大陆碰撞成矿作用：深部动力学机制与成矿

大陆碰撞成矿研究是当今地球科学的重大前沿,但是碰撞带深部动力学机制与成矿之间的联系还不够紧密。目前主要的几种碰撞动力学机制包括大陆俯冲及俯冲相关的陆壳撕裂与回撤、大洋板片断离、上覆岩石圈去根,如对流减薄、拆沉等;而碰撞成矿系统还可以进一步概括总结为岩浆热液成矿系统、变质流体成矿系统和盆地流体成矿系统三类。在此基础上,通过地幔上涌、地壳缩短和地表隆升等各种壳幔响应,将深部动力学与成矿作用进行了成因关联。研究揭示,成矿是地球圈层相互作用导致金属元素循环富集的过程。研究指出,从地球系统科学的角度来研究大陆碰撞成矿作用,有望将矿床学研究提高到新层次,还有望实现“碰撞加工厂”的找矿理想。     

岩石层底部切向应力场及地球大地水准面异常

比较了两种不同方法,即Runcorn 方程和地幔对流模型,计算了作用于岩石层底部的切向应力场.讨论了在三种不同的边界模型下,核-幔边界对于地球大地水准面的贡献.结果表明:模型Ⅰ(自由核-幔边界;摩擦岩石层-地幔边界)和模型Ⅲ(刚性核-幔边界,摩擦岩石层-地幔边界)中,核幔边界对于地球大地水准面的贡献随球谐函数系数的增加而减少,在n≥2时可以忽略下边界的影响.而模型Ⅱ(摩擦核-幔边界,自由表面)中下边界影响不容忽视.证明使用Runcorn 方程计算岩石层底部的切向应力场是一个合理的近似.     

蛇绿岩研究新进展及其定义和分类的再讨论

蛇绿岩保存于大陆造山带中,在地质历史上产于洋--陆过渡区多种构造背景(活动大陆边缘、被动大陆边缘、弧前、弧后、弧内、弧问、板块三联点、转换断层、大陆裂谷或它们的复合背景等)的洋盆扩张脊.蛇绿岩以超镁铁质岩、镁铁质岩为主,是一套彼此具有成因演化亲缘关系的、古洋壳 大洋地幔岩石的有机组合.我们把蛇绿岩概括为产于洋盆扩张脊的洋壳 大洋地幔岩石组合.蛇绿岩按构造背景有红海萌芽型洋壳和大洋中脊型洋壳两个端元,可三分为近陆的活动大陆边缘型--科迪勒拉型、东地中海型(新特提型)过渡型、西地中海型--大洋中脊古特提斯型.科迪勒拉型和东地中海型均属俯冲带(SSZ)上叠型,更近陆靠俯冲带;环太平洋型和被动大陆边缘型属特提斯型.     

我国大陆下地壳的再循环研究取得重大进展

物质的再循环是地球动力学的主要过程之一.大陆下地壳是否能够再循环返回地幔是地球动力学和地球化学一个长期未解决的难题,这一问题对于认识大陆地壳的形成和演化以及壳幔交换作用至关重要.     

中国东北上地幔各向异性研究

利用中国数字地震台网记录的地震波形,观测其穿过地球外核和地幔的SKS波分裂特征,确定了快波S波的振动方向和快慢波时间延迟。这种经过地幔传播的SKS震相的横波分裂主要来源于上地幔的各向异性。可以用应变引起的上地幔中矿物质的结晶优势排列来解释。结果表明,对于所分析的10个观测台站均发现了明显的横波分裂现象。时间延迟在0.4～1.8 s。在东部地区,快波振动方向与中国东北西太平洋俯冲带的板块相对运动方向基本一致,说明俯冲的太平洋板块地幔流对各向异性有重要意义。在西部地区,快波振动方向和时间延迟都具有较大差异,说明其总体形变的起因是复杂的,各向异性受到地幔流和历史构造运动的双重影响。     

中国东北上地幔各向异性研究

利用中国数字地震台网记录的地震波形,观测其穿过地球外核和地幔的SKS波分裂特征,确定了快波S波的振动方向和快慢波时间延迟。这种经过地幔传播的SKS震相的横波分裂主要来源于上地幔的各向异性。可以用应变引起的上地幔中矿物质的结晶优势排列来解释。结果表明,对于所分析的10个观测台站均发现了明显的横波分裂现象。时间延迟在0.4～1.8 s。在东部地区,快波振动方向与中国东北西太平洋俯冲带的板块相对运动方向基本一致,说明俯冲的太平洋板块地幔流对各向异性有重要意义。在西部地区,快波振动方向和时间延迟都具有较大差异,说明其总体形变的起因是复杂的,各向异性受到地幔流和历史构造运动的双重影响。     

