Mantle segmentation along the Oman ophiolite fossil mid-ocean ridge

It has been difficult to relate the segmentation of mid-ocean ridges to processes occurring in the Earth's underlying mantle, as the mantle is rarely sampled directly and chemical variations observed in lavas at the surface are heavily influenced by details of their production as melt extracted from the mantle. Our understanding of such mantle processes has therefore relied on the analysis of pieces of fossil oceanic lithosphere now exposed at the Earth's surface, known as ophiolites. Here we present the phase chemistry and whole-rock major- and trace-element contents of 174 samples of the mantle collected along over 400 km of the Oman Sultanate ophiolite. We show that, when analysed along the fossil ridge, variations of elemental ratios sensitive to the melting process define a three-dimensional geometry of mantle upwellings, which can be related to the segmentation observed in modern mid-ocean ridge environments.     

SHRIMP zircon U-Pb dating for gabbro from the Tiding ophiolite in Tibet

Ophiolite is fragments of oceanic lithosphere formed by obduction and its formation age is very important for reconstructing the formation and evolution of an-cient oceanic basin and the configuration of ancient plate tectonics. The Yarlung Zangbo suture …     

南天山库勒湖蛇绿岩锆石年龄及其地质意义

目的准确测定南天山库勒湖蛇绿岩的形成时代,探讨其地质意义。方法采用激光剥蚀等离子质谱(LA-ICP-MS)法,对库勒湖蛇绿岩中辉长岩锆石进行U-Pb同位素定年。结果辉长岩形成年龄(或结晶年龄)为418.2 Ma±2.6 Ma(MSWD=0.80)。结论南天山库勒湖蛇绿岩的形成时代为末志留世—早泥盆世早期,与古南天山洋盆的俯冲-消减时代相当,它的形成很有可能与古南天山洋盆的俯冲-消减作用有关。     

Earthquake-induced changes in a hydrothermal system on the Juan de Fuca mid-ocean ridge

Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4-11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.     

Geophysical evidence for reduced melt production on the Arctic ultraslow Gakkel mid-ocean ridge

Most models of melt generation beneath mid-ocean ridges predict significant reduction of melt production at ultraslow spreading rates (full spreading rates &<20 mm x yr(-1)) and consequently they predict thinned oceanic crust. The 1,800-km-long Arctic Gakkel mid-ocean ridge is an ideal location to test such models, as it is by far the slowest portion of the global mid-ocean-ridge spreading system, with a full spreading rate ranging from 6 to 13 mm x yr(-1) (refs 4, 5). Furthermore, in contrast to some other ridge systems, the spreading direction on the Gakkel ridge is not oblique and the rift valley is not offset by major transform faults. Here we present seismic evidence for the presence of exceptionally thin crust along the Gakkel ridge rift valley with crustal thicknesses varying between 1.9 and 3.3 km (compared to the more usual value of 7 km found on medium- to fast-spreading mid-ocean ridges). Almost 8,300 km of closely spaced aeromagnetic profiles across the rift valley show the presence of discrete volcanic centres along the ridge, which we interpret as evidence for strongly focused, three-dimensional magma supply. The traces of these eruptive centres can be followed to crustal ages of approximately 25 Myr off-axis, implying that these magma production and transport systems have been stable over this timescale.     

Re-visiting barium isotope compositions of mid-ocean ridge basalts and the implications

Barium (Ba) isotopes can be used as potential tracers for crustal material recycling in the mantle. Determination of the Ba isotope composition of the depleted mantle is essential for such applications. However, Ba isotope data for mantle-derived basalts are still rare. In this study, we reported high-precision Ba isotope data of 30 oceanic basalts including 25 mid-ocean ridge basalts (MORBs) from geochemically and geologically diverse mid-ocean ridge segments and five back-arc basin basalts. The δ138/134Ba values of these samples varied from ?0.06‰ to +0.11‰, with no systematic cross-region variation. Together with published data, we constrained the average δ138/134Ba of global MORBs to +0.05‰±0.09‰ (2 standard deviation, n = 51). Based on depleted MORBs that have (La/Sm)_N < 0.8, low 87Sr/86Sr (< 0.70263), and low Ba/Th < 71.3, we estimated the average δ138/134Ba of the depleted MORB mantle (DMM) as + 0.05‰ ± 0.05‰ (2SD, n = 16) that is significantly lower than the DMM (≈ 0.14‰) reported previously. If a new estimation of the DMM is applied, it is unreasonable to infer that the Ba isotope signatures of the “enriched-type” MORBs (E-MORBs) could be attributed to pervasive sediment recycling in the upper mantle. We, therefore, conclude that the Ba isotope compositions of the E-MORBs could be sourced from the incorporation of subducted altered oceanic crust and/or sediments depending on the Ba isotope composition and other geochemical information of the local mantle.     

