystallization differentiation of watersaturated calc-alkaline basaltic melts with removal of common mafic rock-forming minerals (first of all, amphibole and pyroxene) and accessory phases (apatite, titanite, Ti-magnetite) as cumulus minerals. Crustal assimilation of evolved magmas only insignificantly contributed to the petrogenesis of the dacites.

Takachiho composite volcano, located in south Ky ?? sh ??, Japan, is one of major members of the Kirishima volcano group. The composite volcano consists of four volcanic edifices which are partly overlapping and aligned in E-W direction. On the basis of field geology and tephrochronology, its growth-history can be divided into four stages. During the first stage, a conical stratovolcano, Futagoishi, was formed in the eastern end of the composite volcano. It is com-posed of an alternation of pyroclastic beds and lava flows. The activity of Futagoishi ended before the eruption of Ito pyroclastic flow deposit (22, 000 yBP) from the Aira caldera. The second stage activity started about 10, 000 years ago, after a repose of more than 12, 000 years. It was characterized by vulcanian eruption which continued semi-persistently through a long period of time, probably more than a thousand years. During this period a stratovolcano, Old-Takachiho, was built up on the western flank of Futagoishi, and a large amount of volcanic ash accumulated concentrically around the volcano. The third stage activity was characterized by the eruption of several thin lava flows and scoria from the vent close to Old-Takachiho Volcano. This activity resulted in formation of a small conical stratovolcano, Takachiho-no-mine, on the western slope of Old-Takachiho. At the end of this stage, enormous lava flows were successively effused from the top crater and flowed downward widely. Later lava flows, however, did not reached the foot of the slope due to decrease in effusion rate and piled up around the crater, resulting in the construction of an exogeneous lava dome on the summit. After several hundred years of quiescence, the present stage activity began about 2, 500 years ago. This activity was characterized by sub-plinian eruptions which occurred intermittently and produced thick scoria and ash beds on the eastern foot of the composite vlocano. By repeated eruptions of a great volume of scoria and lava flows, a new vlocano, Ohachi, was born on the western flank of Takachiho-no-mine volcano, and has been active since then. The growth-history of Takachiho composite volcano revealed the following two features: 1) the volcanic activity may have been controlled by the underlying E-W fracture, as suggested by the shifting of the vent position from east to west, and 2) the styles of explosive activity of the composite volcano have gradually changed with time from vulcanian eruption to sub-plinian one.

Linking quantitative measurements of lava flow surface morphology with historical observations of eruptions is an important, but underexploited, route to understanding eruptions of silicic magma. We present here a new, high-resolution digital elevation model (DEM) for the intracaldera Kameni Islands, Santorini, Greece, which reveals the potential of high-resolution imaging (at ∼1 m per pixel) of lava-flow fields by airborne light detection and ranging laser radar (LiDAR). The new DEM has an order-of-magnitude better resolution than earlier models, and reveals a wealth of surface morphological information on the dacite lava flows of the Kameni Islands. In turn, this provides quantitative constraints on the bulk rheology of the emplaced lava flows. When combined with a reanalysis of contemporary eruption accounts, these data yield important insights into the behavior of dacite magma during slow effusive eruptions on Santorini and elsewhere, and allow the development of forecasts for the style and duration of future eruptions.

The Hellyer and Que River high-grade polymetallic volcanic-hosted massive sulfide deposits of western Tasmania are hosted by a sequence of late Middle Cambrian subaqueous mafic to felsic volcanics and volcaniclastics known as the Que-Hellyer Volcanics. These volcanics occur toward the top of a thicker sequence of mainly felsic volcanics, also hosting mineralization, known as the Mount Read Volcanics.The base of the footwall sequence within the Que-Hellyer Volcanics is marked by massive to pillowed basaltic lavas and volcaniclastics of variable thickness. These are conformably overlain by a thick sequence of andesitic, basaltic, and minor dacitic lavas, and associated volcaniclastics. Eruption of these footwall sequences was rapid, with breaks in volcanism represented only by thin discontinuous epiclastic horizons.A hiatus in volcanism followed the emplacement of the footwall sequences. This hiatus is represented by the deposition of locally derived, coarse-grained mass flows. The distribution and geometry of these epiclastics suggest that they accumulated in local syndepositional depressions. Sedimentation was also accompanied by syngenetic Zn-Pb-Cu-Ag-Au-rich mineralization in the central or axial area of these depressions. Rapid burial by further mass flows assisted in the preservation of mineralization. Shallow intrusive to extrusive dacitic lava flows and domes in the host horizon were emplaced during and immediately after mineralization. Autoelastic material from these dacitic domes was then resedimented and incorporated in the epiclastic horizons overlying the orebodies.Eruption of massive to pillowed basaltic, and minor andesitic, lavas of the Hellyer basalt occurred soon after mineralization and was the final phase of volcanism in the Que-Hellyer Volcanics. The basalts were emplaced as extrusions and as shallow intrusions into wet, fine-grained sediments which began to accumulate above the coarse-grained epiclastic horizons. These basalts buried the remaining parts of the sulfide mound not covered by epiclastics. Immediately after the cessation of volcanic activity in the area, a 100-m-thick sequence of black shales and minor sandstones, known as the Que River Shale, was deposited.The general lack of products of explosive volcanism within the Que-Hellyer Volcanics is most likely a function of the composition of the lavas combined with their emplacement in deep water under high confining pressures. Although the absence of such products does not rule out the possibility of emplacement in shallow-water depths, the absence of abundant large-scale tractional sedimentary structures, the presence of the overlying black, anoxic Que River Shale, and the mass-flow characteristics of elastic units favor a relatively deep water setting for mineralization in the Que-Hellyer Volcanics.

