地球的物质运动与地幔热柱多级演化

地球是重力分异和热力对流的对立统一体。重力分异使地内重物质下沉、轻物质上浮，并分划成壳－幔－核结构圈层。核幔间巨大的温度差、压力差、粘度差和速度差的存在，导致源于“超临界层”的热物质流呈柱状上涌形成地幔热柱及其多级演化。由于地球圈层结构及其间的差异，分别在670km、100km深处，即核－幔界面上和岩石圈底部形成地幔亚热柱和幔枝构造。地幔热柱、地幔冷柱共同驱动幔壳运动，并控制着板块运动，形成复杂的大陆（大洋）动力学系统。这种动力学模式越来越得到地球物理学的证实。     

地幔动力系统与演化最新进展评述

评述了９０年代以来地幔动力学研究的一些最新的观测和理论模拟的进展,探讨该领域的几个主要热点问题,包括地幔内部转换带和核幔边界的物理化学性质与演化,俯冲板片热结构及其与地幔的相互作用,热点物理化学性质与地幔柱动力学模拟,地幔对流系统及其对表层地质过程的影响等。这些结果是在多学科交叉研究的背景下取得的。地震层析的结果超越了８０年代取得的大尺度地幔结构,得到了越来越精细的结构,如俯冲板片的结构,６６０ｋｍ间断面的起伏,ＣＭＢ的超低速层和各向异性等。俯冲板片在某些区域平躺在上地幔底部,造成６６０ｋｍ间断面的凹陷。已有明显的迹象表明,俯冲板片至少在某些区域达到了地幔底部,说明下地幔是驱动地表板块运动的地幔对流不可分隔的一部分。全地幔对流模式对地幔中存在不同的地幔地球化学源区的看法提出重大挑战,计算机模拟三维球坐标地幔对流已经成为现实,新的研究正试图把地表板块加入到对流的模拟之中,并再造板块运动的动力学演化史。最后,对这些领域的最新进展提出自己的分析和看法,认为地球动力系统演化研究所面临的难题是地球内部动力状态演变的历史记录问题。而这样的记录,尤其是早期记录,只能从地球表面的造山带和盆地记录中去寻找。认为建立地质记录与?     

板块下的构造及地幔动力学

最新的全球地幔地震层析资料揭示了岩石圈板片可以俯冲到核幔边界，超地幔羽可以从核幔边界上升到地壳上部形成热点。在大陆板块汇聚边界，地幔地震层析图像不仅显示了岩石圈板片的超深俯冲，还保存了拆沉的岩石圈“化石”残片的重要信息。从地幔深部所获取的新资料为全地幔“单层对流“的新模式提供了依据。在介绍上述全球构造研究新动向的基础上，本文强调了研究岩石圈板块必须了解板块下的构造，探索岩石圈板块的驱动力应该从“岩石圈动力学”升华到“地幔动力学”，并提出了大陆板块汇聚边界地幔动力学研究的新思考。     

地幔热柱和大陆的再造

热柱传递出的热约占地幔内热预算的10％左右。以前认为热柱起源于一个温度较高的热边界层,并认为该层极可能在地幔底部,而板块运动则是地幔顶部冷的、致密边界层(大洋岩石圈内)向下俯冲的结果。据此,驱动热柱的构造活动和板块构造活动很大程度上应是互相独立的过程。流体动力学模拟实验显示,地球内部的热柱起始于一个体积很大的“头”的上升,“头”的下部拖着一条窄细的管状“尾巴”并且通过它获得源源不断的热源物质。上升过程中,“头”部体积由于周围地幔的加入而大幅度增长。计算表明,一个起源于     

东北地区地幔热柱构造与成矿成藏作用

东北地区处于古亚洲构造域和西太平洋构造域的叠合部位.该区地幔热柱存在8个地质标志:①南北以西伯利亚板块和华北板块太古代基底(断裂)为界,东西以变质核杂岩体分布为界,基本与日本海毗邻的范围为亚地幔柱的规模;②在佳木斯-牡丹江缝合线内见有科马提岩、苦橄岩,可能为地幔热柱的中心;③水系分布呈放射状及莫霍面分布为松辽盆地幔隆起一致;④典型的热穹隆状态;⑤以花岗岩为主的火成岩省;⑥中生代基性玄武岩基本落入Nb/Ta-Nb图解的地幔柱范围;⑦形成坍塌裂谷特征;⑧岩石圈地幔呈蘑菇云状上涌,地震层析资料证实与太平洋板块俯冲具有相辅相成关系.中生代地幔热柱成矿成藏作用的时空分布具有同时代、同火成岩构造控制,山岭成矿,盆地成藏特征.从He及Ar同位素组成阐明金属矿床与油气田的同源性,分析了幔枝构造是在地表的表现形式,无疑对当今深部找矿及海相油气田的勘查具有理论指导意义.     

