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1.
The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference.  相似文献   

2.
Assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments. Accordingly, newly discovered regions of saline minerals on Mars are high priority targets for exploration. The chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on Earth. However, basaltic weathering largely controls fluid compositions on Mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret Martian evaporites. Here we present a new chemical divide system that predicts evaporite assemblages identified in SNC-type meteorites, ancient evaporites discovered on Meridiani Planum by the Opportunity rover, and Mars Express OMEGA data. We suggest that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among Martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid.  相似文献   

3.
New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe2+ and Fe3+ in Pitzer's ion interaction equations to evaluate Fe redox disequilibrium at Meridiani Planum. Modeling results suggest that evaporation of acidic fluids derived from weathering olivine-bearing basalt should produce Mg, Ca, and Fe-sulfates such as jarosite and melanterite. Calculations that model diagenesis by fluid recharge predict the eventual breakdown of jarosite to goethite as well as the preservation of much of the initial soluble evaporite component at modeled porosity values appropriate for relevant depositional environments (< 0.30). While only one of several possible formation scenarios, this simple model is consistent with much of the chemical and mineralogical data obtained on Meridiani Planum outcrop.  相似文献   

4.
Exploration by the NASA rover Opportunity has revealed sulfate- and hematite-rich sedimentary rocks exposed in craters and other surface features of Meridiani Planum, Mars. Modern, Holocene, and Plio-Pleistocene deposits of the Río Tinto, southwestern Spain, provide at least a partial environmental analog to Meridiani Planum rocks, facilitating our understanding of Meridiani mineral precipitation and diagenesis, while informing considerations of martian astrobiology. Oxidation, thought to be biologically mediated, of pyritic ore bodies by groundwaters in the source area of the Río Tinto generates headwaters enriched in sulfuric acid and ferric iron. Seasonal evaporation of river water drives precipitation of hydronium jarosite and schwertmannite, while (Mg,Al,Fe3+)-copiapite, coquimbite, gypsum, and other sulfate minerals precipitate nearby as efflorescences where locally variable source waters are brought to the surface by capillary action. During the wet season, hydrolysis of sulfate salts results in the precipitation of nanophase goethite. Holocene and Plio-Pleistocene terraces show increasing goethite crystallinity and then replacement of goethite with hematite through time. Hematite in Meridiani spherules also formed during diagenesis, although whether these replaced precursor goethite or precipitated directly from groundwaters is not known. The retention of jarosite and other soluble sulfate salts suggests that water limited the diagenesis of Meridiani rocks.Diverse prokaryotic and eukaryotic microorganisms inhabit acidic and seasonally dry Río Tinto environments. Organic matter does not persist in Río Tinto sediments, but biosignatures imparted to sedimentary rocks as macroscopic textures of coated microbial streamers, surface blisters formed by biogenic gas, and microfossils preserved as casts and molds in iron oxides help to shape strategies for astrobiological investigation of Meridiani outcrops.  相似文献   

5.
Before base surges were described in association with nuclear blasts and explosive volcanic eruptions (especially, the 1980 eruption of Mount St. Helens, Washington), laminar and cross-bedded volcanogenic surge deposits were commonly misinterpreted as being of fluvial or aeolian origin. One well-documented example involves the “water-laid tuffs” in and near the Spor Mountain beryllium mine, Utah; other examples abound. In light of how frequently volcanogenic surge deposits have been misinterpreted on Earth, extreme caution is urged for Mars studies. Contrary to what has been claimed, the markedly cross-bedded, salty deposits at Meridiani Planum on Mars need not have been formed by a combination of aeolian and aqueous processes, and their contained hematitic spherules need not have formed as aqueous concretions. Given the lack of indications of volcanism in the vicinity, and the planet-wide abundance of impact craters, deposition by surges associated with distant impact targets consisting of brine-soaked, locally sulfidic regolith is a reasonable explanation for all features observed, especially if diagenesis and weathering are considered. The uniformly sized and shaped, Ni-enriched blue-gray hematitic spherules would then be some type of vapor condensation spherules (including accretionary lapilli). A similar interpretation is possible for deposits in the Home Plate area, Gusev Crater. Unlike on the dry and atmosphereless Moon, salty impact surge deposits containing spherules should be common, and well-preserved, on Mars.  相似文献   