Relationships between mineralization and silicic volcanism in the Central Andes

Two modified passive samplers were evaluated at multiple field locations. The sampling rate (SR) of the modified polyurethane foam (PUF)-disk passive sampler for total gaseous mercury (TGM) using gold-coated quartz fiber filters (GcQFF) and gaseous oxidized mercury (GOM) using ion-exchange membranes (IEM) were 6.4 ± 1.4 and 15.3 ± 0.3 m(3) day(-1), respectively. The relative percent difference between TGM and GOM concentrations measured by a Tekran system and the passive samplers averaged 19 ± 14 and 13 ± 12% and ranged between 4-44 and 1.5-41%, respectively. The GcQFF and IEM substrates were also evaluated as collection media for surrogate surface dry deposition measurements. Mercury (Hg) concentration and dry deposition gradients were observed using these samplers at an urban/industrial site and compared to a rural/remote site. The Hg dry deposition rates measured by the surrogate surfaces were always higher than those calculated by a widely used inferential modeling method (1.3-50 fold). The Hg dry deposition measured at urban and suburban sites were comparable to those calculated from model. However, they were very different at a rural site, probably due to the low concentrations. Both methods are relatively low cost and will aid in understanding spatial distributions of Hg ambient air concentrations and dry deposition.

<p>Okataina Volcanic Centre is the most recently active of the four major rhyolite eruptive centres in the Taupo Volcanic Zone of New Zealand. Within the Centre lies Haroharo Caldera, a complex of overlapping collapse structures resulting from successive voluminous pyroclastic eruptions from the same general source area. At least four main and possibly two minor caldera-forming eruptions have occurred during the last 250,000 years, although poor exposure means that attempts to interpret the early structural history are highly speculative. Although there is no compelling evidence of structural updoming within Haroharo Caldera, magma resurgence has followed the last major caldera-forming eruption of the Rotoiti Breccia at [greater than or equal to] 42,000 years B.P. Eruption of this magma within the caldera has formed the two large rhyolite lava and pyroclastic piles of the Haroharo Volcanic Complex and Tarawera Volcanic Complex, plus two subsidiary adjacent complexes at Okareka and Rotoma. All these intracaldera eruptives are younger than 20,000 years B.P., with the most recent eruptions from Tarawera; of rhyolite at c. 700 years B.P., and of basalt in 1886 A.D. A considerable amount of earlier work carried out at Okataina was directed mainly at petrology and chemistry of the rhyolites forming the Tarawera and Haroharo Volcanic Complexes. The present study has arisen from a 1:50,000 mapping programme at Okataina and has sought to examine structures and volcanic history in greater detail, and to consider the resulting geological implications for geothermal resources. Caldera boundaries have been mapped, and two major vent lineations are defined, apparently related to fundamental basement fractures which have controlled location of the Tarawera and Haroharo Volcanic Complexes. An intracaldera ring fault is also suggested by the sub-circular arrangement of some young volcanic vents. The Haroharo and Tarawera Complexes are mapped, with locations of source vents, and dating of the major lavas and pyroclastic deposits. All the post-20,000 year eruptives are placed in four main emptive episodes at Haroharo, and five at Tarawera. The near-source pyroclastic surge and flow deposits are 14C dated, and with their associated widespread plinian fall deposits they provide time planes for dating the associated lavas. The emptive episodes generally appear to have been of much shorter duration than the intervening quiescent periods which lasted for thousands of years. All the eruptive episodes at Haroharo involved multiple eruptions from vents spread out over several kilometres along the vent lineations. Similar multiple vent eruptions can be demonstrated for some of the Tarawera eruptive episodes. More than 500 km3 of magma has been erupted from Haroharo Caldera during the last 250,000 years, 80 km3 of which was erupted in the Last 20,000 years. This history suggests that a large magmatic heat source should continue to underlie the Okataina Volcanic Centre. However, very little surface hydrothermal activity occurs within Haroharo Caldera. It is suggested that the large external hydrothermal fields at Tikitere, Waimangu-Waiotapu-Waikite, and possibly Kawerau, are related to Haroharo Caldera heat sources. Presently available data are summarized for hydrothermal fields in and adjacent to Haroharo Caldera, and new analyses are presented for some warm springs discovered within the caldera. Estimates and measurements of chloride fluxes in lakes and rivers are reported. The chloride flux values suggest the occurrence of larger hydrothermal heat flows into lakes and rivers than are apparent at the surface. Measurements of chloride flux in the Tarawera River showed that 280 g s-1 of chloride is added to the river within Haroharo Caldera below the Lake Tarawera outlet. Only 80 g s-1 of this chloride comes from known geothermal sources. A total chloride flux of 760 g s-1 in the Tarawera River passing out of the Okataina Volcanic Centre indicates a minimum geothermal heat flow of 600 MW. Estimates of heat flows in other drainage paths from Haroharo Caldera suggest that minimum total heat flow from the caldera may exceed 1500 MW. A large heat flow from the caldera would appear consistent with the volcanic history. Some suggestions are made for further investigation of the geothermal resources</p>