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1.
http://dx.doi.org/10.1016/j.gsf.2016.07.005   总被引：1，自引：1，他引：0  
The Hadean history of Earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. The Earth-Moon system had many features in common during the birth stage. Solidification of the dry magma ocean at 4.53 Ga generated primordial continents with komatiite. We speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz) down to 50–60 km in depth. The thickness of the mafic KREEP basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100–200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. The primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including Ca, Mg, Fe, Mn, P, K, and Cl which were exposed on the surface of the dry Earth. Around 190 million years after the solidification of the magma ocean, “ABEL bombardment” delivered volatiles including H2O, CO2, N2 as well as silicate components through the addition of icy asteroids. This event continued for 200 Myr with subordinate bombardments until 3.9 Ga, preparing the Earth for the prebiotic chemical evolution and as the cradle of first life. Due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of Hadean plate tectonics.  相似文献   

2.
http://dx.doi.org/10.1016/j.gsf.2016.08.003   总被引：1，自引：1，他引：0  
The primordial crust on the Earth formed from the crystallization of the surface magma ocean during the Hadean. However, geological surveys have found no evidence of rocks dating back to more than 4 Ga on the Earth's surface, suggesting the Hadean crust was lost due to some processes. We investigated the subduction of one of the possible candidates for the primordial crust, anorthosite and KREEP crust similar to the Moon, which is also considered to have formed from the crystallization of the magma ocean. Similar to the present Earth, the subduction of primordial crust by subduction erosion is expected to be an effective way of eliminating primordial crust from the surface. In this study, the subduction rate of the primordial crust via subduction channels is evaluated by numerical simulations. The subduction channels are located between the subducting slab and the mantle wedge and are comprised of primordial crust materials supplied mainly by subduction erosion. We have found that primordial anorthosite and KREEP crust of up to ～50 km thick at the Earth's surface was able to be conveyed to the deep mantle within 0.1-2 Gy by that mechanism.  相似文献   

3.
When plate tectonics began on the Earth has been long debated and here we argue this topic based on the records of Earth-Moon geology and asteroid belt to conclude that the onset of plate tectonics was during the middle Hadean(4.37-4.20 Ga). The trigger of the initiation of plate tectonics is the ABEL Bombardment, which delivered oceanic and atmospheric components on a completely dry reductive Earth, originally comprised of enstatite chondrite-like materials. Through the accretion of volatiles, shock metamorphism processed with vaporization of both CI chondrite and supracrustal rocks at the bombarded location, and significant recrystallization went through under wet conditions, caused considerable eclogitization in the primordial continents composed of felsic upper crust of 21 km thick anorthosite, and 50 km or even thicker KREEP lower crust. Eclogitization must have yielded a powerful slab-pull force to initiate plate tectonics in the middle Hadean. Another important factor is the size of the bombardment. By creating Pacific Ocean class crater by 1000 km across impactor, rigid plate operating stagnant lid tectonics since the early Hadean was severely destroyed, and oceanic lithosphere was generated to have bi-modal lithosphere on the Earth to enable the operation of plate tectonics.Considering the importance of the ABEL Bombardment event which initiated plate tectonics including the appearance of ocean and atmosphere, we propose that the Hadean Eon can be subdivided into three periods:(1) early Hadean(4.57-4.37 Ga),(2) middle Hadean(4.37-4.20 Ga), and(3) late Hadean(4.20-4.00 Ga).  相似文献   

4.
Our blue planet Earth has long been regarded to carry full of nutrients for hosting life since the birth of the planet.Here we speculate the processes that led to the birth of early life on Earth and its aftermath, finally leading to the evolution of metazoans.We evaluate:（1） the source of nutrients,（2） the chemistry of primordial ocean,（3） the initial mass of ocean,and（4） the size of planet.Among the life-building nutrients,phosphorus and potassium play a key role.Only three types of rocks can serve as an adequate source of nutrients:（a） continent-forming TTG（granite）,enabling the evolution of primitive life to metazoans;（b） primordial continents carrying anorthosite with KREEP（Potassium,Rare Earth Elements, and Phosphorus） basalts,which is a key to bear life;（c） carbonatite magma,enriched in radiogenic elements such as U and Th,which can cause mutation to speed up evolution and promote the birth of new species in continental rift settings.The second important factor is ocean chemistry.The primordial ocean was extremely acidic（pH = 1-2） and enriched in halogens（CI,F and others）,S,N and metallic elements（Cd,Cu,Zn,and others）,inhibiting the birth of life.Plate tectonics cleaned up these elements which interfered with RNA.Blue ocean finally appeared in the Phanerozoic with pH = 7 through extensive interaction with surface continental crust by weathering,erosion and transportation into ocean.The initial ocean mass was also important.The birth of life and aftermath of evolution was possible in the habitable zone with 3-5 km deep ocean which was able to supply sufficient nutrients. Without a huge landmass,nutrients cannot be supplied into the ocean only by ridge-hydrothermal circulation in the Hadean.Finally,the size of the planet plays a crucial role.Cooling of massive planets is less efficient than smaller ones,so that return-flow of seawater into mantle does not occur until central stars finish their main sequence.Due to the suitable size of Earth,the dawn of Phanerozoic witnessed the initiation of return-flow of seawater into the mantle,leading to the emergence of huge landmass above sea-level,and the distribution of nutrients on a global scale.Oxygen pump also played a critical role to keep high-PO2 in atmosphere since then,leading to the emergence of ozone layer and enabling animals and plants to invade the land. To satisfy the tight conditions to make the Earth habitable,the formation mechanism of primordial Earth is an important factor.At first,a ’dry Earth’ must be made through giant impact,followed by magma ocean to float nutrient-enriched primordial continents（anorthosite + KREEP）.Late bombardment from asteroid belt supplied water to make 3-5 km thick ocean,and not from icy meteorites from Kuiper belt beyond cool Jupiter.It was essential to meet the above conditions that enabled the Earth as a habitable planet with evolved life forms.The tight constraints that we evaluate for birth and evolution of life on Earth would provide important guidelines for planetary scientists hunting for life in the exosolar planets.  相似文献   