6.
Hydrogen peroxide can form through the interaction of pyrite and anoxic water. The oxidation of pyrite results in the precipitation of sulfates and iron oxides, high redox potentials (~ 1000 mV) and acidic pH (3–4). The oxidative potential of the resultant solution may be responsible for the oxidation of organic compounds, as observed in the subsurface of the Rio Tinto Mars analog. On Mars subsurface migration of groundwater interacting with volcanogenic massive pyrite deposits would have mobilized acidic and oxidizing fluids through large portions of the crust, resulting in the widespread deposition of sulfates and iron oxides. This groundwater could have leached substantial volumes of aquifer material and crustal rocks, thereby erasing any organic compounds possibly down to depths of hundreds of meters. Therefore, the preservation of organic biosignatures must have been severely constrained in the portions of the ancient Martian crust that were exposed to aqueous processes, calling for a redefinition of the future targets in the search for biomolecular traces of life on Mars.  相似文献   

7.
Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.  相似文献   

8.
Analyses of outcrops created by the impact craters Endurance, Fram and Eagle reveal the broad lateral continuity of chemical sediments at the Meridiani Planum exploration site on Mars. Approximately ten mineralogical components are implied in these salt-rich silicic sediments, from measurements by instruments on the Opportunity rover. Compositional trends in an apparently intact vertical stratigraphic sequence at the Karatepe West ingress point at Endurance crater are consistent with non-uniform deposition or with subsequent migration of mobile salt components, dominated by sulfates of magnesium. Striking variations in Cl and enrichments of Br, combined with diversity in sulfate species, provide further evidence of episodes during which temperatures, pH, and water to rock ratios underwent significant change. To first order, the sedimentary sequence examined to date is consistent with a uniform reference composition, modified by movement of major sulfates upward and of minor chlorides downward. This reference composition has similarities to martian soils, supplemented by sulfate anion and the alteration products of mafic igneous minerals. Lesser cementation in lower stratigraphic units is reflected in decreased energies for grinding with the Rock Abrasion Tool. Survival of soluble salts in exposed outcrop is most easily explained by absence of episodes of liquid H2O in this region since the time of crater formation.  相似文献   

9.
The secondary mineral budget on Earth is dominated by clay minerals, Al-hydroxides, and Fe-oxides, which are formed under the moderate pH, high water-to-rock ratio conditions typical of Earth's near-surface environment. In contrast, geochemical analyses of rocks and soils from landed missions to Mars indicate that secondary mineralogy is dominated by Mg (± Fe, Ca)-sulfates and Fe-oxides. This discrepancy can be explained as resulting from differences in the chemical weathering environment of Earth and Mars. We suggest that chemical weathering processes on Mars are dominated by: (1) a low-pH, sulfuric acid-rich environment in which the stoichiometric dissolution of labile mineral phases such as olivine and apatite (± Fe–Ti oxides) is promoted; and (2) relatively low water-to-rock ratio, such that other silicate phases with slower dissolution rates (e.g., plagioclase, pyroxene) do not contribute substantially to the secondary mineral budget at the Martian surface. Under these conditions, Al-mobilization is limited, and the formation of significant Al-bearing secondary phases (e.g., clays, Al-hydroxides, Al-sulfates) is inhibited. The antiquity of rock samples analyzed in-situ on Mars suggest that water-limited acidic weathering conditions have more than likely been the defining characteristic of the Martian aqueous environment for billions of years.  相似文献   

10.
Formation,history and energetics of cores in the terrestrial planets   总被引：1，自引：0，他引：1  
The cores of the terrestrial planets Earth, Moon, Mercury, Venus and Mars differ substantially in size and in history. Though no planet other than the Earth has a conclusively demonstrated core, the probable cores in Mercury and Mars and Earth's core show a decrease in relative core size with solar distance. The Moon does not fit this sequence and Venus may not. Core formation must have been early (prior to ～4 · 109 yr. ago) in the Earth, by virtue of the existence of ancient rock units and ancient paleomagnetism and from UPb partitioning arguments, and in Mercury, because the consequences of core infall would have included extensional tectonic features which are not observed even on Mercury's oldest terrain. If a small core exists in the Moon, still an open question, completion of core formation may be placed several hundred million years after the end of heavy bombardment on tectonic and thermal grounds. Core formation time on Mars is loosely constrained, but may have been substantially later than for the other terrestrial planets. The magnitude and extent of early heating to drive global differentiation appear to have decreased with distance from the sun for at least the smaller bodies Mercury, Moon and Mars.Energy sources to maintain a molten state and to fuel convection and magnetic dynamos in the cores of the terrestrial planets include principally gravitational energy, heat of fusion, and long-lived radioactivity. The gravitational energy of core infall is quantifiable and substantial for all bodies but the Moon, but was likely spent too early in the history of most planets to prove a significant residual heat source to drive a present dynamo. The energy from inner core freezing in the Earth and in Mercury is at best marginally able to match even the conductive heat loss along an outer core adiabat. Radioactive decay in the core offers an attractive but unproven energy source to maintain core convection.  相似文献   

