Geochemistry of recent hydrothermal systems of Mendeleev Volcano, Kuril Islands, Russia

Among the thermal waters of Mendeleev Volcano three main types can be distinguished: sodium-chloride, acid-sulfate and chloride–sulfate–bicarbonate waters. Contents and behaviors of siderophile, chalcophile, lithophile, and rare-earth elements are discussed. These data, together with the result of isotopic studies (oxygen and hydrogen), are used to discuss the origin of these waters. From the three types of thermal waters, only acid-sulfate waters have a significant input on the chemical composition of the surrounding surface waters.     

Structure of hakone caldera as revealed by drilling

Hakone caldera, now 8 by 12 km in diameter, was formed by collapse of a center of a volcano probably 2700 m high. The collapse took place at two separate periods each of which was followed by periods of deep denudation. The central part of the caldera has been covered by a thick pile of lavas of post-caldera cones and domes. For the purpose of finding thermal spring, drilling to depths of a few hundred to one thousand meters was carried out at various points within the caldera except for its central region. The study of the drill cores revealed that the average amount of subsidence at points 2 and 3 km away from the base of the present caldera wall is 600 m and 1200 m respectively, and probably more than 1800 m in the middle of the caldera. Within the caldera, the pre-caldera lavas and pyroclastic rocks are either lacking or much thinner than would be expected. It is concluded therefore that the present topographic depression of the caldera owed its origin to both subsidence and denudation. It is inferred that the subsidence took place along a complicated system of concentric faults combined with tilting of individual fault blocks toward the middle of the caldera. The magma reservoir into which the fault blocks sank probably had a shape of a cupola with a diameter comparable to or a little smaller than the diameter of the caldera.     

Gabbro — quartz diorite inclusions from Izu-Hakone Region,Japan

Gabbro — quartz diorite inclusions, angular to rounded and up to 20 cm in size, have been found as accidental fragments in a mud flow of the Okata basalt group, O-shima Island and in a tuff breccia, Hakone. New analyses are represented for seventeen inclusions and three pyroxenes. It is reasonable to conclude from petrographic and chemical features that the olivine gabbro inclusions were produced by crystal settling from a quartztholeiite magma at the early stage of fractionation within a magma reservior. On the other hand, gabbro and quartz gabbro inclusions are fragments of a small intrusive body within the Tertiary volcanic formation and consist of various amounts of cumulus phases and liquid. Quartz diorite inclusions are also fragments of a plutonic equivalent, but represents a strongly-differentiated liquid phase of the quartz-tholeiite magma.     

Dike swarm in Hakone volcano

Subparallel dikes are exposed on a new road-cut along the foot of the southeastern caldera wall of Hakone Volcano. The dikes are concentrated within a zone 1,915 m wide. Altogether 96 dikes trending generally from NW to SE are seen within a total length of the actual outcrops of 855 m measured at right angles to the trend of the dikes. This implies that there are 215 dikes within the zone of the dike swarm. As the average thickness of the dikes is 2.85 m, the zone was stretched for about 650 m in NE-SW direction owing to the intrusion of the dikes. The dikes tend to converge to a small area near the center of the caldera, and also tend to dip steeply toward the central axis of the zone. It is concluded that the dikes intruded along originally vertical fissures radiating from the central vent of the pre-caldera cone, but the zone of the dike swarm was subjected to bulging with the maximum elevation along its central axis owing to successive intrusion of the dikes from below. Stretching and bulging of the flank of a volcanic cone owing to rise of magma along one of radial fissures were observed during the 1940 eruption of Miyake-zima, Izu Islands, Japan.     

