Mass balance of heavy metals in the seto inland sea, Japan

To discuss the geochemical and environmental behavior of heavy metals in the Seto Inland Sea, the largest semi-enclosed coastal sea in Japan, mass balances of Cu and Zn were studied by application of a simple box model using sedimentation rates and heavy metal contents of core sediments. In 1980, total sedimentary loads of Cu and Zn over the whole area of the Sea were estimated to be 630 and 3500 tons year⁻¹, respectively. Further, the sedimentary loads without (natural) and with human activities were estimated separately to be, respectively, 320 and 310 tons year⁻¹ for Cu and 1800 and 1700 tons year⁻¹ for Zn. Total inputs of Cu and Zn into the Sea, in 1980 were estimated to be 870 and 4250 tons year⁻¹ about one half of this being the result of human activities. 70% of the Cu input and 80% of the Zn input are taken into the sediments. Mean residence times of Cu and Zn in the Sea are calculated to be ≈0.3 and 0.2 years, respectively. Since these values are relatively small compared to the mean residence time of the seawater (0.9 years), Cu and Zn supplied to the Sea are considered to be accumulated rapidly in the sediments.     

Uniformly mantle-like δ<Superscript>18</Superscript>O in zircons from oceanic plagiogranites and gabbros

Lower ocean crust is primarily gabbroic, although 1–2% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater-derived fluids, and thus they may indicate early, high-temperature hydrothermal fluid circulation. To explore seawater–rock interaction prior to and during the genesis of plagiogranite and other late-stage magmas, oxygen-isotope ratios preserved in igneous zircon have been measured by ion microprobe. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Samples originate primarily from drill core acquired during Ocean Drilling Program and Integrated Ocean Drilling Program operations near the Mid-Atlantic and Southwest Indian Ridges. With the exception of rare, distinctively luminescent rims, all zircons from ocean crust record remarkably uniform δ¹⁸O with an average value of 5.2 ± 0.5‰ (2SD). The average δ¹⁸O(Zrc) would be in magmatic equilibrium with unaltered MORB [δ¹⁸O(WR) ~ 5.6–5.7‰], and is consistent with the previously determined value for equilibrium with the mantle. The narrow range of measured δ¹⁸O values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system, and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts. If plagiogranite forms by hydrous partial melting, the uniform mantle-like δ¹⁸O(Zrc) requires melting and zircon crystallization prior to significant amounts of water–rock interactions that alter the protolith δ¹⁸O. Zircons from ocean crust have been proposed as a tectonic analog for >3.9 Ga detrital zircons from the earliest (Hadean) Earth by multiple workers. However, zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments. Many of the >3.9 Ga zircons have mildly elevated δ¹⁸O (6.0–7.5‰), but such values have not been identified in any zircons from the large sample suite examined here. The difference in δ¹⁸O, in combination with newly acquired lithium concentrations and published trace element data, clearly shows that the >3.9 Ga detrital zircons did not originate by processes analogous to those in modern mid-ocean ridge settings.     

Fall,classification, and exposure history of the Mifflin L5 chondrite

The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock‐melted veins are present. The U,Th‐He age is 0.7 Ga, and the K‐Ar age is 1.8 Ga, indicating that Mifflin might have been heated at the time of the 470 Ma L‐chondrite parent body breakup and that U, Th‐He, and K‐Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 30–65 cm. Assuming this exposure geometry, a cosmic‐ray exposure age of 25 ± 3 Ma is calculated from cosmogenic noble gas concentrations. The low ²²Ne/²¹Ne ratio may, however, indicate a two‐stage exposure with a longer first‐stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained.     

Long-traveling landslides in deep snow conditions induced by the 2011 Nagano Prefecture earthquake,Japan

Earthquakes in mountainous areas may produce many landslides that involve abundant snow, but few observations have been made of these hazardous phenomena. The 12 March 2011 north Nagano Prefecture earthquake (M_JMA 6.7) occurred in a mountainous part of Japan that typically has an annual snow cover of more than 2 m, and it induced many snowy landslides. Some of these traveled relatively long distances. We examined the snowy Tatsunokuchi landslide to reconstruct the landsliding processes over deep snow. We infer that the Tatsunokuchi landslide occurred by collapse of a rock debris mass of 5?×?10⁴ m³ that plunged into the abundant snow, forming a mixture of snow and rock debris, which then traveled on top of the snow. Later, the displaced mass included a large amount of snow which was pushed forward at the front and to the sides. The velocity of the landslide was estimated to be approximately 14 m/s. It appears that the displaced mass, having only a small proportion of rock debris, had a low enough density to travel easily on top of the snow. Our observations suggest that there was much liquid water at the base of the displaced mass shortly after the event. Our results suggest that landslides may damage wider areas than expected if they travel over deep snow.     

