Metamorphic zoning and thermal structure of the Dabie ultrahigh- pressure–high-pressure terrane, central China

The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.     

焉耆盆地活动构造的遥感图像特征

焉耆盆地是一个位于天山东南部的山间盆地 ,在遥感图像上 ,可以看到盆地西面的活动构造在地表留下的痕迹 :清楚的线形构造 ,被错开的水系、冲沟等。经分析认为 ,这些活动构造主要由北边的活动褶皱和南边的右行走滑活动断裂组成。野外的实际调查与图像分析的结果十分吻合 ,表明遥感图像在研究活动构造方面是一个十分有力和快速的工具。活动断层的最大水平位移为 2 10m ,最大垂直位移为 2 0m ,主要以走滑作用为主。天山地区活动构造的发育表明印度板块向欧亚板块的碰撞仍在持续     

Relationship among O, Mg isotopes and the petrography of two spinel-bearing compound chondrules

The petrological properties, and O and Al-Mg isotopic compositions of two spinel-bearing chondrules from the Allende CV chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry. A coarse spinel grain in a barred-olivine (BO) chondrule is less enriched in ¹⁶O (Δ¹⁷O ∼ −5‰; Δ¹⁷O = δ¹⁷O - 0.52 δ¹⁸O), whereas smaller spinel grains in a plagioclase-rich chondrule member of a compound chondrule are extremely ¹⁶O-rich (Δ¹⁷O ∼ −17‰) and the spinels have a strongly serrated character. The petrological features and ¹⁶O-enrichments of the spinels in the plagioclase-rich chondrule indicate that spinels originating in coarse-grained Ca-Al-rich Inclusions (CAIs) were incorporated into chondrule precursors and survived the chondrule-forming event. The degree of ¹⁶O-excesses among minerals within each chondrule is correlated to the crystallization sequences. This evidence suggests that the O isotopic variation among minerals may have resulted from incomplete exchange of O isotopes between ¹⁶O-rich chondrule melt and ¹⁶O-poor nebular gas. Aqueous alteration also has changed the O-isotope compositions in the mesostasis. The feldspathic mesostasis in the BO chondrule shows a disturbed Mg-Al isochron indicating that the BO chondrule experienced secondary alteration. While plagioclase in the plagioclase-rich chondrule member of the compound chondrule shows slight ²⁶Mg-excesses corresponding to (²⁶Al/²⁷Al)₀ = [4.6±4.0(2σ)] × 10⁻⁶, nepheline formed by secondary alteration shows no detectable excess. The Al-Mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the Allende parent body.     

Geochemistry of the oldest MORB and OIB in the Isua Supracrustal Belt, southern West Greenland: Implications for the composition and temperature of early Archean upper mantle

Abstract Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate‐type stratigraphy and a duplex structure. Plate history from mid‐oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid‐oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO₂ and Na₂O contents. The Isua Cpx show slightly light (L)REE‐depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least‐altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al₂O₃, and depleted in TiO₂, FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE‐depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg? and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480°C, indicating that the Early Archean mantle was hotter by at most approximately 150°C than the modern mantle.     

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.     

Mechanism for enhanced diffusivity in the deep-sea perpetual salt fountain

The mechanism of enhanced diffusivity occurring in the deep-sea perpetual salt fountain has been investigated experimentally and numerically. Some factors which possibly contribute to the enhanced diffusivity were found to be the pipe oscillation with ocean waves and its baffled wall surface. Field experiments in the ocean (Onagawa Bay of Miyagi, Japan) and numerical simulations were performed to study and confirm the dynamics of the flow and heat transport with enhanced diffusivity occurring in upwelling deep-sea water. The agreement between the field experimental data and the numerical solutions of an oscillating-wall boundary condition imposed on the baffled pipe is encouraging, and it indicates the baffled pipe surface subject to the oscillatory motion leads to the enhanced diffusivity. The buoyancy force and then upwelling velocity can be greatly increased by the enhanced diffusivity. The dominant mechanism is the occurrence of complicated vortices and vortex shedding leading to efficient mixing and enhanced diffusion.     

