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1.
Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P–T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t1 between plume formation and eruption. These relationships are used to determine thermal power and time t1 for the Tunguska syneclise and the Siberian traps as a whole.

The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts.  相似文献   


2.
We present a thermophysical model for interaction between the conduit of a thermochemical plume and horizontal free convection flows in the mantle: The mantle flow incident on the plume conduit melts at the conduit boundary (front part) and crystallizes at its back. Geological data on the intensity of plume magmatism over the last 150 Myr are used to estimate the total thermal power of mantle plumes. A possible scenario for plume-related mantle recrystallization is proposed. Over the lifespan of a thermochemical plume, mantle melts and recrystallizes owing to the motion of the plume source and interaction between the plume conduit and horizontal free convection flows. The plume conduits can melt and recrystallize the entire mantle over a certain period of time. The model for the interaction of drifting plume conduits with mantle flows and the estimated total thermal power of mantle plumes are used to estimate the duration of plume-related melting and recrystallization of the entire mantle. The influence of mantle plumes on the convective structure of the mantle through melting is judged from the model for plume interaction with horizontal mantle flows.  相似文献   

3.
A fluid model for the formation of mantle plumes is proposed. During the emission of gas from the Earth’s core, it accumulates as lenses at the core-mantle boundary. Reaching a critical size, the lenses burst out into the mantle and migrate to the surface. A relatively stationary transmantle fluid flow from the core-mantle boundary arises, which heats the mantle and the layer interacting with it. The flow stops in the base of the hard lithosphere and spreads laterally, causing its melting accompanied by the formation of magma chambers, which, reaching critical sizes, massively intrude and flow out.  相似文献   

4.
Based on laboratory and theoretical modeling results, we present the thermal and hydrodynamical structure of the plume conduit during plume ascent and eruption on the Earth’s surface. The modeling results show that a mushroom-shaped plume head forms after melt eruption on the surface for 1.9 < Ka < 10. Such plumes can be responsible for the formation of large intrusive bodies, including batholiths. The results of laboratory modeling of plumes with mushroom-shaped heads are presented for Ka = 8.7 for a constant viscosity and uniform melt composition. Images of flow patterns are obtained, as well as flow velocity profiles in the melt of the conduit and the head of the model plume. Based on the laboratory modeling data, we present a scheme of a thermochemical plume with a mushroom-shaped head responsible for the formation of a large intrusive body (batholith). After plume eruption to the surface, melting occurs along the base of the massif above the plume head, resulting in a mushroom-shaped plume head. A possible mechanism for the formation of localized surface manifestations of batholiths is presented. The parameters of some plumes with mushroom-shaped heads (plumes of the Altay-Sayan and Barguzin-Vitim large-igneous provinces, and Khangai and Khentei plumes) are estimated using geological data, including age intervals and volumes of magma melts.  相似文献   

5.
地幔柱构造理论研究若干问题及研究进展   总被引：3，自引：0，他引：3  
介绍了目前地幔柱构造理论研究中若干重要问题和最新进展,许多证据显示,地幔柱是严自于核幔边界附近的Ｄ″层发生热扰动并产生地幔柱的热动力源于外地核的不均匀加热作用;一个新启动的地幔柱在穿过整个地幔的缓慢上升过程会形成巨大球状顶冠和狭窄尾柱;地幔柱巨大球状顶冠会导致地壳发生上隆、区域变质作用、地壳深熔作用、构造变形作用和大规模火山作用,形成大陆或大洋溢流玄武岩;地幔柱狭窄尾柱的长期活动会在上覆运动板块上  相似文献   

6.
地幔柱构造、大火成岩省及其地质效应   总被引：61，自引：6，他引：61  
徐义刚 《地学前缘》2002,9(4):341-353
地幔柱是源于核幔边界或上下地幔边界的热异常物质 ,其隐含的巨大能量导致地幔的大规模熔融和大火成岩省的形成。不同时代的科马提岩和苦橄岩的地球化学性质表明地幔柱源区经历了由太古宙时的亏损源区向现代OIB型源区演化的历程 ,可能与壳幔再循环强度的不断增加有关。地幔柱活动和大火成岩事件与大陆裂解 ,全球气候变迁 ,生物灭绝事件 ,磁极倒转和一些大型矿产资源的形成均有密切的联系。文中还介绍了中国开展地幔柱和大火成岩省研究的概况。  相似文献   

