The relationships between drastic changes in Sr isotope ratios of magma sources beneath the NE Japan arc and the spreading of the Japan sea back-arc basin

Summary Based on Sr isotopic data for Tertiary and Quaternary basaltic rocks from the NE Japan arc, relationships are discussed between the temporal variation of magma source characteristics and the opening of the Japan Sea. The basaltic rocks from the trench side and from the transitional zone show initial⁸⁷Sr/⁸⁶Sr ratios (Sr_i ratios) in the range of 0.70411–0.70546 but no temporal variation in Sr_i ratios. The back-arc side basaltic rocks with ages of 29.8 to 15 Ma have Sr_i ratios similar to those of the trench side and the transitional zone, and these values also show no temporal change. In contrast, the basaltic rocks from the back-arc side, with ages younger than 15 Ma, show significantly lower Sri ratios (0.70396 to 0.70290), which are slightly higher than those of N-type MORB. These Sr isotopic features may imply that at least before 15 Ma the magma source regions (the sub-continental mantle) beneath the NE Japan arc had an enriched chemical character and that after 15 Ma, the magma sources for volcanic rocks from the back-arc side show a drastic change in Sr isotopic character, from an enriched nature to a depleted one. The depleted magmas may have been formed as a result of injection of depleted asthenosphere (or of a depleted mantle diapir) into the subcontinental mantle under the back-arc side of the NE Japan arc, during the spreading of the Japan Sea back-arc basin. The middle Miocene basaltic rocks from the back-arc side are characterized by lower contents of LIL elements such as K₂O and Rb compared with those from the trench side, suggesting that during the middle Miocene (syn-opening stage of the Japan Sea) the degree of partial melting may have been higher in the back-arc side mantle than in the trench side mantle. High degree of partial melting in the back-arc side mantle can be attributed to an increasing geothermal gradient in the mantle due to the injection of hot asthenosphere. This injection might also have caused the melting of the lower crust from which the voluminous middle Miocene acidic volcanics in the back-arc side and transitional zone may have been produced.

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Drastische Veränderungen der Sr-Isotopenverhältnisse von Magmenquellen unterhalb des NE japanischen Vulkanbogens und ihre Beziehung zum Spreading des japanischen Back-Arc-Beckens

Zusammenfassung Die Beziehungen zwischen der zeitlichen Veränderung der Charakteristika von Magmenquellen und der Öffnung des Japanischen Meeres werden anhand von Sr-Isotopendaten tertiärer und quartärer basaltischer Gesteine diskutiert. Basaltische Gesteine von der Grabenseite und aus der Übergangszone ergaben initiale⁸⁷Sr/⁸⁶Sr Verhältnisse (Sr_i-Verhältnisse) von 0.70411–0.70546 und lassen keine zeitabhängige Änderung erkennen. Basaltische Besteine aus dem Back-Arc-Bereich mit Altern zwischen 29.8 und ca. 15 Ma zeigen ähnliche Sr_i-Verhältnisse und ebenfalls keine zeitliche Veränderung. Im Gegensatz dazu sind basaltische Gesteine aus dem Back-Arc-Bereich, die jünger als ca. 15 Ma sind, signifikant in ihren Sr_i-Verhältnissen (0.70396–0.70290) erniedrigt. Diese Verhältnisse liegen etwas höher als die von N-MORB. Die Sr-Isotopenergebnisse lassen vermuten, daß zumindest vor ca. 15 Ma der Herkunftsbereich der Magmen (subkontinentaler Mantel) unter dem NE japanischen Vulkanbogen chemisch angereichert war, während die Magmenquellen der jüngeren vulkanischen Gesteine des Back-Arc-Bereiches durch eine drastische Abreicherung charakterisiert sind. Die abgereicherten Magmen könnten, während der Öffnung des japanischen Back-Arc-Beckens, als Folge der Injektion abgereicherter Asthenosphäre (oder eines abgereicherten Manteldiapirs) in subkontinentalen Mantel unterhalb des Back-Arcs des NE japanischen Vulkanbogens, gebildet worden sein. Die miozänen basaltischen Gesteine des Back-Arc-Bereiches sind außerdem durch niedrigere Gehalte an LIL-Elementen, wie z.B. K₂O und Rb charakterisiert. Dies wird als Hinweis auf eine erhöhte Aufschmelzungsrate in diesem Bereich im mittleren Miozän (im Zuge der gleichzeitigen Öffnung des japanischen Meeres) verstanden. Die erhöhte Aufschmelzrate im Mantel des Back-Arc-Breiches wird auf einer Erhöhung des geothermischen Gradienten infolge der Injektion von heißer Asthenosphäre zurückgeführt. Diese Injektion von Asthenosphäre könnte auch der Grund für die Aufschmelzung von Unterkruste und für die Produktion weitverbreiteter saurer miozäner Vulkanite im Back-Arc-Bereich und der Übergangszone sein.