华北陆台早前寒武纪地质演化与地幔对流模式

提出了华北陆台早前寒武纪地质演化及地幔地流模式，太古宙地幔呈南北方向向陆台俯冲，由于放射性热元素在壳下的不断积累，温度逐渐升高，对流速度减慢，从而使地幔对流方向在晚太古代至早元古代发生了转换，由近南北向的挤压俯冲转米为近东西向的壳下地幔下涌，陆台下发生了广泛的ＴＴＧ质重熔，并发生了大幅度的抬升，这种对流状态持续到古生代末。     

福建泉州蛇绿岩及其构造肢解

长乐-南澳断裂带北段分布的一系列基性——超基性岩块可以区分出四类岩石:变质橄栏岩、超镁铁质堆积杂岩、堆晶层状辉长岩和块状辉长岩,它们组成了泉州蛇绿岩,属科迪勒拉型,代表早期边缘海盆地或弧间盆地的上部地幔和洋壳。早白垩世晚期,闽台微大陆与闽浙中生代火山弧碰撞拼贴过程中,泉州蛇绿岩被构造肢解,形成 NW 向逆冲就位的长基超基性推覆岩席和 SE 向剪切逆冲就位的青兰山辉长岩岩块。构造肢解的泉州蛇绿岩构成了深层弧前混杂带。     

Q:东非大裂谷未来会变成海洋吗?

正A:现今地球上有几个处于活动中的裂谷系(右图A),其中最受瞩目的是东非大裂谷,它被称作"地球的伤疤",以其雄伟壮观的地形和众多的火山湖泊而闻名。它的东西两支长度累积近6000千米,裂谷带平均宽度约40~60千米,最大宽度约200千米。裂谷形成于大陆地壳内部。地幔对流理论认为,地幔在大洋中脊上涌形成新的洋壳,而     

新疆吐哈盆地南北缘石碳纪火山岩铂族元素配分及其指示意义

古俯冲带背景下的残余地幔岩的PGEs配分以Pd,Pt明显亏损和Ir,Ru相对富集为特征,而部分熔融产物的玄武岩和基性岩脉则呈现相反的Ir,Ru明显亏损和Pd,Pt相对富集特征。俯冲带中流体的参与一方面增加了部分融程度,有利于Pd,Pt从原始地幔进入熔融相;另一方面,流体的作用使得Pd,Pt相对于Ir,Ru迁移活动性效率提高,造成地幔岩中w(Pd)/w(Ir)和w(Pt)/w(Ir）比例很低。新疆吐鲁番－－哈密盆地南缘觉罗塔格和北缘博格达两个石炭纪火岩带中的玄武岩和基性岩带中的基性岩明显不同,据此可以认为两个岩浆带并不具备古板块俯冲带的构造背景。     

Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume

The compositional differences between mid-ocean-ridge and ocean-island basalts place important constraints on the form of mantle convection. Also, it is thought that the scale and nature of heterogeneities within plumes and the degree to which heterogeneous material endures within the mantle might be reflected in spatial variations of basalt composition observed at the Earth's surface. Here we report osmium isotope data on lavas from a transect across the Azores archipelago which vary in a symmetrical pattern across what is thought to be a mantle plume. Many of the lavas from the centre of the plume have lower 187Os/188Os ratios than most ocean-island basalts and some extend to subchondritic 187Os/188Os ratios-lower than any yet reported from ocean-island basalts. These low ratios require derivation from a depleted, harzburgitic mantle, consistent with the low-iron signature of the Azores plume. Rhenium-depletion model ages extend to 2.5 Gyr, and we infer that the osmium isotope signature is unlikely to be derived from Iberian subcontinental lithospheric mantle. Instead, we interpret the osmium isotope signature as having a deep origin and infer that it may be recycled, Archaean oceanic mantle lithosphere that has delaminated from its overlying oceanic crust. If correct, our data provide evidence for deep mantle subduction and storage of oceanic mantle lithosphere during the Archaean era.     

Multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes

The hypothesis that a single mushroom-like mantle plume head can generate a large igneous province within a few million years has been widely accepted. The Siberian Traps at the Permian-Triassic boundary and the Deccan Traps at the Cretaceous-Tertiary boundary were probably erupted within one million years. These large eruptions have been linked to mass extinctions. But recent geochronological data reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years. This observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal, single-stage plume model suggests. Here we present numerical experiments to demonstrate that the entrainment of a dense eclogite-derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations. Such a process may contribute to multiple episodes of large igneous provinces.     

Mantle plumes： Why the current skepticism？

The present reappraisal of the mantle plume hypothesis is perhaps the most exciting current debate in Earth science. Nevertheless, the fundamental reasons for why it has arisen are often not well understood. They are that 1） many observations do not agree with the predictions of the original model, 2） it is possible that convection of the sort required to generate thermal plumes in the Earth＇s mantle does not occur, 3） so many variants of the original model have been invoked to accommodate conflicting data that the plume hypthesis is in practice no longer testable, and 4） alternative models are viable, though these have been largely neglected by researchers. Regardless of the final outcome, the present vigorous debate is to be welcomed since it is likely to stimulate new discoveries in a way that unquestioning acceptance of the conventional plume model will not.     