7.
Magmatic systems of large continental igneous provinces   总被引：1，自引：1，他引：0  
Large igneous provinces (LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts (during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga. We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations. The first generation plumes were derived from the depleted mantle, whereas the second generation (thermochemical) originated from the core-mantle boundary (CMB). This study mainly focuses on the second (Phanerozoic) type of LIPs, as exemplified by the mid-Paleoproterozoic Jatulian–Ludicovian LIP in the Fennoscandian Shield, the Permian–Triassic Siberian LIP, and the late Cenozoic flood basalts of Syria. The latter LIP contains mantle xenoliths represented by green and black series. These xenoliths are fragments of cooled upper margins of the mantle plume heads, above zones of adiabatic melting, and provide information about composition of the plume material and processes in the plume head. Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world, it is inferred that the heads of the mantle (thermochemical) plumes are made up of moderately depleted spinel peridotites (mainly lherzolites) and geochemically-enriched intergranular fluid/melt. Further, it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths ～20 km. The generation of two major types of mantle-derived magmas (alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads. It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids. The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion. This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.  相似文献   

8.
地幔柱大辩论及如何验证地幔柱假说   总被引：20，自引：1，他引：20  
目前关于地幔柱存在与否的争论主要集中在地幔柱学说的三个假设上:(1)起源于地球核幔边界缓慢上升的细长柱状热物质流;(2)热点下具有异常高温地幔;(3)地幔柱是相对静止的。这三个方面的验证需要今后深部地球物理探测、岩石学和古地磁等学科的综合运用和进一步的工作。文中认为,地幔柱学说依然能合理地解释地球上一级地质现象,反对地幔柱的学者过分强调了一些小尺度的与地幔柱理论不符的细节,而小尺度地壳特征显然还受到其他许多因素的影响。可以从以下5个方面来鉴别老地幔柱:(1)大规模火山作用前的地壳抬升;(2)放射状岩墙群;(3)火山作用的物理特征;(4)火山链的年代学变化;(5)地幔柱产出岩浆的化学组成。研究表明,峨眉山大火成岩省满足其中的3到4个指标,因此地幔柱是形成峨眉山玄武岩的主要动力学机制。  相似文献   

9.
Speculations on the nature and cause of mantle heterogeneity   总被引：8，自引：0，他引：8  
Hotspots and hotspot tracks are on, or start on, preexisting lithospheric features such as fracture zones, transform faults, continental sutures, ridges and former plate boundaries. Volcanism is often associated with these features and with regions of lithospheric extension, thinning, and preexisting thin spots. The lithosphere clearly controls the location of volcanism. The nature of the volcanism and the presence of ‘melting anomalies’ or ‘hotspots’, however, reflect the intrinsic chemical and lithologic heterogeneity of the upper mantle. Melting anomalies—shallow regions of ridges, volcanic chains, flood basalts, radial dike swarms—and continental breakup are frequently attributed to the impingement of deep mantle thermal plumes on the base of the lithosphere. The heat required for volcanism in the plume hypothesis is from the core. Alternatively, mantle fertility and melting point, ponding and focusing, and edge effects, i.e., plate tectonic and near-surface phenomena, may control the volumes and rates of magmatism. The heat required is from the mantle, mainly from internal heating and conduction into recycled fragments. The magnitude of magmatism appears to reflect the fertility, not the absolute temperature, of the asthenosphere. I attribute the chemical heterogeneity of the upper mantle to subduction of young plates, aseismic ridges and seamount chains, and to delamination of the lower continental crust. These heterogeneities eventually warm up past the melting point of eclogite and become buoyant low-velocity diapirs that undergo further adiabatic decompression melting as they encounter thin or spreading regions of the lithosphere. The heat required for the melting of cold subducted and delaminated material is extracted from the essentially infinite heat reservoir of the mantle, not the core. Melting in the upper mantle does not requires the instability of a deep thermal boundary layer or high absolute temperatures. Melts from recycled oceanic crust, and seamounts—and possibly even plateaus—pond beneath the lithosphere, particularly beneath basins and suture zones, with locally thin, weak or young lithosphere. The characteristic scale lengths—150 to 600 km—of variations in bathymetry and magma chemistry, and the variable productivity of volcanic chains, may reflect compositional heterogeneity of the asthenosphere, not the scales of mantle convection or the spacing of hot plumes. High-frequency seismic waves, scattering, coda studies and deep reflection profiles are needed to detect the kind of chemical heterogeneity and small-scale layering predicted from the recycling hypothesis.  相似文献   

