Isotopic portrayal of the Earth's upper mantle flow field

It is now well established that oceanic plates sink into the lower mantle at subduction zones, but the reverse process of replacing lost upper-mantle material is not well constrained. Even whether the return flow is strongly localized as narrow upwellings or more broadly distributed remains uncertain. Here we show that the distribution of long-lived radiogenic isotopes along the world's mid-ocean ridges can be used to map geochemical domains, which reflect contrasting refilling modes of the upper mantle. New hafnium isotopic data along the Southwest Indian Ridge delineate a sharp transition between an Indian province with a strong lower-mantle isotopic flavour and a South Atlantic province contaminated by advection of upper-mantle material beneath the lithospheric roots of the Archaean African craton. The upper mantle of both domains appears to be refilled through the seismically defined anomaly underlying South Africa and the Afar plume. Because of the viscous drag exerted by the continental keels, refilling of the upper mantle in the Atlantic and Indian domains appears to be slow and confined to localized upwellings. By contrast, in the unencumbered Pacific domain, upwellings seem comparatively much wider and more rapid.     

南海成因：岩石圈破裂与俯冲带相互作用新认识

 南海深部计划与国际大洋钻探航次取得了一系列创新进展与重大突破：1）发现南海陆缘岩石圈减薄之初未出现地幔蛇纹岩出露，且岩浆迅速出现；2）新提出南海不是“小大西洋”，而是“板缘张裂”盆地，与经典的大西洋型“板内张裂”陆缘模式不同；3）揭示南海受到俯冲带的强烈控制，提出俯冲诱发地幔上涌并影响南海岩浆活动。     

Evolution and diversity of subduction zones controlled by slab width

Subducting slabs provide the main driving force for plate motion and flow in the Earth's mantle, and geodynamic, seismic and geochemical studies offer insight into slab dynamics and subduction-induced flow. Most previous geodynamic studies treat subduction zones as either infinite in trench-parallel extent (that is, two-dimensional) or finite in width but fixed in space. Subduction zones and their associated slabs are, however, limited in lateral extent (250-7,400 km) and their three-dimensional geometry evolves over time. Here we show that slab width controls two first-order features of plate tectonics-the curvature of subduction zones and their tendency to retreat backwards with time. Using three-dimensional numerical simulations of free subduction, we show that trench migration rate is inversely related to slab width and depends on proximity to a lateral slab edge. These results are consistent with retreat velocities observed globally, with maximum velocities (6-16 cm yr(-1)) only observed close to slab edges (<1,200 km), whereas far from edges (>2,000 km) retreat velocities are always slow (<2.0 cm yr(-1)). Models with narrow slabs (< or =1,500 km) retreat fast and develop a curved geometry, concave towards the mantle wedge side. Models with slabs intermediate in width ( approximately 2,000-3,000 km) are sublinear and retreat more slowly. Models with wide slabs (> or =4,000 km) are nearly stationary in the centre and develop a convex geometry, whereas trench retreat increases towards concave-shaped edges. Additionally, we identify periods (5-10 Myr) of slow trench advance at the centre of wide slabs. Such wide-slab behaviour may explain mountain building in the central Andes, as being a consequence of its tectonic setting, far from slab edges.     

High mixing rates in the abyssal Southern Ocean

Mixing of water masses from the deep ocean to the layers above can be estimated from considerations of continuity in the global ocean overturning circulation. But averaged over ocean basins, diffusivity has been observed to be too small to account for the global upward flux of water, and high mixing intensities have only been found in the restricted areas close to sills and narrow gaps. Here we present observations from the Scotia Sea, a deep ocean basin between the Antarctic peninsula and the tip of South America, showing a high intensity of mixing that is unprecedented over such a large area. Using a budget calculation over the whole basin, we find a diffusivity of (39 plus minus 10) x 104[?]m2[?]s-1, averaged over an area of 7 x 105[?]km2. The Scotia Sea is a basin with a rough topography, situated just east of the Drake passage where the strong flow of the Antarctic Circumpolar Current is constricted in width. The high basin-wide mixing intensity in this area of the Southern Ocean may help resolve the question of where the abyssal water masses are mixed towards the surface.     