The influence of ridge migration on the magmatic segmentation of mid-ocean ridges

The Earth's mid-ocean ridges display systematic changes in depth and shape, which subdivide the ridges into discrete spreading segments bounded by transform faults and smaller non-transform offsets of the axis. These morphological changes have been attributed to spatial variations in the supply of magma from the mantle, although the origin of the variations is poorly understood. Here we show that magmatic segmentation of ridges with fast and intermediate spreading rates is directly related to the migration velocity of the spreading axis over the mantle. For over 9,500 km of mid-ocean ridge examined, leading ridge segments in the 'hotspot' reference frame coincide with the shallow magmatically robust segments across 86 per cent of all transform faults and 73 per cent of all second-order discontinuities. We attribute this relationship to asymmetric mantle upwelling and melt production due to ridge migration, with focusing of melt towards ridge segments across discontinuities. The model is consistent with variations in crustal structure across discontinuities of the East Pacific Rise, and may explain variations in depth of melting and the distribution of enriched lavas.     

Evidence for seismogenic fracture of silicic magma

It has long been assumed that seismogenic faulting is confined to cool, brittle rocks, with a temperature upper limit of approximately 600 degrees C (ref. 1). This thinking underpins our understanding of volcanic earthquakes, which are assumed to occur in cold rocks surrounding moving magma. However, the recent discovery of abundant brittle-ductile fault textures in silicic lavas has led to the counter-intuitive hypothesis that seismic events may be triggered by fracture and faulting within the erupting magma itself. This hypothesis is supported by recent observations of growing lava domes, where microearthquake swarms have coincided with the emplacement of gouge-covered lava spines, leading to models of seismogenic stick-slip along shallow shear zones in the magma. But can fracturing or faulting in high-temperature, eruptible magma really generate measurable seismic events? Here we deform high-temperature silica-rich magmas under simulated volcanic conditions in order to test the hypothesis that high-temperature magma fracture is seismogenic. The acoustic emissions recorded during experiments show that seismogenic rupture may occur in both crystal-rich and crystal-free silicic magmas at eruptive temperatures, extending the range of known conditions for seismogenic faulting.     

U-Pb zircon dating of Early Paleozoic gabbro from the Nantinghe ophiolite in the Changning-Menglian suture zone and its geological implication

The Nantinghe ophiolite is located in the northern part of the Changning-Menglian suture zone in southeast Tibet. It is composed of meta-peridotite, cumulative gabbro, meta-gabbro, plagioclase amphibolite and meta-basalt. Zircon U-Pb dating of the cumulative gabbro gives concordant ages of 473.0±3.8 Ma and 443.6±4.0 Ma respectively, indicating the early and late episodes of mafic magmatisms during the Paleo-Tethys oceanic rifting. The 16 LA-ICPMS zircon U-Pb analyses of meta-gabbro yield a weight mean age of 439±2.4Ma. The gabbro shows relatively low contents of SiO2 (46.46%-52.11%), TiO2 (0.96%-1.14%) and K2O (0.48%-0.75%). Its trace element distribution patterns are partly similar to those of the mid-ocean ridge basalts, and part is depleted in high field strength elements such as Nb, Ta, Zr, Hf and Ti. These features suggest that the mafic rocks were probably formed in a MORB-like or backarc rift basin setting. The zircon U-Pb age of gabbro is consistent with a late crystallization age of the cumulative gabbro from the Nantinghe ophiolite, suggesting that the Paleo-Tethys oceanic basin was opened during 444-439 Ma, possibly as a backarc basin. It is the first precise age which defines the formation time of the early Paleozoic ophiolite in the Changning-Menglian suture zone. These geochronological and geochemical characteristics of the Nantinghe ophiolite are consistent with those from the Guoganjianianshan and Taoxinghu of the Longmu Co-Shuanghu suture in the Qiangtang region. Thus, we suggest that the both Changning-Menglian and Longmu Co-Shuanghu sutures were probably transformed from the relic oceanic crust of the uniform Paleo-Tethys, which likely represents the original and main Paleo-Tethys oceanic basin.     