10.
Plate subduction and mantle plumes are two of the most important material transport processes of the silicate Earth. Currently, a debate exists over whether the subducted oceanic crust is recycled back to the Earth's surface through mantle plumes, and can explain their derivation and major characteristics. It is also puzzling as to why plume heads have huge melting capacities and differ dramatically from plume tails both in size and chemical composition. We present data showing that both ocean island basalt and mid-ocean ridge basalt have identical supra-primitive mantle mean Nb/U values of ～46.7, significantly larger than that of the primitive mantle value. From a mass balance calculation based on Nb/U?we have determined that nearly the whole mantle has evolved by plate subduction-induced crustal recycling during formation of the continental crust. This mixing back of subducted oceanic crust, however, is not straightforward, because it generally would be denser than the surrounding mantle, both in solid and liquid states. A mineral segregation model is proposed here to reconcile different lines of observation. First of all, subducted oceanic crustal sections are denser than the surrounding mantle, such that they can stay in the lower mantle, for billions of years as implied by isotopic data. Parts of subducted oceanic crust may eventually lose a large proportion of their heavy minerals, magnesian-silicate-perovskite and calcium-silicate-perovskite, through density segregation in ultra-low-velocity zones as well as in very-low-velocity provinces at the core-mantle boundary due to low viscosity. The remaining minerals would thus become lighter than the surrounding mantle, and could rise, trapping mantle materials, and forming mantle plumes. Mineral segregation progressively increases the SiO2 content of the ascending oceanic crust, which enhances flux melting, and results in giant Si-enriched plume heads followed by dramatically abridged plume tails. Therefore, ancient mineral-segregated subducted oceanic crust is likely to be a major trigger and driving force for the formation of mantle plumes.  相似文献   

11.
核—幔相互作用及其地球动力学意义   总被引：1，自引：0，他引：1  
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。  相似文献   

12.
Laboratory and numerical experiments simulating the heat transfer and flow structure of thermochemical mantle plumes provide insights into the mechanisms of plume eruption onto the surface depending on the relative thermal power of plumes Ka = N/N1, where N and N1 are the heat transferred from the plume base to the plume conduit and the heat transferred from the plume conduit to the surrounding mantle, respectively, under steady thermal conduction. There are three main types of plumes according to the Ka criterion: (i) plumes with low thermal power (Ka < 1.15), which fail to reach the surface, (ii) plumes with intermediate thermal power (1.15 < Ka < 1.9), which occur beneath cratons and transport melts from depths below 150 km, where diamond is stable (diamondiferous plumes), and (iii) plumes with a mushroom-shaped head (1.9 < Ka < 10), which are responsible for large intrusive bodies, including batholiths. The volume of erupted melt and the depth from which the melt is transported to the surface are estimated for plumes of types (ii) and (iii). The relationship between the plume head area (along with the plume head diameter) and the relative thermal power is obtained. The relationship between the thickness of the block above the plume head and the relative thermal power is derived. On the basis of the results obtained, the geodynamic-regime diagram of thermochemical mantle plumes, including the plumes with Ka > 10, has been constructed.  相似文献   

13.
This paper presents the numerical models built for convection in a three-component mantle with heavy matter in the form of the D“ layer and a light highly viscous supercontinent. The models explain deformation of the heavy layer by mantle flows with hot provinces concentrating on the mantle bottom. The role played by supercontinents in plume generation is also explained, as well as the regularities of how plumes, which produce hot spots, traps, and basaltic plateaus on the Earth’s surface and ore diamond deposits in the lithosphere, are generated on the mantle bottom.  相似文献   