A correlation between mid-ocean-ridge basalt chemistry and distance to continents

To fully understand the structure and dynamics of the Earth's convecting mantle, the origins of temperature variations within the mantle need to be resolved. Different hypotheses have been proposed to account for these temperature variations: for example, heat coming from the decay of radioactive elements or heat flowing out of the Earth's core. In addition, theoretical studies suggest that the thermal properties of continental masses can affect mantle convection, but quantitative data that could allow us to test these models are scarce. To address this latter problem, we have examined the chemistry of mid-ocean-ridge basalt--which reflects the temperature of the source mantle--as a function of the distance of the ridge from the closest continental margin. No correlation is observed for oceanic ridges close to subduction zones or hotspots; subduction zones probably inhibit thermal transfer between the mantle beneath continents and ocean, whereas hotspots influence the major-element chemistry of ridge basalts, which makes their interpretation with respect to mantle temperature more difficult. However, we do observe a significant correlation for mid-oceanic basalts from the Atlantic and Indian oceans. From this, we conclude that the location of continental masses relative to active ridges influences the large-scale thermal structure of the mantle and we estimate that the mantle cools by 0.05 to 0.1 degrees C per kilometre from the continental margins.     

Origin of the Changbai intraplate volcanism in Northeast China： Evidence from seismic tomography

Seismic images of the mantle beneath the active Changbai intraplate volcano in Northeast China determined by teleseismic travel time tomography are presented. The data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations. The results show a columnar low-velocity (-3%) anomaly extending to 400 km depth under the Changbai volcano. High velocity anomalies are visible in the mantle transition zone, and deep earthquakes occur at depths of 500--600 km under the region,suggesting that the subducting Pacific slab is stagnant in the transition zone, as imaged clearly also by global tomography.These results suggest that the Changbai intraplate volcano is not a hotspot like Hawaii but a kind of back-arc volcano related to the upwelling of hot asthenospheric materials associated with the deep subduction and stagnancy of the Pacific slab under northeast Asia.     

全球地表温度大气遥相关路径研究

基于复杂网络方法,分析不同区域地表温度之间存在的相关关系及其时滞,建立了体现大气遥相关的全球地表温度网络,进而给出地表温度网络遥相关路径．研究表明:网络连接的空间距离在3 500和7 000 km处有1个峰值,这与大气Rossby波的1/2和1倍波长一致．地表温度网络中,影响传播的主导节点在北半球分布在东亚、向西延伸的北太平洋、美国东海岸及邻接的北大西洋地区;在南半球分布在50° S纬度带．遥相关现象在南半球比北半球更显著,典型遥相关路径与不同的环流作用有明确对应:1）北太平洋中部到墨西哥的连接反映了西风带的作用;2）北大西洋传播到非洲北部、格陵兰岛到里海的连接,均属于连接北大西洋到欧亚大陆的跨欧亚波列的一部分;3）俄罗斯喀拉海到北太平洋的连接与北大西洋涛动（NAO）密切关联;4）南半球的连接反映了大气西风带和Rossby波的影响．大气遥相关路径分析有利于深化对地表温度变化的认识,可为减缓气候全球变化提供理论基础．      

Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge

The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.     

Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface

At the northern Cascadia margin, the Juan de Fuca plate is underthrusting North America at about 45 mm x yr(-1) (ref. 1), resulting in the potential for destructive great earthquakes. The downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake (thought to have been in 1700) occurred before instrumental recording. Thermal and deformation studies indicate that, off southern Vancouver Island, the interplate interface is presently fully locked for a distance of approximately 60 km downdip from the deformation front. Great thrust earthquakes on this section of the interface (with magnitudes of up to 9) have been estimated to occur at an average interval of about 590 yr (ref. 3). Further downdip there is a transition from fully locked behaviour to aseismic sliding (where high temperatures allow ductile deformation), with the deep aseismic zone exhibiting slow-slip thrust events. Here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to a broad reflection band at greater depth where aseismic slip is thought to be occurring. This change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones.     

Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua

Resolving flow geometry in the mantle wedge is central to understanding the thermal and chemical structure of subduction zones, subducting plate dehydration, and melting that leads to arc volcanism, which can threaten large populations and alter climate through gas and particle emission. Here we show that isotope geochemistry and seismic velocity anisotropy provide strong evidence for trench-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua. This finding contradicts classical models, which predict trench-normal flow owing to the overlying wedge mantle being dragged downwards by the subducting plate. The isotopic signature of central Costa Rican volcanic rocks is not consistent with its derivation from the mantle wedge or eroded fore-arc complexes but instead from seamounts of the Galapagos hotspot track on the subducting Cocos plate. This isotopic signature decreases continuously from central Costa Rica to northwestern Nicaragua. As the age of the isotopic signature beneath Costa Rica can be constrained and its transport distance is known, minimum northwestward flow rates can be estimated (63-190 mm yr(-1)) and are comparable to the magnitude of subducting Cocos plate motion (approximately 85 mm yr(-1)). Trench-parallel flow needs to be taken into account in models evaluating thermal and chemical structure and melt generation in subduction zones.     