11.
Iron-rich spherules (> 90% Fe2O3 from electron microprobe analyses) ∼10-100 μm in diameter are found within sulfate-rich rocks formed by aqueous, acid-sulfate alteration of basaltic tephra on Mauna Kea volcano, Hawaii. Although some spherules are nearly pure Fe, most have two concentric compositional zones, with the core having a higher Fe / Al ratio than the rim. Oxide totals less than 100% (93-99%) suggest structural H2O and/or OH− 1. The transmission Mössbauer spectrum of a spherule-rich separate is dominated by a hematite (α-Fe2O3) sextet whose peaks are skewed toward zero velocity. Skewing is consistent with Al3+ for Fe3+ substitution and structural H2O and/or OH− 1. The grey color of the spherules implies specular hematite. Whole-rock powder X-ray diffraction spectra are dominated by peaks from smectite and the hydroxy sulfate mineral natroalunite as alteration products and plagioclase feldspar that was present in the precursor basaltic tephra. Whether spherule formation proceeded directly from basaltic material in one event (dissolution of basaltic material and precipitation of hematite spherules) or whether spherule formation required more than one event (formation of Fe-bearing sulfate rock and subsequent hydrolysis to hematite) is not currently constrained. By analogy, a formation pathway for the hematite spherules in sulfate-rich outcrops at Meridiani Planum on Mars (the Burns formation) is aqueous alteration of basaltic precursor material under acid-sulfate conditions. Although hydrothermal conditions are present on Mauna Kea, such conditions may not be required for spherule formation on Mars if the time interval for hydrolysis at lower temperatures is sufficiently long.  相似文献   

12.
Randall S Perry and Mark A Sephton discuss a target material for Mars Sample Return: rocks coated with desert varnish, a mineral layer that records past surface climates on Earth, and probably on Mars too.  相似文献   

13.
Thermodynamic conditions (first of all, temperature) are the main dynamic factors in the transformation process of ferrous to ferric iron (TFFI). TFFI usually takes place within a temperature range of 473–843 K (most active at temperatures above 673 K) and does not require presence of the oxidizing agents above 673 K. Analysis of the chemical composition of different rocks and minerals indicates that only for some sedimentary rocks is the relative content of ferrous iron oxide less than its value in magnetite, and this value is minimal for oceanic sediments. The relative content of ferrous iron oxide in oceanic magmatic rocks exceeds this value in continental magmatic rocks and depends on the rate of rock cooling. An investigation of the role of the titanium oxide content of different rocks on stability of ferrous iron oxide against its transformation to ferric iron oxide shows that a significant correlation (r = 0.79) does exist between the relative content of ferrous iron oxide and ratio of TiO2/Fe2O3. Temperature within the solar nebula at location of the Earth was within the temperature range of the TFFI. During the Earth accretion and its early evolution, ferric iron oxide was unstable and most likely did not exist. The first magnetic minerals containing ferric iron could have appeared only after the Earth’s surface had cooled below ∼843 K. The formation of the first Algoma-type banded iron formations could be used as a marker of the Earth’s surface cooling below ∼843 K.  相似文献   

14.
The Eoarchean Nulliak supracrustal rocks in the Saglek Block of northeastern Labrador, Canada, contain some of the world's oldest carbonate rocks. This work attempted to reveal the origin of the carbonate rocks and estimate the surface environmental conditions of the early Earth based on their occurrence and geochemistry. They occur together with mafic and ultramafic rocks in Pangertok Inlet and St. John's Harbour South, whereas they are interlayered with pelitic rock layers with quartzofeldspathic mineral assemblages in St. John's Harbour East and Big Island. The geological occurrence suggests that the formers were formed around hydrothermal fields, whereas the latters were deposited near a continental margin. Some carbonate rocks have high SiO2, Al2O3, and Zr contents, indicating that the silicification and involvement of detrital materials influenced their composition; thus, pure carbonate rocks were selected using a combined filter of the SiO2, TiO2, Al2O3, Zr, and Ba contents. The selected carbonate rocks have positive La, Eu, Gd, Y, U, Pb, and Sr anomalies, negative Nb, Zr, and Hf anomalies, and relatively small enrichment in heavy rare earth elements (HREEs). The La and Y anomalies suggest that they originated from chemical sediments precipitated from seawater. On the other hand, the small HREE-enrichment suggests that REEs were mainly dissolved as REE-carbonate complexes in seawater or that the riverine influxes were dominated by the detritus of Eoarchean continental crusts, presumably composed of HREE-depleted TTG. The U anomaly suggests that uranium was more dissolved than Th as U-bearing carbonate complexes in seawater. The Nulliak carbonate rocks also show a positive correlation between Y and Eu anomaly values, suggesting that the precipitation of iron-oxyhydroxide causing the Y anomaly was more significant near the hydrothermal fields than the continental margin, consistent with an alkaline hydrothermal model.  相似文献   

