Geochemical characteristics of hydrothermal plumes above the TAG and Broken Spur fields of the Mid-Atlantic Ridge

The paper reports data on the distribution of solute (Mn, Zn, Cu, Pb, and Cd) and particulate (Fe, Mn, Zn, Cu, Pb, Ni, and Co) species of metals in hydrothermal plumes above the active TAG and Broken Spur hydrothermal fields (26° N and 29° N in the MAR rift valley, respectively). Sediment-trap data on fluxes of hydrothermal-sedimentary material in the areas indicate that (i) the predominant Zn source for the metalliferous sediments at the TAG field is material precipitating from a plume of neutral buoyancy, and (ii) the predominant source of Fe and Co is redeposited ore material coming from the area of extensive settling of sulfides.     

BrO formation in volcanic plumes

Volcanoes have only recently been recognized as a potentially major source of reactive bromine species to the atmosphere, following from the detection of bromine monoxide (BrO) in the plume emitted by Soufrière Hills Volcano, Montserrat. However, BrO is not expected to be emitted in significant quantity from magma, presenting a puzzle regarding its formation. We report here new field measurements of the tropospheric plume emitted by Mt. Etna, Italy, which provide the first direct evidence of fast oxidation of halogen species in a volcanic plume, and lead to an explanation of how BrO is generated from magmatic HBr emissions. We show that the timescale of BrO formation (a few minutes after emission into the atmosphere) is consistent with rapid heterogeneous halogen chemistry involving sulphate aerosol in the plume. The model highlights considerable complexity to the oxidative chemistry of volcanic plumes.     

Thermochemical mantle plumes

Doklady Earth Sciences -     

Microbial populations in contaminant plumes

Received, May 1999/Revised, October 1999/Accepted, October 1999     

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.     

Biogeochemistry of landfill leachate plumes

The literature has been critically reviewed in order to assess the attenuation processes governing contaminants in leachate affected aquifers. Attenuation here refers to dilution, sorption, ion exchange, precipitation, redox reactions and degradation processes. With respect to contaminants, focus is on dissolved organic matter, xenobiotic organic compounds, inorganic macrocomponents as anions and cations, and heavy metals. Laboratory as well as field investigations are included. This review is an up-date of an earlier comprehensive review. The review shows that most leachate contamination plumes are relatively narrow and do not in terms of width exceed the width of the landfill. The concept of redox zones being present in the plume has been confirmed by the reported composition of the leachate contaminated groundwater at several landfills and constitutes an important framework for understanding the behavior of the contaminants in the plume as the leachate migrates away from the landfill. Diverse microbial communities have been identified in leachate plumes and are believed to be responsible for the redox processes. Dissolved organic C in the leachate, although it appears to be only slowly degradable when the volatile organic acids are gone, apparently acts as substrate for the microbial redox processes. Several xenobiotic organic compounds have been found to be degradable in leachate contaminated groundwater, but degradation rates under anaerobic redox conditions have only been determined in a few cases. Apparently, observations in actual plumes indicate more extensive degradation than has been documented in the laboratory. The behavior of cations in leachate plumes is strongly influenced by exchange with the sediment, although the sediment often is very coarse and sandy. Ammonium seems to be subject to anaerobic oxidation, but the mechanisms are not yet understood. Heavy metals do not seem to constitute a significant pollution problem at landfills, partly because the heavy metal concentrations in the leachate often are low, and partly because of strong attenuation by sorption and precipitation. Although complexation of heavy metals with dissolved organic matter is significant, the heavy metals are in most cases still strongly attenuated in leachate-polluted aquifers. The information available on attenuation processes has increased dramatically during the last 15 a, but the number of well-documented full scale leachate plumes are still few and primarily from sandy aquifers. Thus, the diversity of attenuation processes in leachate plumes is probably not yet fully understood. Apparently, the attenuation processes in leachate plumes may for many contaminants provide significant natural remediation, limiting the effects of the leachate on the groundwater to an area usually not exceeding 1000 m from the landfill.     