Lateral variation of basalt magma type across continental margins and Island Arcs

Quaternary basalt magmas in the Circum-Pacific belt and island arcs and also in Indonesia change continuously from less alkalic and more siliceous type (tholeiite) on the oceanic side to more alkalic and less siliceous type (alkali olivine basalt) on the continental side. In the northeastern part of the Japanese Islands and in Kamchatka, zones of tholeiite, high-alumina basalt, and alkali olivine basalt are arranged parallel to the Pacific coast in the order just named, whereas in the southwestern part of the Japanese Islands, the Aleutian Islands, northwestern United States, New Zealand, and Indonesia, zones of high-alumina basalt and alkali olivine basalt are arranged parallel to the coast. In the Izu-Mariana, Kurile, South Sandwich and Tonga Islands, where deep oceans are present on both sides of the island arcs, only a zone of tholeiite is represented. Thus the lateral variation of magma type is characteristic of the transitional zone between the oceanic and continental structures. Because the variation is continuous, the physico-chemical process attending basalt magma production should also change continuously from the oceanic to continental mantle. Suggested explanations for the lateral variation assuming a homogeneous mantle are: 1) Close correspondence between the variations of depth of earthquake foci in the mantle and of basalt magma type in the Japanese Islands indicates that different magmas are produced at different depths where the earthquakes are generated by stress release: tholeiite at depths around 100 km, high-alumina basalt at depths around 200 km, and alkali olivine basalt at depths greater than 250 km. 2) Primary olivine tholeiite magma is produced at a uniform level of the mantle (100–150 km), and on the oceanic side of the continental margin, it leaves the source region immediately after its production and forms magma reservoirs at shallow depths, perhaps in the crust, where it undergoes fractionation to produce SiO₂-oversaturated tholeiite magma, whereas on the continental side, the primary magma forms reservoirs near the source region and stays there long enough to be fractionated to produce alkali olivine basalt magma, and in the intermediate zone, the primary magma forms reservoirs at intermediate depths where it is fractionated to produce high-alumina basalt magma.     

Dissolved noble gases in the porewater of lacustrine sediments as palaeolimnological proxies

This paper presents results from the numerical modelling of the transport of atmospheric noble gases (He, Ne, Ar, Kr, Xe), tritiated water and ³He produced by radioactive decay of ³H, in unconsolidated lacustrine sediment. Two case studies are discussed: (1) the evolution of ³H and ³He concentrations in the sediment porewater of Lake Zug (Switzerland) from 1953 up to the present; and (2) the response of dissolved atmospheric noble gas concentrations in the sediment porewater of a subtropical lake to an abrupt climatic change that occurred some 10 kyr before the present. (1) Modelled ³H and ³He porewater concentrations are compared with recent data from Lake Zug. An estimate of the effective diffusion coefficients in the sediment porewater is derived using an original approach which is also applicable also to lakes for which the historical ³H and ³He concentrations in the water column are unknown. (2) The air/water partitioning of atmospheric noble gases is sensitive to water temperature and salinity, and thus provides a mechanism by which these environmental variables are recorded in the concentrations of atmospheric noble gases in lakes. We investigate the feasibility of using noble gas concentrations in the porewater of lacustrine sediments as a proxy for palaeoenvironmental conditions in lakes. Numerical modelling shows that heavy noble gases in sediment porewater, because of their comparatively small diffusion coefficients and the strong temperature sensitivity of their equilibrium concentrations, can preserve concentrations corresponding to past lake temperatures over times on the order of 10 kyr. Noble gas analysis of sediment porewaters therefore promises to yield valuable quantitative information on the past environmental states of lakes.     

Development of sampling downscaling: a case for wintertime precipitation in Hokkaido

This study has developed sampling downscaling (SmDS), in which dynamical downscaling (DDS) is executed for a few of period selected from a long-term integration by general circulation model based on an observed statistical relationship between large-scale climate and regional-scale precipitation. SmDS expectedly produces climatology and frequency distribution of precipitation over a nested region with reducing computational cost, if a global-scale climate pattern mostly controls regional-scale weather statistics. Here SmDS was attempted for wintertime precipitation over Hokkaido, Japan, because a linkage between snowfall and sea-level pressure patterns has been known by Japanese synopticians and it can be detected by singular value decomposition (SVD) analysis on wintertime inter-annual variability during the period from 1980/1981 to 2009/2010 for precipitation over Hokkaido and moisture flux convergence around there. DDS for the full period over the same domain was also performed for comparison with SmDS. SmDS selected two winters from the top and two winters from the bottom of the projection onto the first SVD mode. It was found that, comparing with the full DDS, SmDS indeed provided unbiased statistics for average but exaggerated extreme statistics such as heavy rainfall frequency. It was also shown that the sampling in the SmDS method was much more effective than the random sampling.