14.
Zvi Garfunkel   《Lithos》2008,100(1-4):49-65
Models of continental flood basalt (CFB) formation are evaluated by examining their implications for the setting, mainly temperature and depth, of melting which is assumed to result from adiabatic decompression. Most attractive is the model of melting in upwelling bodies (probably plume heads) rising to the base of the continental lithosphere. This constrains the melting to 120–150 km or deeper (continental lithospheric thickness) and thus the plume potential temperatures to ≥ 300 °C higher than ambient mantle. The primary melts should be hot, MgO-rich, modified during ascent, and assimilate components of the lithosphere, which can provide the continental-like geochemical signature of many CFB. Circulation within the upwellings and presence of eclogite patches also influence magma generation and composition. Dehydration melting when plumes heat the lowermost lithosphere can generate CFB only if the source region contains ca. 15% hydrous minerals beneath the entire area covered by flood volcanics, which is difficult to justify. On the other hand, assimilation of “continental” chemical components from large parts of the lithosphere does not require such extreme metasomatism. Decompression melting under strongly thinned rifted lithosphere requires lower potential temperatures of the rising material and lesser modification of the primary magmas than the plume head model of CFB formation. Available observations do not support the contemporaneity of flood volcanism with rifts having the required sizes and histories, but more information is needed to further test this model. On the other hand, magma production can assist rift initiation and lithospheric rupture, so subsequent thinning can explain the common formation of volcanic rifted margins immediately following CFB emplacement. Ancient LIP should record the same processes as seen in young CFB.  相似文献   

15.
Based on the compositions of melt inclusions and coexisting minerals from meymechites and alkali picrites, the temperatures and pressures of the ascending material of the Siberian plume were estimated at the level of the lithosphere-asthenosphere boundary. The melts trapped in olivine show high contents of titanium and other incompatible elements. The rocks crystallized under high oxygen fugacity conditions. The calculated compositions of primary magmas are similar to the compositions of near-solidus melts derived from a dry fertile lherzolite at 7 GPa. The estimated potential temperature is close to 1650°C, which is much higher than the potential temperature of plumes that generate the primary basaltic magmas of mid-ocean ridges. The obtained data show that, during the activity of the giant magma-generating system of the Siberian trap province, hot peridotite masses ascended probably from the core-mantle boundary up to the base of the continental lithosphere. Our results are at odds with the suggestion that the basalt flows of the Siberian and other large igneous provinces are not related to mantle plumes.  相似文献   

16.
The paper considers the conditions and mechanisms of the formation of linear volcanic structures in the Brazil Basin, South Atlantic. Among these objects, those related to the ascent of deep mantle plumes predominate. It is shown that the ascent of melts from plume sources leads to the formation of (a) hot spot tracks in the form of linear volcanic ridges and (b) active hot lines in the form of submarine mountain chains with trends differing from those of hot spot tracks and with a more variable character of the age distribution of volcanic rocks. Fault tectonics affects the character of plume activity. In addition, plume material from a hot spot area is dragged by a moving plate as a flow or a sublithospheric lens, which leads to the long-term existence of particular independent segments of linear structures and sometimes to late volcanism reactivation within their limits. Decompression melting of the asthenospheric mantle in zones where thin lithosphere undergoes tension causes the formation of passive hot lines. The main mantle source for the considered volcanic rocks was a mixture of DMM and HIMU mantle components, with the latter abruptly dominating. In marginal oceanic regions, the EM1 component is also present (the EM2 component is found more rarely) within fragments of tectonically delaminated continental mantle that was trapped by the oceanic mantle during the breakup of Gondwana.  相似文献   

17.
板块俯冲起始与大陆地壳演化   总被引：1，自引：0，他引：1  
组成大陆地壳的物质主要来自两个地质过程:地幔柱活动和板块俯冲。目前大多数研究认为板块俯冲起始于30多亿年前。在板块俯冲起始之前,基性的初始地壳物质受热重熔是大陆地壳生长的主要方式,其中,地幔柱活动是关键。地幔柱不仅向地壳输送玄武质岩浆,同时导致已有玄武质岩石和沉积岩通过部分熔融向中酸性岩石转化。当原始岩石圈强度足够大时,地幔柱会导致岩石圈倾斜、破裂,产生下滑力,诱发板块俯冲。板块俯冲引发岩浆活动,产生大量的岩浆岩,如岛弧安山岩、弧后盆玄武岩等。这些岩浆岩通过喷发、侵位,再经由块体拼贴、增生等过程加入到大陆地壳,是大陆地壳生长的主要途径。同时,板内岩浆活动乃至地幔柱活动等也与板块俯冲有直接或者间接的联系。俯冲再循环物质促进地幔柱发育,也为大陆地壳的生长提供物源和热能。与此同时,大陆地壳不断风化剥蚀,其中一部分沉积物随俯冲板块再循环到地幔,而板块俯冲过程也通过俯冲剥蚀等过程,将仰冲盘岩石圈物质刮削带入地幔。这些是大陆地壳消减的主要途径。目前大陆地壳增生和消减基本处于动态平衡。  相似文献   