俯冲工厂和大陆物质的俯冲再循环研究

板块的俯冲系统可以比拟为一个工厂。再循环研究强调对俯冲物质各种组分的行为、去向的追踪和定量分析。沉积物俯冲和俯冲侵蚀作用导致陆壳物质返回地幔,初步估算表明,大陆物质返回地幔的速率与岩浆活动导致陆壳生长的速率在数量上大体相当,晚近时期陆壳的净增长速率可能近于零。大洋岛玄武岩地化特征上的多样性提示,沉入下地幔的板片可能从深部卷入地幔柱的源区。俯冲再循环过程对地壳、地幔的动力学和演化产生深刻影响。     

Telecommunication interactions between microplates and basal mantle LLSVPs.SCIEI北大核心CSCD

基于海洋地质地球物理观测建立的板块构造理论意味着板块和浅部地幔共同演化,然而地幔底部尤其是大型横波低速异常区(LLSVP)与板块(尤其微板块)运动和演化之间是否存在关联仍有争议。一些研究认为LLSVP长期保持稳定,而另一些模型则认为它与各级板块存在相互作用。为此,本文通过总结前人成果,并基于近期发表的板块重建和地幔对流模型进行进一步分析,探讨微板块运动和LLSVP的演化关系。模拟结果表明,微板块与大板块类似,俯冲后通常会下沉至核幔边界。微幔块会推动地幔底部热的物质聚集并形成大的热化学结构。该热化学结构与层析成像揭示的LLSVP基本吻合。下地幔径向流速场和温度场的二阶结构与地表速度场散度的二阶结构随时间的移动轨迹相似,表明深浅部圈层的耦合演化,但是下地幔结构演化一般会滞后于浅表。在微幔块推挤之下,地幔柱优先沿着地幔底部热化学结构的边缘形成,且有时会被推至热化学结构的内部。地幔柱上升至浅部后,能够导致岩石圈弱化甚至裂解或板块边界跃迁,形成微板块。因此,地幔底部LLSVP不是稳定或静止的,而是与微板块动态协同演化,并通过地幔柱与浅表板块边界发生遥相关,从而控制微板块生成场所。     

幔枝构造及其成矿控矿作用

幔枝构造(mantle branch structure)——地幔热柱演化的第三级单元,它不仅控制着陆内造山带的形成和演化,而且控制着金、银、铜、铅、锌等内生成矿物质的迁移通道和储集场所。 地幔柱是源于地球核幔边界附近并穿透地幔向上运移的热幔物质流,可以地幔底界(2900km)、上地幔底界(670km)和地壳底界(100km)为限划分一、二、三次柱(S.Maruyama,1994)。现代科学研究认为,由于地球内部结构分层及内、外温差的存在,地球的地幔对流一直存在,并成为其主要物质运动形     

大洋板块俯冲带地震波各向异性及剪切波分裂的成因机制

大洋板块俯冲带是许多重要地质作用(例如脱水、部分熔融、岩浆和地震活动)发生的场所.对位于俯冲带之上的地震台站所检测到的不同剪切波的数据解析,可以获得源于上覆板块、地幔楔、俯冲板块和板下地幔的地震波各向异性的关键信息.本文系统总结了世界各地大洋俯冲带的剪切波分裂样式,对目前国际上流行的大洋俯冲带的地震波各向异性的主要成因...     

热柱、超级热柱及其成因的研究进展

从20世纪地幔热柱假说问世,经过30多年的发展,在地幔热柱的全球分布、鉴别特征、形态学和成因理论方面都有了长足的进步。特别是通过下地幔不均匀性的研究,发现了下地幔中存在的超级热柱和下地幔底层中的超低速带,为探讨热柱成因提供了重要依据。     

Lost primordial continents

We investigate the bulk density variations of some representative compositions for the lower mantle based on the pressure–volume–temperature equation of state of the constituent mineral phases. The density variations of pyrolite, harzburgite, mid-ocean ridge basalt (MORB), tonalite–trondhjemite–granodiorite (TTG), and anorthosite are studied at a temperature of 300 K and at lower mantle pressures. The density of MORB is greater than that of pyrolite throughout the lower mantle, while the density of harzburgite is slightly lower than that of pyrolite. The density of anorthosite is comparable to that of pyrolite in the lower mantle in general, and greater in the lowermost mantle, while the density of TTG is lower than pyrolite throughout the lower mantle. The above results have important implications for the fate of primordial continents, TTG and anorthosite crust. While subducted TTG might be stagnant in the mantle transition zone, dense subducted anorthositic crust could be expected to sink to the core–mantle boundary (CMB) and thus might be a major component of the D" layer immediately above the CMB. Thus, we propose that significant bodies of continental material could be present in the mantle in the transition zone and immediately above the CMB, in addition to the continents on the Earth's surface.     

核幔边界D″区的地震学研究进展

核幔边界层在地球演化过程中扮演着极为重要的作用,是人们认识地球的主要研究对象之一。文中综述了近十多年来对核幔边界D″区开展地震学研究的主要方法及成果,内容涵盖了核幔边界D″区上部间断面、不均匀性、各向异性和超低速层等4个主要研究对象。结合多个相关学科的研究进展,从对热-化学-动力学三方面的耦合作用的分析,来探讨形成D″区各种观测现象的原因和机制,以及在此基础上提出的动力学演化模型。最后简要叙述了有关核幔边界研究的地震学、矿物实验、理论分析及计算科学的发展方向和挑战。随着对D″区认识的不断更新,逐步揭示地表观测到的可能受核幔边界因素控制的多种地质及地球物理现象。     

Cold Thermal Anomalous Structure within Lower Mantle and Its Geodynamic Implications

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｔｅｍｐｅｒａｔｕｒｅｓｔｒｕｃｔｕｒｅｏｆｔｈｅｅａｒｔｈｉｎｔｅｒｉｏｒｈａｓａｐｒｏ ｆｏｕｎｄｓｃｉｅｎｔｉｆｉｃｓｉｇｎｉｆｉｃａｎｃｅｆｏｒｔｈｅｓｔｕｄｙｏｎｔｈｅｄｙｎａｍｉｃｅｖｏｌｕｔｉｏｎｐｒｏｃｅｓｓｏｆｔｈｅｅａｒｔｈ .Ｉｎｇｅｎｅｒａｌ,ｔｈｅｃｒｕｓｔａｌａｎｄｌｉｔｈｏｓｐｈｅｒｉｃｔｅｍ ｐｅｒａｔｕｒｅｆｉｅｌｄｉｓｄｅｔｅｒｍｉｎｅｄｂｙｔｈｅｅｑｕａｔｉｏｎｏｆｔｈｅｒｍａｌｃｏｎｄｕｃｔｉｏｎｗｉｔｈｔｈｅｓｕｒｆａｃｅｈｅａ…     

下地幔及核幔边界结构及地球动力学

新一代高分辨率下地幔及核幔边界的地震层析成像,改变了我们对全球构造模式及地球动力过程的认识。古海洋岩石圈板片一直俯冲到下地幔底部,其残留体在核幔边界积累,并支持了地幔整体对流模式。位于核幔边界上的D″层有着十分复杂而精细的结构。紧靠核幔边界的地幔一侧发现了超低速层(ULVZ),它们可能是D″层内的局部熔融物,是引起地表热点的上升地幔柱的源头。     

Thermodynamical constraints on the crystallization of a deep magma-ocean on Earth

It has been argued that the crystallization of the magma ocean (MO) after the Moon-forming impact led to the formation of a basal magma ocean (BMO). We search which primordial conditions of pressure, temperature and chemical composition could be compatible with such scenario, based on thermodynamical constraints. The major requirement is an early formation of a viscous layer (VL) of mantle material (i.e. bridgmanite (Bg)) at mid lower-mantle depth, which could insulate thermally and chemically the BMO from the rest of the mantle. To produce such VL, Bg grains should be: (i) neutrally buoyant at mid lower-mantle depths, (ii) sufficiently abundant to produce an efficient insulating layer, and (iii) aggregated to the boundary layer from above and below. The first and the second require a large amount of MO crystallization, up to more than 45%, even in the most favorable case of all Fe partitioning into the melt. The latter is very questionable because the Bg grains have a very small settling velocity. We also investigate different scenarios of MO crystallization to provide constraints on the resulting core temperature. Starting from a fully molten Earth, a temperature as high as ～4725 K could be found at the core–mantle boundary (CMB), if the Bg grains settle early atop the CMB. Such a basal layer of Bg can efficiently decouple from each other the cooling rates of the core and the mantle above the VL. If the settling velocity of Bg grains is too low and/or the MO is too turbulent, such basal VL may not form. In this case, the CMB temperature after MO solidification should stabilize at ～4350 K. At this temperature, enough Bg grains are crystallized to make the mushy mantle viscous at any mantle depth.     

核—幔相互作用及其地球动力学意义

核－幔边界是地球内部反差最大的一个边界,也是最重要的边界,界区的核－幔相互作用过程在整个地球动力学系统中起着重要的作用。本文在概述了该－边界基本特征的基础上,系统地总结了有关核幔物质的化学反应和Ｄ”层的形成,核－幔间的质量传递及其对地幔对流之贡献,核－幔耦合与动量传递等的研究成果及有关问题。     

Mass anomalies and the structure of the earth

Details of the Earth's geoid and gravity fields are summarized and examined. A set of 9274 centerpoints of 5 ° cubes (referred to as bloblets) represents subducted slab locations. This set, developed from reconstructed plate history, was provided by the first author of Lithgow-Berttelloni et. al. [1998] and is the best available estimate of locations of subduction material in the Earth's mantle. Two global mass solutions offered here utilize 1) only those bloblets in the outer 800 km, and 2) only those bloblets in the outer 1400 km. Since each bloblet location represents the center of a 5-degree cube [a larger volume than appropriate for a fragment of subducted lithosphere] it was necessary in the 800 km depth limit model to reduce their density to 0.004 grams/cc, and by increasing bloblet density six times at 797.5 km depth to simulate the piling up of slab material beneath the 670 km boundary. The 1400 km depth limit model [commensurate with evidence of slab penetration into the lower mantle from seismic tomography] required estimating densities for the bloblets at nine different mantle depths. An additional four point-masses at 3000 km depth (to simulate CMB topography, unrelated to dynamic topography) completes the mass models. Both these models show reasonable agreement to patterns and magnitudes for degrees 2–10, 3–10, 4–10, 2–3, 3, and 2 geoid fields with both geometric and hydrostatic flattening. These models support an assessment that topography at the core mantle boundary (CMB) may be produced by processes within the core rather than from within the mantle. Possible causes for the CMB topography are discussed.     

Experimental study of reaction between perovskite and molten iron to 146 GPa and implications for chemically distinct buoyant layer at the top of the core

Partitioning of oxygen and silicon between molten iron and (Mg,Fe)SiO₃ perovskite was investigated by a combination of laser-heated diamond-anvil cell (LHDAC) and analytical transmission electron microscope (TEM) to 146 GPa and 3,500 K. The chemical compositions of co-existing quenched molten iron and perovskite were determined quantitatively with energy-dispersive X-ray spectrometry (EDS) and electron energy loss spectroscopy (EELS). The results demonstrate that the quenched liquid iron in contact with perovskite contained substantial amounts of oxygen and silicon at such high pressure and temperature (P–T). The chemical equilibrium between perovskite, ferropericlase, and molten iron at the P–T conditions of the core–mantle boundary (CMB) was calculated in Mg–Fe–Si–O system from these experimental results and previous data on partitioning of oxygen between molten iron and ferropericlase. We found that molten iron should include oxygen and silicon more than required to account for the core density deficit (<10%) when co-existing with both perovskite and ferropericlase at the CMB. This suggests that the very bottom of the mantle may consist of either one of perovskite or ferropericlase. Alternatively, it is also possible that the bulk outer core liquid is not in direct contact with the mantle. Seismological observations of a small P-wave velocity reduction in the topmost core suggest the presence of chemically-distinct buoyant liquid layer. Such layer physically separates the mantle from the bulk outer core liquid, hindering the chemical reaction between them.     

Superplume, supercontinent, and post-perovskite: Mantle dynamics and anti-plate tectonics on the Core–Mantle Boundary

The Western Pacific Triangular Zone (WPTZ) is the frontier of a future supercontinent to be formed at 250 Ma after present. The WPTZ is characterized by double-sided subduction zones to the east and south, and is a region dominated by extensive refrigeration and water supply into the mantle wedge since at least 200 Ma. Long stagnant slabs extending over 1200 km are present in the mid-Mantle Boundary Layer (MBL, 410–660 km) under the WPTZ, whereas on the Core–Mantle Boundary (CMB, 2700–2900 km depth), there is a thick high-V anomaly, presumably representing a slab graveyard. To explain the D″ layer cold anomaly, catastrophic collapse of once stagnant slabs in MBL is necessary, which could have occurred at 30–20 Ma, acting as a trigger to open a series of back-arc basins, hot regions, small ocean basins, and presumably formation of a series of microplates in both ocean and continent. These events were the result of replacement of upper mantle by hotter and more fertile materials from the lower mantle.The thermal structure of the solid Earth was estimated by the phase diagrams of Mid Oceanic Ridge Basalt (MORB) and pyrolite combined with seismic discontinuity planes at 410–660 km, thickness of the D″ layers, and distribution of the ultra-low velocity zone (ULVZ). The result clearly shows the presence of two major superplumes and one downwelling. Thermal structure of the Earth seems to be controlled by the subduction history back to 180 Ma, except in the D″ layer. The thermal structure of the D″ layer seems to be controlled by older slab-graveyards, as expected by paleogeographic reconstructions for Laurasia, Gondwana and Rodinia back to 700 Ma.Comparison of mantle tomography between the Pacific superplume and underneath the WPTZ suggests the transformation of a cold slab graveyard to a large-scale mantle upwelling with time. The Pacific superplume was born from the coldest CMB underneath the 1.0–0.75 Ga supercontinent Rodinia where huge amounts of cold slabs had accumulated through collision-amalgamation of more than 12 continents. A high velocity P-wave anomaly on a whole-mantle scale shows stagnant slabs restricted to the MBL of circum-Pacific and Tethyan regions. The high velocity zones can be clearly identified within the Pacific domain, suggesting the presence of slab graveyards formed at geological periods much older than the breakup of Rodinia. We speculate that the predominant subduction occurred through the formation period of Gondwana, presumably very active during 600 to 540 Ma period, and again from 400 to 300 Ma during the formation of the northern half of Pangea (Laurasia). We correlate the three dominant slab graveyards with three major orogenies in earth history, with the emerging picture suggesting that the present-day Pacific superplume is located at the center of the Rodinian slab graveyard.We speculate the mechanism of superplume formation through a comparison of the thermal structure of the mantle combined with seismic tomography under the Western Pacific Triangular Zone (WPTZ), Laurasia (Asia), Gondwana (Africa), and Rodinia (Pacific). The coldest mantle formed by extensive subduction to generate a supercontinent, changes with time of the order of several hundreds of million years to the hottest mantle underneath the supercontinent. The Pacific superplume is tightly defined by a steep velocity gradient on the margin, particularly well documented by S-wave velocity. The outermost region of the superplume is characterized by the Rodinia slab graveyard forming a donut-shape. We develop a petrologic model for the Pacific superplume and show how larger plumes are generated at shallower depths in the mantle. We link the mechanism of formation of the superplume to the presence of the mineral post-perovskite, the phase transformation of which to perovskite is exothermic, and thus aids in transporting core heat to mantle, and finally to planetary space by plumes.We summarize the characteristics of tectonic processes operating at the CMB to propose the existence of an “anti-crust” generated through “anti-plate tectonics” at the bottom of the mantle. The chemistry of the anti-crust markedly contrasts with that of the continental crust overlying the mantle. Both the crust and the anti-crust must have increased in volume through geologic time, in close relation with the geochemical reservoirs of the Earth. The process of formation of a new superplume closely accompanies the process of development of anti-crust at the bottom of mantle, through the production of dense melt from the partial melting of recycled MORB, observed now as the ULVZ. When CMB temperature is recovered to near 4000 K through phase transformation, the recycled MORB is partially melted imparting chemical buoyancy of the andesitic residual solid which rises up from CMB, leaving behind the dense melt to sink to CMB and thus increase the mass of anti-crust. These small-scale plumes develop to a large-scale superplume through collision and amalgamation with time. When all recycled MORBs are consumed, it is the time of demise of superplume. Immediately above the CMB, anti-plate tectonics operates to develop anti-crust through the horizontal movement of accumulated slab and their partial melting. Thus, we speculate that another continent, or even a supercontinent, has developed through geologic time at the bottom of the mantle.We also evaluate the heating vs. cooling models in relation to mantle dynamics. Rising plumes control not only the rifting of supercontinents and continents, but also the Atlantic stage as seen by anchored ridge by hotspots in the last 200 Ma in the Atlantic. Therefore, we propose that the major driving force for the mantle dynamics is the heat supplied from the high-T core, and not the slab pull force by cooling. The best analogy for this is the atmospheric circulation driven by the energy from Sun.     

核幔边界动力学——地球自转十年尺度波动

总结了地球核幔边界动力学有关研究的新进展,如核幔边界的特性、核幔边界的地形起伏、核幔边界附近地幔对流格局和地球外核顶部的流场等。从地核—地幔之间的耦合出发,讨论了地球自转十年尺度的波动问题,简述了核幔之间电磁耦合、粘滞耦合和地形耦合对十年尺度波动影响的基本理论。提出在该项研究中应以综合分析为基础,开展多学科的联合、交叉研究的途径以深化对地球自转十年尺度波动机理的认识。