线性层结盐水中的羽流运动特性

采用线性层结盐水发生装置以及喷嘴向下以重盐水注入盐水层结的方法,模拟线性层结盐水中羽流的发展过程;应用时间分辨粒子图像测速技术（PIV）,研究线性层结盐水中羽流内部流场结构。羽流的发展过程可以定性地归纳为3个阶段:入射阶段、中性浮力层出现阶段和中性浮力层稳定阶段。实验结果显示:羽流流速从中心区域向边缘逐步递减;羽流在下沉到最大深度后回升到中性浮力层,并向四周扩散;在羽流内部及边界存在大量复杂的涡旋结构,说明在羽流发展的过程中,始终伴随着羽流与周围环境水体之间的掺混作用。重新拟合确定了羽流最大穿透距离公式中的量纲一常数C,取值为4.03。     

Energy sources for mantle plumes

Mantle plumes are the consequence of a focussed transfer of mass from mantle to core and are agents of outgassing from the mantle. The energy of propagation is derived from released energy of their formation, decreases in density, and partial fluidization. This energy is dissipated during distension and rupture of the overlying crust.

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Zusammenfassung Mantelplumes haben ihren Ursprung in einer gezielten Massenverfrachtung vom Mantel zum Kern und führen zur Mantelentgasung. Die Fortpflanzungsenergie entsteht aus frei werdender Bildungsenergie, Dichteverminderung und teilweiser Verflüssigung. Die Energie verflüchtigt sich bei Ausdehnung und Rißbildung der überlagernden Kruste.

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Résumé Plumes en le manteau sont derivées d'un transfert de masse dès le manteau au noyau. Elles sont agents de la dégassification du manteau. L'énergie de la propagation est derivée de l'énergie de la formation liberée, des décroissements de la densité, et de la fluidification partielle. Cette énergie est dissipée pendant la dilatation et la rupture de la croûte s'étendante là-dessus.

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Plumes , . : , . .

     

Mantle plumes and their record in Earth history

In recent years, the study of mantle plumeshas become a “hot spot” in earth sciences;enormous papers and data have been published, and many mantle plume-related questions have arisen: Did mantle plumes in the Precambrian behave the same way as they do today? What are the roles of plumes in the evolution of the earth? What are the effects of mantle plumes on global changes and metallogenesis? These issues are frequently encountered and hotly debated in a wide range of earth science fields such as tectonics geochemistry, geophysics, metallogenesis,     

中条裂谷的幔柱构造与成矿

中条山地区的地球物理场具有“三低一高”(低波速、低密度、低黏度及高地热异常)特点,变质核杂岩中赋存有科马堤岩并具“双层”结构,火成岩呈“双峰”状态,变质期的古地温为522～608℃,压力0.3～0.73GPa,地热梯度23℃/km,表明中条山地区为新太古代-元古宙的大陆裂谷—地幔热柱构造.其成矿作用存在着“四代同堂”的成矿模式,成矿期与岩浆侵入期的时差甚小;成矿空间上,大多数矿床沿着裂谷内两组断裂呈X型“对称式”展布,有益元素矿物组合及成矿温度自中心向周围呈有规律降低趋势.由于在新太古代以前,富铜的扬子板块俯冲至华北板块之下,促使中条裂谷的地壳加厚,形成丰富的成矿物质场,因为地幔热柱脉动式隆升,热液在运移过程中不断萃取地壳的铜质,故成为区域内独特的铜矿集中区.     