15.
岩石红外比辐射率简易测定方法及意义   总被引：1，自引：1，他引：0       下载免费PDF全文
利用地表辐射 (亮温 )求取地面温度需要通过岩石比辐射率进行换算。地球上的岩石千差万别 ,即使岩石学上分类一致的岩石 ,比辐射率也可以显著不同 ,需要大量的比辐射率时 ,现有测量方法显得过于复杂或环境要求过高 ,发展简便的比辐射率测量方法十分必要。实际上 ,在室温情况下 ,比辐射率为常数 ,可以通过测量一系列的温度和相应的辐射值 ,仪器本身的辐射和环境辐射可直接当作未知量参加反演 ,最终利用最小二乘法拟合获得比辐射率 ,大大降低了测量的难度。文中测量了1 6种岩石标本的比辐射率 ,方差一般保持在 0 .0 1左右 ,大多数 <0 .0 1 ;线性拟合的相关系数均 >0 99  相似文献   

16.
The formation of hematite and goethite concretions in different sedimentary rocks including sandstones is an important diagenetic process in the geologic history of the Earth. Its interpretation can also contribute to understanding the diagenetic history of Martian iron hydroxide concretions. A case study of iron-rich concretions from Estonian Middle Devonian sandstones exposed in ancient river valleys in southeastern Estonia was carried out based on the results of mineralogical, petrographical, geochemical, petrophysical and magnetic analyses. It was found that the high Fe2O3(total) content (25.0–39.5%), high magnetic susceptibility, bulk and grain density, very low porosity, corrosion and fracturing of the quartz grains of the platy iron concretions are in contrast with properties of the Devonian host sandstones. However the ferrous iron content (measured as FeO) of iron-rich concretions was as low as in the other Devonian rocks, suggesting an oxidizing environment and arid climate during the cementation by iron-hydroxides. The fracturing of quartz grains cemented by iron hydroxides could take place at near-surface conditions including vadose and phreatic zones in arid climate with high evaporation rates. Such climatic conditions have been reported for the Baltic region during Devonian, Upper Permian and Triassic times. We have found that goethite is prevalent in the cement, replacing clay and carbonate minerals. We assume that this iron-rich cement is originated from the mobilization of iron in host sandstones by groundwater, associated with tectonic activity at the end of the Middle Devonian, evidenced by fracturing in Devonian outcrops and caves. Although this mobilization could occur under reducing conditions, precipitation of goethite and hematite for the cementation could take place in oxidizing environment along bedding planes close to the surface during short sedimentation breaks. Another possible time for the formation of iron concretions could be Permian, under the condition of both arid climate and tectonic activity.  相似文献   

17.
SmNd isotopic data for mineral separates from the ferroan anorthosite 60025 define a precise isochron of 4.44 ± 0.02Ga age. This age is roughly 110 m.y. younger than the formation of the first large solid objects in the solar nebula, as recorded by the radiometric ages of the differentiated meteorites. In the magma ocean model for early lunar differentiation, ferroan anorthosites are the first crustal rocks to form on the Moon. If the Moon is as old as the oldest meteorites, the relatively young age determined for 60025 implies either that the magma ocean did not form synchronously with lunar formation, or that the magma ocean required over 100 m.y. before reaching the stage of ferroan anorthosite crystallization. Alternatively, we propose that the accumulated body of radiogenic isotope data for lunar rocks permit the Moon to be as young as 4.44–4.51 Ga. If so, isotopic evidence for chemical differentiation on the Earth at about this same time suggests that the formation of the Moon is reflected in the chemical evolution of the Earth. This, in turn, is consistent with the idea that the materials that now make up the Moon were derived from the Earth, perhaps ejected by collision between the Earth and another very large planetesimal during the final stages of accumulation of the terrestrial planets. Terrestrial origin models for the Moon lessen the requirement that the Earth and Moon each have near chondritic relative abundances of the refractory elements and could require that certain chemical and isotopic characteristics of both bodies be considered in the framework of the chemical mass-balance of the combined Earth-Moon system.  相似文献   