18.
This paper reviews the Mesozoic continental flood basalts (CFBs) associated with the break-up and dispersal of Gondwana from 185-60 Ma, the conditions for melt generation in mantle plumes and within the continental mantle lithosphere, and possible causes for lithospheric extension. The number of CFB provinces within Gondwana is much less than the number of mantle plumes that are likely to have been emplaced beneath it in the 300 Ma prior to its initial break-up. Also, the difference between the age of the peak of CFB volcanism and that of the oldest adjacent ocean crust decreases with the age of volcanism during the break-up and dispersal of Gondwana. The older CFBs of Karoo and Ferrar appear to have been derived largely from source regions within the mantle lithosphere. It is only in the younger Paranâ-Etendeka and Deccan CFBs that there are igneous rocks with major, trace element and radiogenic isotope ratios indicative of melting within a mantle plume. These younger CFBs are also clearly associated with hot spot traces on the adjacent ocean floor. The widespread 180 Ma magmatic event is attributed to partial melting within the lithosphere in response to thermal incubation over 300 Ma. In the case of the Ferrar (Antarctica) this was focussed by regional plate margin forces. The implication is that supercontinents effectively self-destruct in response to the build up of heat and resultant magmatism, since these effects significantly weaken the lithosphere and make it more susceptible to break-up in response to regional tectonics. The younger CFB of Paranâ-Etendeka was generated, at least in part, because the continental lithosphere had been thinned in response to regional tectonics. While magmatism in the Deccan was triggered by the emplacement of the plume, that too may have been beneath slightly thinned lithosphere.  相似文献   

19.
The shape of a plume conduit produced by melting solid paraffin block above a local heat source was studied experimentally as a function of the relative thermal power of the plume Ka= N/N1, where N1 is the power of the plume source and N1 is the power corresponding to the amount of heat transferred by conduction through the plume conduit to the surrounding solid paraffin block. The limiting power of the plume source at which the plume erupts at the Earth’s surface (Nlim1= (1.35–1.60) × 1010 W) and the power at which the mushroom-shaped plume head formed at the base of the refractory layer (Nlim2= (1.78–1.90) × 1010 W) with no horizontal mantle flow were determined. The dependence of the diameter of the base of the plume on the Ka number was established. The Ka value and the diameter of the plume base were determined for the Hawaiian and Iceland plumes, for the plume responsible for the formation of the Tunguska syneclise and for the McKenzie and Central Atlantic continental plateau-basalt provinces and for the Ontong Java and Manihiki oceanic lava plateaus.  相似文献   

20.
地球内部物理和演化的几个核心论题：Ⅱ地球动力体系   总被引：1，自引：0，他引：1  
生成于岩石圈底部的“大陆根”与地幔羽的形成过程有关,其主要证据来自3－D地震成象和实验、数值模拟结果。地幔上涌和地幔下涌分别代表高温、低速带和低温、高速带。长波长的地幔构造与表层构造特征相关,地球内部边界层－热边界层或化学边界层将对全球动力体系产生直接或间接的效应。因此,深入研究这些边界层的结构、形态、热力学和物理化学特性,对解决地幔整体对流与成层对流体系中某些相冲突的问题具有关键意义。全球地震成象和深源地震资料表明,某些破碎的早期俯冲板片可能连续或间断性地下沉到核－幔边界处,并返回到起源于该边界层的地幔羽中。今后的任务不是重提地幔整体对流或是成层对流的问题．而是如何建立两者的统一模式。整体地幔对流体系在时间和空间演化过程中与成层对流、局部小规模对流或次生对流相伴生的理论、实验和数值模拟将是地球动力学研究的主要趋势。  相似文献   

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