A case for mantle plumes

The existence of at least several plumes in the Earth＇s mantle can be inferred with few assumptions from well-established observations. As well, thermal mantle plumes can be predicted from well-established and quantified fluid dynamics and a plausible assumption about the Earth＇s early thermal state. Some additional important observations, especially of flood basalts and rift-related magmatism, have been shown to be plausibly consistent with the physical theory. Recent claims to have detected plumes using seismic tomography may comprise the most direct evidence for plumes, but plume tails are likely to be difficult to resolve definitively and the claims need to be well tested. Although significant questions remain about its viability, the plume hypothesis thus seems to be well worth continued investigation. Nevertheless there are many non-plate-related magmatic phenomena whose association with plumes is unclear or unlikely. Compositional buoyancy has recently been shown potentially to substantially complicate the dynamics of plumes, and this may lead to explanations for a wider range of phenomena, including “headless” hotspot tracks, than purely thermal plumes.     

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。     

Mesozoic plate-motion history below the northeast Pacific Ocean from seismic images of the subducted Farallon slab

The high-resolution seismic imaging of subducted oceanic slabs has become a powerful tool for reconstructing palaeogeography. The images can now be interpreted quantitatively by comparison with models of the general circulation of the Earth's mantle. Here we use a three-dimensional spherical computer model of mantle convection to show that seismic images of the subducted Farallon plate provide strong evidence for a Mesozoic period of low-angle subduction under North America. Such a period of low-angle subduction has been invoked independently to explain Rocky Mountain uplift far inland from the plate boundary during the Laramide orogeny. The computer simulations also allow us to locate the largely unknown Kula-Farallon spreading plate boundary, the location of which is important for inferring the trajectories of 'suspect' terrain across the Pacific basin.     

The influence of a chemical boundary layer on the fixity,spacing and lifetime of mantle plumes

Seismological observations provide evidence that the lowermost mantle contains superposed thermal and compositional boundary layers that are laterally heterogeneous. Whereas the thermal boundary layer forms as a consequence of the heat flux from the Earth's outer core, the origin of an (intrinsically dense) chemical boundary layer remains uncertain. Observed zones of 'ultra-low' seismic velocity suggest that this dense layer may contain metals or partial melt, and thus it is reasonable to expect the dense layer to have a relatively low viscosity. Also, it is thought that instabilities in the thermal boundary layer could lead to the intermittent formation and rise of mantle plumes. Flow into ascending plumes can deform the dense layer, leading, in turn, to its gradual entrainment. Here we use analogue experiments to show that the presence of a dense layer at the bottom of the mantle induces lateral variations in temperature and viscosity that, in turn, determine the location and dynamics of mantle plumes. A dense layer causes mantle plumes to become spatially fixed, and the entrainment of low-viscosity fluid enables plumes to persist within the Earth for hundreds of millions of years.     

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.     

Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt

A substantial uncertainty in the Earth's global geochemical water cycle is the amount of water that enters the deep mantle through the subduction and recycling of hydrated oceanic lithosphere. Here we address the question of recycling of water into the deep mantle by characterizing the volatile contents of different mantle components as sampled by ocean island basalts and mid-ocean-ridge basalts. Although all mantle plume (ocean island) basalts seem to contain more water than mid-ocean-ridge basalts, we demonstrate that basalts associated with mantle plume components containing subducted lithosphere--'enriched-mantle' or 'EM-type' basalts--contain less water than those associated with a common mantle source. We interpret this depletion as indicating that water is extracted from the lithosphere during the subduction process, with greater than 92 per cent efficiency.     

Supercontinent cycles and the calculation of absolute palaeolongitude in deep time

Traditional models of the supercontinent cycle predict that the next supercontinent--'Amasia'--will form either where Pangaea rifted (the 'introversion' model) or on the opposite side of the world (the 'extroversion' models). Here, by contrast, we develop an 'orthoversion' model whereby a succeeding supercontinent forms 90° away, within the great circle of subduction encircling its relict predecessor. A supercontinent aggregates over a mantle downwelling but then influences global-scale mantle convection to create an upwelling under the landmass. We calculate the minimum moment of inertia about which oscillatory true polar wander occurs owing to the prolate shape of the non-hydrostatic Earth. By fitting great circles to each supercontinent's true polar wander legacy, we determine that the arc distances between successive supercontinent centres (the axes of the respective minimum moments of inertia) are 88° for Nuna to Rodinia and 87° for Rodinia to Pangaea--as predicted by the orthoversion model. Supercontinent centres can be located back into Precambrian time, providing fixed points for the calculation of absolute palaeolongitude over billion-year timescales. Palaeogeographic reconstructions additionally constrained in palaeolongitude will provide increasingly accurate estimates of ancient plate motions and palaeobiogeographic affinities.