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With 6 Figures     

Detection of a new interstellar molecule, H2CN

We have detected a new interstellar molecule, H2CN (methylene amidogen), in the cold, dark molecular cloud TMC-l. The column density of H2CN is estimated to be approximately 1.5 x 10(11) cm-2 by assuming an excitation temperature of 5 K. This column density corresponds to a fractional abundance relative to H2 of approximately 1.5 x 10(-11). This value is more than three orders of magnitude less than the abundance of the related molecule HCN in TMC-1. We also report a tentative detection of H2CN in Sgr B2(N). The formation mechanism of H2CN is discussed. Our detection of the H2CN molecule may suggest the existence of a new series of carbon-chain molecules, CH2CnN (n = 0, 1, 2,...).     

Crustal structure and tectonics of the Hidaka Collision Zone, Hokkaido (Japan), revealed by vibroseis seismic reflection and gravity surveys

This study is the first integrated geological and geophysical investigation of the Hidaka Collision Zone in southern Central Hokkaido, Japan, which shows complex collision tectonics with a westward vergence. The Hidaka Collision Zone consists of the Idon'nappu Belt (IB), the Poroshiri Ophiolite Belt (POB) and the Hidaka Metamorphic Belt (HMB) with the Hidaka Belt from west to east. The POB (metamorphosed ophiolites) is overthrust by the HMB (steeply eastward-dipping palaeo-arc crust) along the Hidaka Main Thrust (HMT), and in turn, thrusts over the Idon'nappu Belt (melanges) along the Hidaka Western Thrust (HWT). Seismic reflection and gravity surveys along a 20-km-long traverse across the southern Hidaka Mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts, back thrusts, frontal thrust-and-fold structures, and duplex structures. The main findings are as follows. (1) The HMT, which dips steeply at the surface, is a listric fault dipping gently at a depth of 7 km beneath the eastern end of the HMB, and cutting across the lithological boundaries and schistosity of the Hidaka metamorphic rocks. (2) A second reflector is detected 1 km below the HMT reflector. The intervening part between these two reflectors is inferred to be the POB, which is only little exposed at the surface. This inference is supported by the high positive Bouguer anomalies along the Hidaka Mountains. (3) The shallow portion of the IB at the front of the collision zone has a number of NNE-dipping reflectors, indicative of imbricated fold-and-thrust structures. (4) Subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile. These reflectors may correspond to the velocity boundary (5.9–6.6 km/s) previously obtained from seismic refraction profiling in the northern Hidaka Mountains. (5) These crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the Early Tertiary.     

Spessartine Garnets in a Manganiferous Carbonate Formation from Nsuta, Ghana

Abstract: Manganese carbonate ore beds and host rock manganiferous phyllites at the Nsuta mine, western Ghana, contain well developed garnet crystals. Individual crystals are idioblastic, sometimes porphyroblastic, and homogeneous, and are associated with rhodochrosite (with or without kutnahorite), quartz and muscovite. The conspicuous absence of chlorite in garnet-rich assemblages, and of garnet in chlorite-rich rocks, suggest chemical constraints may have been important in the formation of the two minerals. Gondite bands within carbonate ores are interpreted to have resulted from localised processes in which manganese carbonates, in environments rich in alumino-silicate minerals, may have been completely exhausted during metamorphic reactions.     

The 1989 submarine eruption off eastern Izu Peninsula,Japan: ejecta and eruption mechanisms

The submarine eruption of a new small knoll, which was named Teishi knoll, off eastern Izu Peninsula behind the Izu-Mariana arc occurred in the evening of 13 July 1989. This is the first historic eruption of the Higashi-Izu monogenetic volcano group. The eruption of 13 July followed an earthquake swarm near Ito city starting on 30 June. There were subsequent volcanic tremors on 11 and 12 July, and the formation of the Teishi knoll on the 100 m deep insular shelf 4 km northeast of Ito city. There were five submarine explosions, which were characterized by intermittent domelike bulges of water and black tephra-jets, which occurred within 10 min on 13 July. Ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria, highly vesiculated pumice, and lithic material. Petrographical features suggest that the pumice was produced by vesiculation of reheated wet felsic tuff of an older formation. The Teishi knoll, before the eruption, was a circular dome, 450 m across and 25 m high, with steep sides and a flat summit. Considerations of submarine topographic change indicate the knoll was raised by sill-like intrusion of 10⁶ m³ of magma beneath a 30 m thick sediment blanket. This shallow intrusion is assumed to have started on 11 July when volcanic tremors were observed for the first time, but there was no indications of violent interaction between wet host sediments and intruding magma. The submarine eruption of 13 July appears to have been Friggered by a major lowering of the magma-column. The basalt scoria, having crystal-contents of more than 60%, is assumed to be derived from the cooled plastic margin of the shallow intrusive body. However, glassy scoria, which would indicate the interaction between hot fluidal magma and external water, was not observed. A scenario for the 1989 submarine eruption is as follows. When rapid subsidence of the hot interior of the intrusive magma occurred, reduced pressure caused the implosion of cooled plastic magma, adjacent pressurized, hot host material, and wet sediment. The mixing of these materials triggered the vigorous vapor explosions.     