Seismic tomography of the northwest Pacific and its geodynamic implications

High-resolution tomographic images across Japan Trenh-Changhai Mountains-lDong Ujimqinqi are displayed, showing the morphological feature of the subducted slab in the norhwestem Pacific margin and the eharaeter istics of lithosphere stmctures under the Changhai Mountains and the Da Hinggan Mnuntains. The Pacific plate began to penetrate into the deeper mantle after it subducted to the 660 km discontinuity with an underthmsting angle of 26°, but did not continue to mnve furrther westward. In contrast, there appeared a remarkable thermal upwelling zone to the west of the downward plate. In addition, the evidence frnm the subduction time and time lag between the subduetion and eon sequent magmatism indicates that there is no direct genetic correlatiom between the Mesoznic magmatism in eastern China ami subduction of the Pacific plate. In this work. we also emphasize that what the tomographic images reflect is the pre sent structure in the deep earth interior, which should preserve some Mesozoic lithospheric structure characteristics. In summary, we attribute the Mesozoic intense magmatic evolution in north China to the intraplate asthenosphere upwelling zone.     

利用SS前驱波研究纳斯卡–南美俯冲带周边上地幔间断面起伏及其动力学意义

利用USArray的远震记录资料, 对1万多条地震记录的SS前驱波进行共反射点叠加分析(叠加范围是半径为5°的球冠), 得到纳斯卡板块南部及南美板块西部(120°W—60°W, 30°S—5°N)上地幔间断面的起伏形态。由于数据很好地覆盖了研究区域, 叠加范围更小, 因此比以往的结果具有更高的空间分辨率。在Galapagos, Easter和 San Felix这3个热点下方观测到的间断面起伏形态符合地幔深部存在高温异常的假设。还观测到: 在东太平洋洋隆南段下方, 410 km间断面深度增加; 在俯冲带下方, 410 km和 660 km间断面深度增加。     

A discontinuity in mantle composition beneath the southwest Indian ridge

The composition of mid-ocean-ridge basalt is known to correlate with attributes such as ridge topography and seismic velocity in the underlying mantle, and these correlations have been interpreted to reflect variations in the average extent and mean pressures of melting during mantle upwelling. In this respect, the eastern extremity of the southwest Indian ridge is of special interest, as its mean depth of 4.7 km (ref. 4), high upper-mantle seismic wave velocities and thin oceanic crust of 4-5 km (ref. 6) suggest the presence of unusually cold mantle beneath the region. Here we show that basaltic glasses dredged in this zone, when compared to other sections of the global mid-ocean-ridge system, have higher Na(8.0), Sr and Al2O3 compositions, very low CaO/Al2O3 ratios relative to TiO2 and depleted heavy rare-earth element distributions. This signature cannot simply be ascribed to low-degree melting of a typical mid-ocean-ridge source mantle, as different geochemical indicators of the extent of melting are mutually inconsistent. Instead, we propose that the mantle beneath approximately 1,000 km of the southwest Indian ridge axis has a complex history involving extensive earlier melting events and interaction with partial melts of a more fertile source.     

Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic Ocean

Submarine hydrothermal venting along mid-ocean ridges is an important contributor to ridge thermal structure, and the global distribution of such vents has implications for heat and mass fluxes from the Earth's crust and mantle and for the biogeography of vent-endemic organisms. Previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow-spreading ridges (ridges with full spreading rates <2 cm x yr(-1)-which make up 25 per cent of the global ridge length), and that such vent systems would be hosted in ultramafic in addition to volcanic rocks. Here we present evidence for active hydrothermal venting on the Gakkel ridge, which is the slowest spreading (0.6-1.3 cm x yr(-1)) and least explored mid-ocean ridge. On the basis of water column profiles of light scattering, temperature and manganese concentration along 1,100 km of the rift valley, we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites. Our discovery of such abundant venting, and its apparent localization near volcanic centres, requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow-spreading ridges.     