Physicochemical parameters of deep-seated mantle plumes

Thermodynamic analysis of experimental data has demonstrated that FeO activity in silicate melts identical in composition to natural magmas can be described by the regular-solution model, which takes into account interactions of all cations with Si and interaction of Ca with Al. Using this model, we propose an oxygen barometer for spinel + magma phase association. In contrast to the earlier proposed methods for estimation of oxygen chemical potential, this barometer can work in the PJ-domain close to the liquidus of magmatic process. The new oxygen barometer has been applied to magmas related to mantle plume activity, including Siberian meimechites, Hawaiian picrites, and picrites from the Emeishan large igneous province (LIP) and Greenland. We have shown that most magmas related to the activity of deep-seated mantle plumes are characterized by a higher relative chemical potential of oxygen than magmas of mid-ocean ridges. Thermodynamically calculated stability fields of rocks with different carbon-containing phases show that under PJ-conditions of the lower mantle, the ascending mantle plumes are characterized by relatively high oxygen fugacity. Formation of diamond in the lower mantle requires more oxidizing conditions as compared with the major part of this geosphere, where the presence of Fe-Ni alloy is predicted. We have put forward a hypothesis that the main reason for the oxygen fugacity increase in particular domains of the lower mantle is a shift of redox equilibria toward a decrease in the amount of Fe-Ni alloy, up to its disappearance, with temperature growth.     

Alkaline plumes of continents and oceans

Series of continental and oceanic alkaline associations have been compared. Comparison confirms that alkaline plumes originated from the Earth’s liquid core under the continents and, less often, under the oceans. The spatial distribution of alkaline complexes has been analyzed in terms of the plume magmatism theory. Analysis suggests that the zoning and lateral migration of alkaline magmatic centers in alkaline provinces were determined by the migration of an alkaline plume (multiplume) and its alkaline basaltic, alkaline ultramafic, carbonatitic, kimberlitic, and other derivates.Two components are well pronounced in the chemical history of alkaline plume magmatism. The first is the foidaphile component, which persists in all igneous and metasomatic rocks of various alkaline complexes. It includes elements associated with Na and K: rare alkali metals, alkaline earth metals, radioactive elements, rare earths, and others. They make up the important part of the plume that might have separated from the liquid core. The second component is rock-forming mantle–lithospheric, which formed in the asthenosphere during the mixing of mantle and lithospheric sources while the plume ascended to the Earth’s surface.     

Geodynamic regimes of thermochemical mantle plumes

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.     

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.     

南秦岭古生代热水沉积盆地与热水沉积成矿

扬子地块北部被动边缘的南秦岭古生代沉积盆地中,发育一套自早古生代—中生代以来的碳酸盐岩夹细碎屑岩沉积建造,形成规模巨大独具特色的以铅锌金为主的多金属成矿带。伸展构造体制下形成的裂陷或断陷型盆地中,正常水成沉积与热水沉积同盆共存。正常水成沉积中叠加的热水沉积是一个"突发事件或灾变事件",具有特殊的物质组成和产态。通过对区内沉积成矿盆地的识别、分级,二级沉积盆地中边缘部位常发育多个三级构造热水沉积成矿盆地,它受控于沉积盆地中的同生断裂,具有沉积岩相、热水沉积岩组合、显著成矿作用及物化探异常广布的特点。三级构造热水沉积成矿盆地是矿床定位的构造空间,四级热水沉积洼地为矿体(矿层)的容纳空间。区内热水沉积岩主要为重晶石(毒重石)岩、硅质岩、钠长石岩和铁碳酸盐岩类,铅锌重晶石等矿产多产于热水沉积岩中或上盘。热水沉积形成一般由早期的热水喷发交代→主期热水喷流→晚期热水喷气演变。早期的热水喷发交代往往沿矿液喷发通道,形成网脉状、角砾状矿化;主期热水喷流主要形成多金属及热水喷流相,形成块状、条带状、层纹状矿石或热水沉积岩;晚期热水喷气主要形成浸染状矿石和热水喷气岩石。     

煤层甲烷碳同位素在煤层气勘探中的地质意义--以沁水盆地为例

根据沁水煤层气甲烷碳同位素的组成与分布特征,从煤层甲烷碳同位素在煤层气解吸-扩散-运移中的分馏效应,结合水文地质条件和构造条件,讨论了煤层甲烷碳同位素在煤层气勘探中的地质意义,认为沁水煤层气δ13C₁值不仅总体上较高,而且随埋深增大而增高,说明沁水煤层气存在因煤层抬升而卸压所导致的煤层气解吸-扩散-运移效应,从而形成了该区甲烷碳同位素在平面上的分带现象。