Three-year investigations into sperm whale-fall ecosystems in Japan

We report the first study of sperm whale‐fall ecosystems, based on mass sinking of whale carcasses at shelf depths in the northwest Pacific. We conducted three observations over a 2‐year period on replicate sperm‐whale carcasses implanted at depths of 219–254 m off the southern part of Japan from July 2003 to August 2005. The study was made possible by a mass stranding of sperm whales in January 2002, and the subsequent sinking of 12 carcasses in the waters off Cape Nomamisaki. Dense aggregations of unique chemosynthesis‐based fauna had formed around the whale carcasses after 18 months (July 2003). The mytilid mussel Adipicola pacifica was the most abundant macrofaunal species and covered most of the exposed bone surfaces. The general composition of the fauna was similar to that of deep‐water reducing habitats, but none of the species appearing in this study has been found at hydrothermal vents, cold seeps or deep‐water whale falls. A new species of lancelet, which was the first record of the subphylum Cephalochordata from reducing environments, a new species of Osedax; a rarely encountered benthic ctenophore, and a rare gastropod species were discovered at this sperm whale‐fall site. Benthic communities were similar across all the carcasses studied, although the body sizes of the whales were very different. The succession of epifaunal communities was relatively rapid and the sulphophilic stage was considerably shorter than that of other known whale falls.     

Very low frequency Ocean Bottom ambient seismic noise and coupling on the shallow continental shelf

Sources of very low frequency (0.01 to 1.0 Hz) ambient seismic noise in the shallow (<100 m) water continental margin sediments are investigated using Ocean Bottom Seismometers (OBS). The predominant seismic motions are found to be due to surface gravity (water) waves and water-sediment interface waves. Actual experimental measurements of seabed acceleration and hydrodynamic pressure are given, including side by side comparisons between buried and plate-mounted OBS units. OBS-sediment resonant effects are found to be negligible at the low frequencies under investigation. Wherever there exists relative motion between the seabed and the water, however, an exposed OBS is subject to added mass forces that cause it to move with the water rather than the sediments. Calculations based on measured seabed motions show that a neutral density, buried seismometer has superior sediment coupling charactersitics to any exposed OBS design.     

Behaviors of 210Pb and 210Po around hydrothermal vents in the Okinawa Trough

Vertical profiles for the uranium-series radioisotopes 210Pb and 210Po were obtained at thetwo hydrothermal vent sites, the Iheya Ridge and the Minami-Ensei Knoll, in the Mid-Okinawa Trough in 1993 and 1994, respectively. In 1995, both radioisotopes were measured at the Minami-Ensei Knoll a-gain. At the Iheya Ridge, where the hydrothermal activity is not active as reflected by the CH_4, and 222Rn data, both the total 210Pb and 210Po activities show deficiency relative to their parents, and the mean residence time of 210Pb and 210Po is approximately equal to 20 and 2-5 a, respectively. At the Minami-Ensei Knoll, which is characterized by black smokers, the total 210Pb(0.167 × 10~(-3)-2.5 × 10~(-3)_ Bq/kg) around the plumes is deficient relative to 226Ra but the total 210Po activities (1.83 × 10 ~(-3) - 2.83 × 10~(-3) Bq/kg) are in excess relative to 210Pb. The 210Po activities are higher than those in the East China Sea and the Okinawa Trough and excess 210Po has been found. The 210Pb/226Ra and 210Po/     

Processes causing the chlorophylla maximum in the tidal front in Iyo-nada,japan

A field survey was undertaken to reveal the structure of the tidal front in Iyo-nada. An obvious tidal front was found between the mixed region around Hayasui Straits and the stratified region in Iyo-Nada. Its structure was typical in the eastern part and was influenced by the river discharge in the western part. An intense chlorophylla maximum was found in the subsurface layer of the eastern typical front. Analysis using TS diagram suggested that, around the eastern front, there was an intrusion from the mixed water to the middle layer of the stratified water. This intrusion was supposed to be an important process supplying nutrients from the mixed water to the subsurface of the frontal region and causing the intense chlorophylla maximum.