太平洋火山特征与深部成因机制

太平洋板块边界和内部均发育大量火山，是研究地球火山的天然实验场。综述了太平洋火山特征与深部成因机制，表明研究人员对地球不同环境下的火山（包括大洋中脊、俯冲带岛弧、板内地幔柱等）进行了系统性研究，分别构建了减压熔融、俯冲板片脱水与富水地幔楔熔融、地幔柱高温熔融的经典模式。但目前学界对于板内非地幔柱型火山的深部岩浆起源以及浅部喷发通道等重要科学问题仍缺乏清晰的认识。未来需要采用创新观测手段，开展多学科交叉研究以取得突破。     

Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts

The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher (187)Os/(188)Os ratios than the depleted upper-mantle value and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.     

Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere

Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10-12?km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine's high-pressure polymorphs, wadsleyite and ringwoodite, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle (deeper than 1,000?km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250?km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes) west of and within the subducting Tonga slab and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100?p.p.m. by weight). We conclude that subducting slabs are essentially dry at depths below 400?km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.     

江淮流域夏季降水异常及与全球中低纬海温异常关系的诊断研究

对1951－1999年中国夏季江淮流域降水异常与海温异常关系的分析表明,前期及同期各季节三大洋海表温度异常（SSTA）与长江流域降水异常的关系是非常显著的,而对淮河流域降水异常总体上的影响较小,前期冬季SSTA的影响显著区主要有：热带印度洋、黑潮、热带中东太平洋和大西洋,各关键区海温异常对亚洲夏季风的影响特征为：当前期冬季赤道印度洋、黑潮、赤道大西洋和热带东太平洋海表温度异常升高（降低）,当年夏季印度西南季风和东亚热带辐合带减弱（加强）,副热带高压位置偏南（北）,副热带辐合带加强（减弱）,长江流域易发生洪涝（干旱）,相关显著性分析表明,前冬赤道印度洋和黑潮区的海温异常对中国夏季降水的影响更为显著。     

汤加-克马德克俯冲带的地质构造与地震火山特征

2022年1月15日西南太平洋的洪阿哈阿帕伊岛海底火山发生了爆炸式的剧烈喷发，吸引了全球的关注。洪阿哈阿帕伊岛海底火山位于汤加-克马德克俯冲带，综合前期研究结果，对汤加-克马德克俯冲带的地质构造特征、地震和火山分布进行初步分析，发现：（1）从汤加-克马德克俯冲带弧前向海方向直到俯冲的太平洋板块，构造上主要表现为大规模正断层。（2）路易斯维尔海山链的俯冲将汤加-克马德克俯冲带分为北部的汤加俯冲带和南部的克马德克俯冲带，沿汤加俯冲带板块汇聚率为67～84 mm/a，沿克马德克俯冲带板块汇聚率为41～58 mm/a，板块俯冲速度的差异造成汤加俯冲带和克马德克俯冲带目前俯冲深度的不同。（3）在路易斯维尔海山链以北，太平洋板块上覆沉积物厚度不足0.4 km，而在南侧达到1 km左右，由于俯冲板块上覆沉积物厚度的差异而造成北部的汤加俯冲带和南部的克马德克俯冲带孕育地震能力的差异。这些认识对研究该俯冲带的火山喷发机制、大地震成因机理及其灾害风险具有重要意义。     

Water content in the early Cretaceous lithospheric mantle beneath the south-central Taihang Mountains: implications for the destruction of the North China Craton

Based on studies of the water content of the early Cretaceous Feixian high-magnesium basalts in the eastern part of the North China Craton （NCC）, it has been suggested that the early Cretaceous lithospheric mantle of the eastern NCC was highly hydrous （〉1,000 ppm, HeO wt.） and that this high water content had significantly reduced the vis- cosity of the lithospheric mantle and provided a prerequisite for the destruction of the NCC. The eastern part of the NCC had undergone multistage subduction of oceanic plates from the south, north, and east sides since the early Paleozoic, and these events may have caused the strong hydration of the NCC lithospheric mantle. To determine which subduction had contributed most to this hydration, we measured the water contents of the peridotite xenoliths hosted by the early Cretaceous high-magnesium diorites of Fushan in the south- central part of the Taihang Mountains. Our results demon- strate that the water content of the early Cretaceous litho- spheric mantle beneath the south part of the Taihang Mountains was ~ 40 ppm and significantly lower than that of the contemporary lithospheric mantle beneath the eastern part of the NCC. Thus, the hydration of the early Cretaceous lithospheric mantle of the eastern part of the NCC can be ascribed to the subduction of the Pacific plate from the west side. Thus, the main dynamic factor in the destruction of the NCC was likely the subduction of the Pacific plate.