18.
Geobiology is a new discipline on the crossing interface between earth science and life science,and aims to understand the interaction and co-evolution between organisms and environments.On the basis of the latest international achievements,the new data presented in the Beijing geobiology forum sponsored by Chinese Academy of Sciences in 2013,and the papers in this special issue,here we present an overview of the progress and perspectives on three important frontiers,including geobiology of the critical periods in Earth history,geomicrobes and their responses and feedbacks to global environmental changes,and geobiology in extreme environments.Knowledge is greatly improved about the close relationship of some significant biotic events such as origin,radiation,extinction,and recovery of organisms with the deep Earth processes and the resultant environmental processes among oceans,land,and atmosphere in the critical periods,although the specific dynamics of the co-evolution between ancient life and paleoenvironments is still largely unknown.A variety of geomicrobial functional groups were found to respond sensitively to paleoenvironmental changes,which enable the establishment of proxies for paleoenvironmental reconstruction,and to play active roles on the Earth environmental changes via elemental biogeochemical cycles and mineral bio-transformations,but to be deciphered are the mechanisms of these functional groups that change paleoenvironmental conditions.Microbes of potential geobiology significance were found and isolated from some extreme environments with their biological properties partly understood,but little is known about their geobiological functions to change Earth environments.The biotic processes to alter or modify the environments are thus proposed to be the very issue geobiology aims to decipher in the future.Geobiology will greatly extend the temporal and spatial scope of biotic research on Earth and beyond.It has great potential of application in the domains of resource exploration and global change.To achieve these aims needs coordinative multidisciplinary studies concerning geomicrobiology and related themes,database and modeling of biogeochemical cycles,typical geological environments,and coupling of biological,physical,and chemical processes.  相似文献   

19.
The geochemical study of the Earth's mantle provides important constraints on our understanding of the formation and evolution of Earth, its internal structure, and the mantle dynamics. The bulk Earth composition is inferred by comparing terrestrial mantle rocks with chondrites, which leads to the chondritic Earth model. That is, Earth has the same relative proportions of refractory elements as that in chondrites, but it is depleted in volatiles. Ocean island basalts(OIB) may be produced by mantle plumes with possible deep origins; consequently, they provide unique opportunity to study the deep Earth. Isotopic variations within OIB can be described using a limited number of mantle endmembers, such as EM1, EM2 and HIMU, and they have been used to decipher important mantle processes. Introduction of crustal material into the deep mantle via subduction and delamination is important in generating mantle heterogeneity; however, there is active debate on how they were sampled by mantle melting, i.e.,the role of olivine-poor lithologies in the OIB petrogenesis. The origin and location of high 3He/4He mantle remain controversial,ranging from unprocessed(or less processed) primitive material in the lower mantle to highly processed materials with shallow origins, including ancient melting residues, mafic cumulates under arcs, and recycled hydrous minerals. Possible core-mantle interaction was hypothesized to introduce distinctive geochemical signatures such as radiogenic 186 Os and Fe and Ni enrichment in the OIB. Small but important variations in some short-lived nuclides, including 142 Nd, 182 W and several Xe isotopes, have been reported in ancient and modern terrestrial rocks, implying that the Earth's mantle must have been differentiated within the first 100 Myr of its formation, and the mantle is not efficiently homogenized by mantle convection.  相似文献   

20.
Geophysical arguments against plate tectonics in a hotter Earth, based on buoyancy considerations, require an alternative means of cooling the planet from its original hot state to the present situation. Such an alternative could be extensive flood volcanism in a more stagnant-lid like setting. Starting from the notion that all heat output of the Earth is through its surface, we have constructed two parametric models to evaluate the cooling characteristics of these two mechanisms: plate tectonics and basalt extrusion/flood volcanism. Our model results show that for a steadily (exponentially) cooling Earth, plate tectonics is capable of removing all the required heat at a rate of operation comparable to or even lower than its current rate of operation, contrary to earlier speculations. The extrusion mechanism may have been an important cooling agent in the early Earth, but requires global eruption rates two orders of magnitude greater than those of known Phanerozoic flood basalt provinces. This may not be a problem, since geological observations indicate that flood volcanism was both stronger and more ubiquitous in the early Earth. Because of its smaller size, Mars is capable of cooling conductively through its lithosphere at significant rates, and as a result may have cooled without an additional cooling mechanism. Venus, on the other hand, has required the operation of an additional cooling agent for probably every cooling phase of its possibly episodic history, with rates of activity comparable to those of the Earth.  相似文献   

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