Geochronology and Nd and Pb isotope characteristics of gabbro dikes in the Luobusha ophiolite, Tibet

We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clinopyroxene separates from one of the rocks yields a Middle Jurassic age of (177±31) Ma (with an initial ε_Nd(t) = +8), which provides a significant bound on the time of formation of the Luobusha ophiolite. The Nd and Pb isotopic characteristics of the dike indicate an Indian-Ocean-type isotopic affinity, and we conclude that the Luobusha ophiolite formed in an oceanic setting during the Middle Jurassic.     

Evidence of recent volcanic activity on the ultraslow-spreading Gakkel ridge

Seafloor spreading is accommodated by volcanic and tectonic processes along the global mid-ocean ridge system. As spreading rate decreases the influence of volcanism also decreases, and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates (<1.5 cm yr(-1)). Here we present three-dimensional sonar maps of the Gakkel ridge, Earth's slowest-spreading mid-ocean ridge, located in the Arctic basin under the Arctic Ocean ice canopy. We acquired this data using hull-mounted sonars attached to a nuclear-powered submarine, the USS Hawkbill. Sidescan data for the ultraslow-spreading (approximately 1.0 cm yr(-1)) eastern Gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley. The western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999, suggesting that an axial eruption was imaged shortly after its occurrence. These findings demonstrate that eruptions along the ultraslow-spreading Gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought.     

Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

Early-crystallizing chromian spinel(Cr-spinel) in the Nagqu ophiolite has high Os and low Re contents,and it is resistant to alteration during serpentinization,weathering and metamorphism.The chemical composition of primitive magma is preserved in Cr-spinel,which makes it suitable for determining the initial Os-isotope composition of the mantle source.This study presents Cr-spinel Os isotopes and zircon U-Pb ages for cumulate dunite and gabbro,respectively,in the same cumulate section of the ophiolite at Nagqu in Tibet.The results shed light on the formation and evolution of lithospheric mantle.The Nagqu ophiolite is located in the central part of the Bangong-Nujiang suture zone.It is a remnant of the Neotethyan oceanic crust,and contains cumulate dunite and gabbro.Zircon from the gabbro yielded a weighted mean 206 Pb/238 U age of 183.7±1 Ma.Cr-spinel exhibits Os values of 0.2 to 0.3,suggesting that the mantle source for the dunite is similar to that of carbonaceous chondrites.Thus,the Tibetan lithosphere is primarily a relic of Tethyan oceanic lithosphere,which has formed by the transformation of the normal asthenospheric mantle in the Mesozoic.This is the first study to combine the spinel Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of the mantle source for the ophiolite.     

泄水建筑物中突扩突跌体型侧墙上的水力特性

为保证池水建筑物侧墙的稳定与安全,通过模型试验研究了突扩突跌体型的侧墙上的水力特性。采用比尺为1¨30的水工模型,在高流速条件下观测了侧墙水流流态、通气量、掺气浓度及时均压力分布。试验细部结构特征,发现侧空腔内的反射回漩流和突扩弧形面上的白色蚀带。侧墙等压线分析发现:侧墙水流冲击带是引发或影响各种侧墙水力特性的主导因素,冲击带水流向上游反射,缩小了侧空腔的有效通气面积;冲击带水流的折冲作用,可能诱发侧墙空化空蚀破坏。     

Evidence for lateral gene transfer between Archaea and bacteria from genome sequence of Thermotoga maritima.