A possible anorthositic continent of early Mars and the role of planetary size for the inception of Earth-like life

The Moon has an anorthositic primordial continental crust. Recently anorthosite has also been discovered on the Martian surface. Although the occurrence of anorthosite is observed to be very limited in Earth's extant geological record,both lunar and Martian surface geology suggest that anorthosite may have comprised a primordial continent on the early Earth during the first 600 million years after its formation. We hypothesized that differences in the presence of an anorthositic continent on an Earthlike planet are due to planetary size. Earth likely lost its primordial anorthositic continent by tectonic erosion through subduction associated with a kind of proto-plate tectonics(PPT). In contrast, Mars and the Moon, as much smaller planetary bodies, did not lose much of their anorthositic continental crust because mantle convection had weakened and/or largely stopped, and with time, they had appropriately cooled down. Applying this same reasoning to a super-Earth exoplanet suggests that, while a primordial anorthositic continent may briefly form on its surface, such a continent will be likely transported into the deep mantle due to intense mantle convection immediately following its formation. The presence of a primordial continent on an Earth-like planet seems to be essential to whether the planet will be habitable to Earth-like life. The key role of the primordial continent is to provide the necessary and sufficient nutrients for the emergence and evolution of life. With the appearance of a "trinity" consisting of(1) an atmosphere,(2) an ocean, and(3) the primordial continental landmass, material circulation can be maintained to enable a "Habitable Trinity" environment that will permit the emergence of Earth-like life. Thus, with little likelihood of a persistent primordial continent, a super-Earth affords very little chance for Earth-like life to emerge.     

Fragility curves for expressway embankments based on damage datasets after recent earthquakes in Japan

The seismometer network of the Japanese expressway system was enhanced following the 1995 Kobe earthquake. Based on earthquake information from the instruments of the seismometer network, a traffic control is performed directly after the event because of the potential for damage to expressway structures. Expressways serve as vital trunk lines of transportation and are important for the restoration of damage-stricken areas. Therefore, earthquake-induced damage to expressway structures should be estimated as soon as possible. Expressway embankments were seriously damaged during recent earthquakes, such as the 2004 Niigata Chuetsu earthquake. The present study constructs the fragility curves of expressway embankments in Japan in order to estimate the damage distribution immediately after an earthquake. Damage datasets for expressways are compiled for the 2003 Northern-Miyagi earthquake, the 2003 Tokachi-oki earthquake, the 2004 Niigata Chuetsu earthquake, and the 2007 Niigata Chuetsu-oki earthquake. The spatial distributions of the peak ground velocity (PGV) are estimated for these four earthquakes in order to evaluate the relationship between the damage ratio of expressway embankments and the PGV. Statistical analysis is then conducted in order to draw the fragility curves for expressway embankments. Based on the fragility curves, major damage that disrupts ordinary expressway traffic may occur when the peak ground velocity exceeds approximately 35.0 cm/s. The fragility curves constructed in the present study are helpful for predicting the damage distribution on expressways soon after an earthquake, which enables efficient traffic control and rapid disaster response.     

An examination of the presence and topography of the D＇＇ discontinuity under the Russia–Kazakhstan border region using seismic waveform data from a deep earthquake in Spain

The D＇＇ layer,which is located atop the core–mantle boundary,has long been an area of focus for global seismology studies. A widely used approach to study the discontinuities in the D＇＇ layer involves the use of the SdS phases between the S and ScS phases,which requires that certain stringent conditions be satisfied with respect to an epicentral distance and earthquake depth. Therefore,this approach is only practical for investigating the presence and topography of velocity interfaces in certain local regions around the world. The Russia–Kazakhstan border region has been a ‘‘blind spot＇＇ with respect to this detection method. The seismic network deployed in the northeastern margin of the Tibetan Plateau has recorded relatively clear SdS phases for the MS6.3 earthquake that occurred in Spain on April 11,2010,allowing this blind spot to be studied. This paper compares the observed waveforms and synthetics and uses the travel times of the relevant phases to obtain a D＇＇ discontinuity depth between2,610 and 2,740 km in the examined area. This study provides the first results regarding the depth of the D＇＇ layer discontinuity for this region and represents an important addition to the global studies of the D＇＇ layer.     

Geology of the Kokchetav UHP-HP metamorphic belt, Northern Kazakhstan

Abstract Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure–high-pressure (UHP–HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich-like; that is, the UHP–HP units are separated from underlying low-P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top-to-the-south sense of shear along the top, and top-to-the-north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks.