地幔岩流体包裹体的稀土元素初步研究

地幔流体REE的研究有助于了解地幔区域化学不均一性、地幔的富集与亏损等地幔地球化学特征。当前对地幔流体的REE研究，主要是通过对比富CO2包裹体与贫CO2包裹体的地幔岩或地幔矿物的测试分析来间接获得信息。本尝试运用电感耦合等离子体质谱（ICP-MS）和热爆方法，直接测定了长白山地区的地幔捕虏体中流体-熔体包裹体REE含量。初步研究显示流体-熔体包裹体中富含REE，尤其LREE相对富集；REE组成曲线右倾，Eu弱正异常，与地幔岩的寄主玄武岩REE组成特征相似，反映源区地幔岩的交代特征。     

Do We Really Need Mantle Components to Define Mantle Composition?

We discuss the concept of components in the Earth's mantle startingfrom a petrological and geochemical approach, but adopting anew method of projection of geochemical and isotopic data. Thisallows the compositional variability of magmatic associationsto be evaluated in multi-dimensional space, thus simultaneouslyaccounting for a large number of compositional variables. Wedemonstrate that ocean island basalts (OIB) and mid-ocean ridgebasalts (MORB) are derived from a marble-cake mantle, in whichdifferent degrees of partial melting of recycled lithosphere,which are heterogeneous in age and composition, contribute tothe magma genesis. This view is supported by the variabilityin the geochemical and isotopic signatures of OIB that are observedon the scale of a single ocean island as well as on that ofan ocean, mostly varying between two extreme compositions, thatare not strictly related to the commonly accepted mantle components(DMM, EMI, EMII, HIMU). Rather they are a distinctive featureof the mantle source sampled at each ocean island and are stronglydependent on the Pb isotope system. We recommend a change inperspective in studies of MORB–OIB geochemistry from onebased on physically distinct mantle components to a model basedon the existence of a marble-cake-like upper mantle. Althoughresembling the statistical upper mantle, this model impliesthat geochemical homogenization can be attained only withinthe limits of local mantle composition, so that a world-wideuniform depleted reservoir cannot be sampled by simply extendingthe volume of the region undergoing partial melting. KEY WORDS: geochemistry; isotope; mantle; OIB     

Compositions of Upper Mantle Fluids Beneath Eastern China: Implications for Mantle Evolution

The composition of gases trapped in olivine, orthopyroxene and clinopyroxene in Iherzolite xenoliths collected from different locations in eastern China has been measured by the vacuum stepped-heating mass spectrometry. These xenoliths are hosted in alkali basalts and considered as residues of partial melting of the upper mantle, and may contain evidence of mantle evolution. The results show that various kinds of fluid inclusions in Iherzolite xenoliths have been released at distinct times, which could be related to different stages of mantle evolution. In general, primitive fluids of the upper mantle (PFUM) beneath eastern China are dominated by H2, CO2 and CO, and are characterized by high contents of H2 and reduced gases. The compositions of PFUM are highly variable and related to tectonic settings. CO, CO2 and H2 are the main components of the PFUM beneath cratons; the PFUM in the mantle enriched in potassic metasomatism in the northern part of northeastern China has a high content of H2, while CO2 a     

地幔捕虏体中的流体-熔体包裹体

地幔流体的研究现已成为国内外前沿课题。地幔岩捕虏体中的流体-熔体包裹体是地幔流体的直接证据,通过对它们的研究可以直接获取地授流体的信息。包裹体按相态特征主要有三类：二氧化碳流体包裹体、二氧化碳-硅酸盐熔体包裹体、硫化物-熔体流体包裹体。本总结了地幔岩中流体-熔体包裹体的基本特征、微量元素地球化学、硫化物-熔体包裹体和二氧化碳流体包裹体稳定同位素特征的研究进展状况。讨论认为：地幔流体是由C、H、O、S等元素的挥发份和硅酸盐熔体组成;上地幔流体在化学成分上明显富含CO2、硫化物、LILE和BEE,它引起地幔交代作用和地授部分熔融;上地授流体的分布存在不均匀性,其组成也存在地区性差异。     

Mantle plumes and plate motions

This paper elaborates the hypothesis that convection plumes may be rising from the lower mantle to spread out in the asthenosphere and drive lithospheric plates about and thus possibly provide the primary mechanism which governs the behaviour of the earth's surface. The paper notes some characteristics of plumes and identifies more than thirty by the hot spots which overlie them. Most lie close to mid-ocean ridges and have produced aseismic ridges trending away from them on either plate. A few have been overridden by plates to produce single, isolated chains of seamounts and islands. One plume may have uplifted the Colorado Plateau. Such distinctions serve to identify five types of hot spots.Most plates are in motion over the lower mantle. They are considered to be driven by the plumes, but their paths are influenced by interactions with other plates. Some temporarily become more or less stationary relative to the lower mantle. It is held that stationary plates, of which Africa and Southeast Asia may be present examples, develop special characteristics among which much volcanism, epeirogenic uplift, rifting and the development of basins and swells are diagnostic.It is well-known that if two plates approach one another at a subduction zone that a continental plate generally overrides an oceanic one. It is here suggested that the question of which plate is more nearly stationary over the mantle is important and determines the character of the continental margin. It is held that, if a continental plate advances over an oceanic one which is fixed over the mantle, a migrating marginal trench and mountains of Andean type with huge batholiths will form on the leading edge of the continent. On the other hand, if a continental plate is fixed and one or more oceanic plates are advancing and sliding under it, island arcs (and, when a collision with another continent occurs, mountains of Appalachian type) will form along each coast towards which a plate is advancing.     

地幔热柱及其成矿作用研究

成矿作用研究表明 ,在地球的演化历史中 ,成矿数量有从少到多 ,聚矿能力有由弱到强的演化趋势。中生代为全球规模的成矿大爆发阶段 ,究其原因可能与地球的发展演化密切相关。地球形成的早期 ,由于地球物质尚未充分分异 ,成矿强度不大。当然 ,有部分在地球演化早期形成的矿床 ,在后来的多次构造改造过程中被改造迁移 ,甚至消失。中生代地球进入了一个强烈的地幔热柱活动时期 ,聚集于D”层及外核的成矿物质可通过地幔热柱多级演化向上迁移 ,并在幔枝构造的有利构造扩容带中成矿 ,幔枝构造则成为中生代主要的成矿控矿构造类型     

Petrology of Mantle Xenoliths from Harrat al Kishb: The Mantle beneath Western Saudi Arabia

Tertiary to Recent continental rifting and sea floor spreadingformed the Red Sea. Mantle xenoliths from the Saudi ArabianRed Sea margin provide an opportunity to study the mantle beneaththe flanks of this young ocean basin. The Harrat al Kishb mantlexenolith suite consists of Cr-diopside group spinel harzburgiteand lherzolite mantle wall rock, and a variety of pyroxenitesproduced by crystallization from mafic magmas within the mantle.The pyroxenites include two texturally distinct varieties ofCr-diopside group spinel websterites, and Al-augite group spinelpyroxenite, garnet-spinel websterite, and garnet-bearing spinelclinopyroxenite. All Harrat al Kishb xenoliths are deformedto some degree and many are recrystallized. Mineral exsolutionand zoning textures indicate reequilibration to decreasing temperatureconditions. Several xenoliths provide evidence for metasomaticprocesses in the mantle beneath western Saudi Arabia. Estimates of peridotite temperatures are 900–980?C withpressure bracketed between 13 and 19 kb. Al-augite spinel pyroxenitesyield temperatures of 1050–1070?C. Garnet-spinel websteritesyield temperatures and pressures in the range 1000–1030?C,13.8–16.5 kb. These P-T estimates show that mantle temperatures are elevatedwell above those predicted by low surface heat flow measurements.Mantle heating associated with rifting is young enough thatsurface heat flow has not yet equilibrated. The xenolith dataare consistent with a model of asthenosphere upwelling beneaththe Red Sea rift. Comparison of xenolith data with existingseismic refraction data reveals a coherent picture of the compositionof the western Saudi Arabian lithosphere.     

In-situ Moissanite in Dunite: Deep Mantle Origin of Mantle Peridotite in Luobusa Ophiolite, Tibet

We report the discovery of an in-situ natural moissanite as an inclusion in the Cr-spinel from the dunite envelope of a chromitite deposit in Luobusa ophiolite, Tibet. The moissanite occurs as a twin crystal interpenetrated by two quadrilateral signal crystals with sizes of 17 μm× 10 μm and 20 μm× 7 μm, respectively. The moissanite is green with parallel extinction. The absorption peaks in its Raman spectra are at 967-971 cm-1, 787-788 cm-1, and 766 cm-1. The absorption peaks in the infrared spectra are at 696 cm-1, 767 cm-1, 1450 cm-1, and 1551 cm-1, which are distinctly different from the peaks for synthetic silicon carbide. Moissanites have been documented to form in ultra-high pressure, high temperature, and extremely low fO2 environments and their 13C-depleted compositions indicate a lower mantle origin. Combined with previous studies about other ultra-high pressure and highly reduced minerals in Luobusa ophiolite, the in-situ natural moissanite we found indicates a deep mantle origin of some materials in the mantle sequence of Luobusa ophiolite. Further, we proposed a transformation model to explain the transfer process of UHP materials from the deep mantle to ophiolite sequence and then to the supra-subduction zone environment. Interactions between the crown of the mantle plume and mid-ocean ridge are suggested to be the dominant mechanism.     

东亚地幔流动速度研究

用高分辨率地震体波速度成像以及相关的地球物理资料,计算地幔垂直和切向流动形式及流动速度,得到东亚及西太平洋大体分为3个区域物质流动大致方式:东亚边缘裂谷系和西太平洋边缘海为下部会聚上部扩散的地幔上升流,尤其对于南海来说,大致勾勒出南海地区构造特征——从上到下的大体结构可能是上部呈“工”字型结构、中间为圆柱型、底部呈发散形的地幔上升流;西伯利亚地幔物质上部会聚下部扩散的“人”字型下降流;青藏高原—缅甸—印度尼西亚特提斯俯冲带上部会聚下部扩散的“人”字型地幔下降流。东亚西太平洋分为3个区域地幔对流与地表的西太平洋构造域、古亚洲构造域和特提斯构造域相吻合。东亚中部的物质在地幔深部无明显流动趋势。地幔上升流起源于核幔边界,主要表现在下地幔和上地幔下部,到上地幔顶部和地表与现代热点位置符合。地幔垂直流动速度每年1～4cm,切向速度为每年1～10cm。     

地幔岩包体中斜方辉石的GOD化现象

对我国东部四个地点碱性玄武岩中地幔岩样品的研究分析表明，地幔岩包体被带上来的过程中，斜方辉石比单斜辉石更易发生部分熔融，而形成熔体玻璃（Ｇｌａｓｓ）、橄榄石（Ｏｌｉｖｉｎｅ）和铬透辉石（ＣｈｒｏｍｅＤｉｐｓｉｄｅ）集合体，此作用可称为ＧＯＤ化。ＧＯＤ化沿包体边部最发育，向内逐渐减弱，以至消失。当玻璃极富Ｋ２Ｏ时，出现强烈ＧＯＤ化。在强烈ＧＯＤ化外面的寄主岩石中，还形成销沸石和／或方沸石聚集体。斜方辉石的ＧＯＤ化不仅是由于升温降压而导致的不一致熔融所致，而且还伴有物质的交换（带入ＣａＯ＋Ｎａ２Ｏ＋Ｋ２Ｏ，带出ＳｉＯ２＋ＭｇＯ）。     

Geochemical Records of Mantle Processes in Mantle Xenoliths from Three Cenozoic Basaltic Volcanoes in Eastern China

Rare earth element (REE) contents, and Sr and Nd isotopic compositions were measured for three suites of mantle xenoliths from the Kuandian, Hannuoba and Huinan volcanoes in the north of the Sino-Korean Platform. From the correlations of Yb contents with Al/Si and Ca/Si ratios, the peridotites are considered to be the residues of partial melting of the primitive mantle. The chondrite-normalized REE compositions are diverse, varying from strongly LREE-depleted to LREE-enriched, with various types of REE patterns. Metasomatic alteration by small-volume silicate melts, of mantle peridotites previously variably depleted due to fractional melting in the spinel peridotite field, can account for the diversity of REE patterns. The Sr/ Ba versus La/Ba correlation indicates that the metasomatic agent was enriched in Ba over Sr and La, suggestive of its volatile-rich signature and an origin by fluid-triggered melting in an ancient subduction zone. The Sr and Nd isotopic compositions of these xenoliths, even from     

Characteristics of Upper Mantle Activity in the South China Sea Region and the Indochina Mantle Plume

According to computed results of the mantle traction field beneath the lithosphere based on satellite-modelled gravity anomalies of different degrees, it has been revealed that the three types of mantle convection on different scales existing in the South China Sea region is the key factor controlling the geodynamics. The mantle convection models on large and middle scales have been proved by natural seismic S-wave tomographic data and interpreted by using the present mantle plume concept In consideration of other relevant geological and geophysical data, the authors put emphasis on discussing the expression form, origin and age of the Indochina mantle plume and its important effect on the conversion of organic matter and hydrocarbon accumulation in Cenozoic basins.     

Mantle convection and early crustal evolution

The existence of peridotitic komatiites in the Archaean suggests that the Archaean mantle was significantly hotter than the modern mantle. This evidence is contradicted by estimates of Archaean continental geothermal gradients, based on the pressure and temperature recorded in metamorphic rocks, which suggest that there is no marked difference between Archaean and modern continental geothermal gradients.Numerical modelling shows that small changes in the mantle temperature can have an important influence on convection. If the average temperature of the upper mantle is increased by 200°C, convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe. The crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction. Later, when the mantle temperature falls to 100°C above its present level, the upper mantle partial melt zone contracts away from subduction areas.It is suggested that the first primitive felsic magmas were generated at subduction zones. The appearance of these magmas at ～3.8 Ga permitted the formation of buoyant continents and eventually led to crustal thickening. As a consequence of this thickening the proto-continents, consisting of a bimodal suite of basalts and sodic granodiorites, contained two types of latent energy: (1) radioactive energy held in elements such as Th, K and U; and (2) potential energy resulting from the elevation of the continents above sea level. The potential energy of the continents led to sedimentation. The increase in the rate of sedimentation during the Archaean resulted from increased crustal buoyancy. At the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust. This caused a major metamorphic, metasomatic and crustal melting event which produced the potassic granites of the late Archaean. Once the radioactive elements had been removed from the lower crust, that region of the continent become tectonically stable. The Proterozoic shelf sediments were deposited at the margins of these stable cratons.Convection models of the Archaean mantle show hot diapirs rising from the boundary layer above the core—mantle interface. We suggest that these diapirs began to melt at a depth of ～ 450 km, giving rise to komatiitic magmas. This model requires the average temperature of the Archaean upper mantle to be ～ 100°C above that of the modern mantle. The similarity between Archaean and modern continental geothermal gradients can be explained if Archaean continents formed above subduction zones.Raising the temperature of the Archaean mantle by 100°C (1) halves the thickness of the oceanic lithosphere, (2) increases the oceanic geothermal gradient at the mid-point of a convection cell, (3) decreases the viscosity of the mantle by at least an order of magnitude. The combination of these effects produces a marked decrease in the strength of the Archaean lithosphere and mantle. Thus the form of Archaean tectonics can be expected to have been very different from modern tectonics.     

地幔流体与铀成矿模式

近年来的研究显示．火山岩型和花岗岩型铀矿具有早、晚两期铀矿化．其中早期铀矿化具有地幔流体成矿作用的特征。本讨论了地幔流体及其成矿作用以及早期铀矿化的特征，提出了地幔流体铀成矿模式．强调构成成矿热液主体的∑CO2、U和H2O分别来自不同的源区．∑CO2来自地幔流体，U主要来自地幔流体上升途径的围岩．H2O主要来自地幔流体及其上升途径的围岩。     

大火成岩省及地幔动力学

大火成岩省由一个体积巨大的、连续的、以富镁铁岩石占优势的喷出岩及其伴生的侵入岩组成，是一个全球现象。它包括大陆溢流玄武岩和伴生的侵入岩，火山被动边缘玄武岩，大洋高原、海岭、海山群和洋盆溢流玄武岩。Ontong Java和Kerguelen-Broken Ridge大洋高原、北大西洋火山被动边缘、德干和哥伦比亚河大陆溢流玄武岩是3个主要大火成岩省的典型代表。各种不同的大火成岩省在时空分布及组成上都具有相似性，它们具有非常大的体积、高的喷发速率，岩石类型以拉斑玄武岩为主。大火成岩省代表了地球上已知的最大的火山岩浆活动，记录了物质和能量从地球内部向外的大量转换。大火成岩省难以用板块构造来解释，可用热柱模式来解释，通常被认为是与来自下地幔的热柱“头”有关。大火成岩省是地球动力学过程在地壳的表现，因此大火成岩省参数可作为边界条件去反演地幔动力学过程。     

交代岩石圈地幔与金成矿作用

交代岩石圈地幔与大型金矿床之间的成因联系受到越来越多的关注.研究成矿金属在地幔源区的富集程度和幔源岩浆中的金含量,以及金从地幔源区释放、迁移并大规模富集成矿的机制和过程可以帮助我们更好地认识交代岩石圈地幔对金富集成矿的重要作用.金是高度亲铜元素,同时还具有流体活动性,在地幔岩浆作用、岩浆热液演化和富集成矿等诸多过程中的行为较为复杂.主要从金的地球化学行为出发,通过梳理金在各类地幔岩石和幔源岩浆中的分布,以及岩浆热液中金的主要行为及其受控因素,探讨交代岩石圈地幔对巨量金成矿的关键控制机制.主要认识包括:（1）交代岩石圈地幔可能是形成大规模热液型金矿床的重要源区,但金在源区的异常富集可能并不是成矿的必要条件;（2）地幔交代组分（特别是挥发分）有助于金从地幔源区中有效释放、并通过跨岩石圈尺度的深大断裂迁移富集;（3）富挥发分的岩浆热液中金的富集沉淀过程远比一个富金的地幔源区对大规模金成矿作用的贡献更大.因此,深刻理解岩石圈地幔长期演化过程中金在地幔交代和岩浆-热液演化过程中的行为与富集机制是解析大型金矿床成因的关键.      

Mantle peridotites from the Western Pacific

We review petrographical and petrological characteristics of mantle peridotite xenoliths from the Western Pacific to construct a petrologic model of the lithospheric mantle beneath the convergent plate boundary. The peridotite varies from highly depleted spinel harzburgite of low-pressure origin at the volcanic front of active arcs (Avacha of Kamchatka arc and Iraya of Luzon–Taiwan arc) to fertile spinel lherzolite of high-pressure origin at the Eurasian continental margin (from Sikhote-Alin through Korea to eastern China) through intermediate lherzolite–harzburgite at backarc side of Japan island arcs. Oxygen fugacity recorded by the peridotite xenoliths decreases from the frontal side of arc to the continental margin. The sub-arc type peridotite is expected to exist beneath the continental margin if accretion of island arc is one of the important processes for continental growth. Its absence suggests replacement by the continental lherzolite at the region of backarc to continental margin. Asthenospheric upwelling beneath the continental region, which has frequently occurred at the Western Pacific, has replaced depleted sub-cratonic peridotite with the fertile spinel lherzolite. Some of these mantle diapirs had opened backarc basins and strongly modified the lithospheric upper mantle by metasomatism and formation of Group II pyroxenites.     

Mantle Plumes and Their Geologic Manifestations

This paper interprets the conditions of plume formation with a focus on interpretations of origin at the core-mantle boundary. We also discuss important geological manifestations including formation of alkaline-basalt oceanic islands, kimberlite pipes, and plateau-basalt fields. The paper concentrates on the Siberian traps that formed between 249 and 245 Ma, one of the world's largest known plateau-basalt fields, and on the magmatic zonation and ore mineralization associated with the field and that are interpreted as forming from a Siberian mantle plume. One part of the field consists of undifferentiated low-K tholeiite basalts in the Tunguska syncline and contains only minor associated ore deposits. Major, unique Cu-Ni and PGE deposits are associated with differentiated alkaline basalt and with mafic-ultramafie intrusions in the northwestern Siberian platform. Angara-Ilim-type magnetite deposits occur in the southern part of the plateau-basalt province and are related to interaction of mafic magmas with overlying carbonate and evaporite sedimentary rocks. Permian and Triassic alkaline granitoid and bimodal magmatic rocks occur in fold-and-thrust belts that frame the southern, southwestern, and northwestern margins of the main trap area. The paper also presents new geochemical and geochronological data for A-granites for the Taymyr region that are synchronous with traps. For both the southern and northern framing areas, porphyry Cu deposits interpreted as derived from a mantle-crust system are associated with subalkaline and alkaline granitoids.     

Mantle plumes from top to bottom

Hotspots include midplate features like Hawaii and on-axis features like Iceland. Mantle plumes are a well-posed hypothesis for their formation. Starting plume heads provide an explanation of brief episodes of flood basalts, mafic intrusions, and radial dike swarms. Yet the essence of the hypothesis hides deep in the mantle. Tests independent of surface geology and geochemistry to date have been at best tantalizing. It is productive to bare the current ignorance, rather than to dump the plume hypothesis. One finds potentially fruitful lines of inquiry using simple dynamics and observations. Ancient lithospheric xenoliths may reveal heating by plumes and subsequent thermal equilibration in the past. The effect at the base of the chemical layer is modest 50-100 K for transient heating by plume heads. Thinning of nonbuoyant platform lithosphere is readily observed but not directly attributable to plumes. The plume history in Antarctica is ill constrained because of poor geological exposure. This locality provides a worst case on what is known about surface evidence of hotspots. Direct detection of plume tail conduits in the mid-mantle is now at the edge of seismic resolution. Seismology does not provide adequate resolution of the deep mantle. We do not know the extent of a chemically dense dregs layer or whether superplume regions are cooler or hotter than an adiabat in equilibrium with the asthenosphere. Overall, mid-mantle seismology is most likely to give definitive results as plume conduits are the guts of the dynamic hypothesis. Finding them would bring unresolved deep and shallow processes into place.