The 1,860,725-base-pair genome of Thermotoga maritima MSB8 contains 1,877 predicted coding regions, 1,014 (54%) of which have functional assignments and 863 (46%) of which are of unknown function. Genome analysis reveals numerous pathways involved in degradation of sugars and plant polysaccharides, and 108 genes that have orthologues only in the genomes of other thermophilic Eubacteria and Archaea. Of the Eubacteria sequenced to date, T. maritima has the highest percentage (24%) of genes that are most similar to archaeal genes. Eighty-one archaeal-like genes are clustered in 15 regions of the T. maritima genome that range in size from 4 to 20 kilobases. Conservation of gene order between T. maritima and Archaea in many of the clustered regions suggests that lateral gene transfer may have occurred between thermophilic Eubacteria and Archaea.     

Discovery of very-high-energy gamma-rays from the Galactic Centre ridge

The source of Galactic cosmic rays (with energies up to 10(15) eV) remains unclear, although it is widely believed that they originate in the shock waves of expanding supernova remnants. At present the best way to investigate their acceleration and propagation is by observing the gamma-rays produced when cosmic rays interact with interstellar gas. Here we report observations of an extended region of very-high-energy (> 10(11) eV) gamma-ray emission correlated spatially with a complex of giant molecular clouds in the central 200 parsecs of the Milky Way. The hardness of the gamma-ray spectrum and the conditions in those molecular clouds indicate that the cosmic rays giving rise to the gamma-rays are likely to be protons and nuclei rather than electrons. The energy associated with the cosmic rays could have come from a single supernova explosion around 10(4) years ago.     

Possible seismic reflector in the lower crust： Evidence from fabrics and experiments of seismic velocity on layered gabbro at high temperature and high pressure

Lattice preferred orientations (LPO) of plagioclase and augite are measured on layered gabbro from the Panxi region, Sichuan Province. The LPO concentration [010] of plagioclase and [100] of augite are perpendicular to the foliation, which indicates a kind of growth fabric associated with crystallizing habits of minerals when the magma is solidifying under the compaction. Calculated seismic velocities based on LPO data of minerals give rise to rather strong anisotropy 5.81% and 5.54% for compressional seismic wave (Vp) and shear seismic wave (Vs), respectively. The experiments at high temperature and high pressure show that the P-wave velocity of layered gabbro is 6.4-6.97 km/s with the maximum Vp anisotropy 5.22% and the Poisson‘s ratio is between 0.28-0.31. According to the comparison of fabrics with seismic velocities of layered gabbro, it is suggested that the large-scale layered intrusive body or the similar layered geological body may exist in the lower crust of this area. Such a layered intrusive body which has strong seismic anisotropy may be the seismic reflector in the lower crust.     

Redox evolution of a degassing magma rising to the surface

Volatiles carried by magmas, either dissolved or exsolved, have a fundamental effect on a variety of geological phenomena, such as magma dynamics and the composition of the Earth's atmosphere. In particular, the redox state of volcanic gases emanating at the Earth's surface is widely believed to mirror that of the magma source, and is thought to have exerted a first-order control on the secular evolution of atmospheric oxygen. Oxygen fugacity (f(O2) estimated from lava or related gas chemistry, however, may vary by as much as one log unit, and the reason for such differences remains obscure. Here we use a coupled chemical-physical model of conduit flow to show that the redox state evolution of an ascending magma, and thus of its coexisting gas phase, is strongly dependent on both the composition and the amount of gas in the reservoir. Magmas with no sulphur show a systematic f(O2) increase during ascent, by as much as 2 log units. Magmas with sulphur show also a change of redox state during ascent, but the direction of change depends on the initial f(O2) in the reservoir. Our calculations closely reproduce the H2S/SO2 ratios of volcanic gases observed at convergent settings, yet the difference between f(O2) in the reservoir and that at the exit of the volcanic conduit may be as much as 1.5 log units. Thus, the redox state of erupted magmas is not necessarily a good proxy of the redox state of the gases they emit. Our findings may require re-evaluation of models aimed at quantifying the role of magmatic volatiles in geological processes.