Abiki oscillations in Sakitsu Bay,west Kyushu,Japan

Sakitsu and Yokaku bays in Amakusa in west Kyushu, Japan, experienced inundation damage in the February 2009 meteotsunami (Abiki) event. The oscillation characteristics of both bays are investigated by taking field measurements and conducting numerical experiments with regard to flood mitigation with the aim to reduce the flood impact during Abiki events. A continuous wavelet transform and bandpass filtering both of the pressure and water level indicated that a sequence of pressure disturbances, as small as 1.0 hPa, caused the large amplified oscillation within Sakitsu Bay. When a sequence of ocean long waves entered the bay, a surf beat evolved in the early stages. Subsequently, the sea level began to undergo large amplitude oscillations, and there was a secondary peak of oscillation with a period of around 24 min, as seen in both field measurements and numerical experiments. A surf beat with the period of 12 min formed in Yokaku Bay owing to the continuous incidence of ocean waves with period of 12 min, but its wave period was almost half of that of the natural period of the bay. This surf beat may have entered Sakitsu Bay with natural period of 11.8 min and caused large water-level fluctuations.     

Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex

The 560–550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subduction–accretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub-greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote-amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high-pressure zone III to lower grade zones II and I to the west and east. The P – T conditions estimated from zoned amphibole indicate an anticlockwise P – T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na–Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high-pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.     

High precision oxygen three‐isotope analyses of anhydrous chondritic interplanetary dust particles

Abstract– Oxygen three‐isotope ratios of three anhydrous chondritic interplanetary dust particles (IDPs) were analyzed using an ion microprobe with a 2 μm small beam. The three anhydrous IDPs show Δ¹⁷O values ranging from ?5‰ to +1‰, which overlap with those of ferromagnesian silicate particles from comet Wild 2 and anhydrous porous IDPs. For the first time, internal oxygen isotope heterogeneity was resolved in two IDPs at the level of a few per mil in Δ¹⁷O values. Anhydrous IDPs are loose aggregates of fine‐grained silicates (≤3 μm in this study), with only a few coarse‐grained silicates (2–20 μm in this study). On the other hand, Wild 2 particles analyzed so far show relatively coarse‐grained (≥ few μm) igneous textures. If anhydrous IDPs represent fine‐grained particles from comets, the similar Δ¹⁷O values between anhydrous IDPs and Wild 2 particles may imply that oxygen isotope ratios in cometary crystalline silicates are similar, independent of crystal sizes and their textures. The range of Δ¹⁷O values of the three anhydrous IDPs overlaps also with that of chondrules in carbonaceous chondrites, suggesting a genetic link between cometary dust particles (Wild 2 particles and most anhydrous IDPs) and carbonaceous chondrite chondrules.     

Internal structure of the Nojima Fault zone from the Hirabayashi GSJ drill core

Abstract The internal structures of the Nojima Fault, south-west Japan, are examined from mesoscopic observations of continuous core samples from the Hirabayashi Geological Survey of Japan (GSJ) drilling. The drilling penetrated the central part of the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake) ( M 7.2). It intersected a 0.3 m-thick layer of fault gouge, which is presumed to constitute the fault core (defined as a narrow zone of extremely concentrated deformation) of the Nojima Fault Zone. The rocks obtained from the Hirabayashi GSJ drilling were divided into five types based on the intensities of deformation and alteration: host rock, weakly deformed and altered granodiorite, fault breccia, cataclasite, and fault gouge. Weakly deformed and altered granodiorite is distributed widely in the fault zone. Fault breccia appears mostly just above the fault core. Cataclasite is distributed mainly in a narrow (≈1 m wide) zone in between the fault core and a smaller gouge zone encountered lower down from the drilling. Fault gouge in the fault core is divided into three types based on their color and textures. From their cross-cutting relationships and vein development, the lowest fault gouge in the fault core is judged to be newer than the other two. The fault zone characterized by the deformation and alteration is assumed to be deeper than 426.2 m and its net thickness is > 46.5 m. The fault rocks in the hanging wall (above the fault core) are deformed and altered more intensely than those in the footwall (below the fault core). Furthermore, the intensities of deformation and alteration increase progressively towards the fault core in the hanging wall, but not in the footwall. The difference in the fault rock distribution between the hanging wall and the footwall might be related to the offset of the Nojima Fault and/or the asymmetrical ground motion during earthquakes.