5.
The paper is focused on the evolution of the Earth starting with the planetary accretion and differentiation of the primordial material (similar in composition to CI chondrites) into the core and mantle and the formation of the Moon as a result of the impact of the Earth with a smaller cosmic body. The features of the Hadean eon (ca. 4500–4000 Ma) are described in detail. Frequent meteorite-asteroid bombardments which the Earth experienced in the Hadean could have caused the generation of mafic/ultramafic primary magmas. These magmas also differentiated to produce some granitic magmas, from which zircons crystallized. The repeated meteorite bombardments destroyed the protocrust, which submerged into the mantle to remelt, leaving refractory zircons, indicators of the Early Earth’s geologic conditions, behind.The mantle convection that started in the Archean could possibly be responsible for the Earth’s subsequent endogenous evolution. Long-living deep-seated mantle plumes could have promoted the generation of basalt-komatiitic crust, which, thickening, could have submerged into the mantle as a result of sagduction, where it remelted. Partial melting of the thick crust, leaving eclogite as a residue, could have yielded tonalite-trondhjemite-granodiorite (TTG) melts. TTG rocks are believed to compose the Earth’s protocrust. Banded iron bodies, the only mineral deposits of that time, were produced in the oceans that covered the Earth.This environment, recognized as LID tectonics combined with plume tectonics, probably existed on the Earth prior to the transitional period, which was marked by a series of new geologic processes and led to a modern-style tectonics, involving plate tectonics and plume tectonics mechanisms, by 2 Ga. The transitional period was likely to be initiated at about 3.4 Ga, with the segregation of outer and inner cores, which terminated by 3.1 Ga. Other rocks series (calc-alkaline volcanic and intrusive) rather than TTGs were produced at that time. Beginning from 3.4-3.3 Ga, mineral deposits became more diverse; noble and siderophile metal occurrences were predominant among ore deposits. Carbonatites, hosting rare-metal mineralization, could have formed only by 2.0 Ga. From 3.1 to 2.7 Ga, there was a period of “small-plate” tectonics and first subduction and spreading processes, which resulted in the first supercontinent by 2.7 Ga. Its amalgamation indicates the start of superplume-supercontinent cycles.Between 2.7 and 2.0 Ga, the D″ layer formed at the core-mantle interface. It became a kind of thermal regulator for the ascending already tholeiitic mantle plume magmas. All deep-seated layers of the Earth and large low-velocity shear provinces, called mantle hot fields, partially melted enriched EM-I and EM-II mantles, and the depleted recent asthenosphere mantle, which is parental for midocean-ridge basalts, were finally generated by 2 Ga. Therefore, an interaction of all Earth’s layers began from that time.  相似文献   

6.
行星地球不均一成因和演化的理论框架初探   总被引：4，自引：0，他引：4  
地球是太阳系的一部分 ,研究地球的成因和演化必须要与太阳系的形成结合起来。文章在综合最新的地球化学、地球物理和天体化学研究资料的基础上 ,对地球的不均一成因进行了理论上的推导。对星子学说、地球的多阶段堆积模型和地球化学不均一性以及它们的相互关系进行了论述 ,从行星演化的角度阐述地球不均一成因的理论框架。根据行星起源的星子学说 ,以及天体化学、地球化学和深部地质地球化学和地球物理资料的多重限制 ,行星地球的增生经历了两个主要阶段 ,即原地球的形成阶段和晚期星子堆积形成上地幔镶饰层阶段。早前寒武纪岩石的铅、钕、氧同位素的研究表明 ,在地球形成的初期就存在化学不均一性 ,而这种不均一性很可能代表初始堆积星子化学组成的差异  相似文献   

7.
Relatively soon (0.2 Ga) after the Earthformed, it is likely that major oceans appeared in ahot (100°C) reducing environment where carbondioxide was probably the dominant atmospheric gas,with PCO2, values reaching perhaps in excess of 10atm. During the Hadean Eon between 4.3 and 3.8 Ga BP,major changes in the concentration of atmosphericCO2 and associated temperature changes had aprofound influence on the carbonate geochemistry ofthe Hadean Ocean. Although no rocks are known to havesurvived prior to the Archean Eon, it is stillpossible to calculate approximate values for importantseawater parameters during the Hadean Eon based onother sources of information and reasonableassumptions about processes such as weatheringreactions.Our calculations are based on a linear temperaturechange from 100°C to 70°C and logPCO2 change from 1 to -1.5 over the Hadean Eon. Over this range in temperature and P CO2, theinfluence of T is relatively small, but changes inP CO2 result in large compositional variations inthe carbonate chemistry of Hadean seawater. In theearly Hadean, seawater pH was probably about5.8 ± 0.2, DIC may have reached close to 130 mM,and alkalinity was perhaps close to 30 mM. By thelate Hadean, seawater pH probably had changed to closeto neutral (6.8), and DIC and alkalinity were closerto present-day values. Even large uncertainties inNa+ + Cl-, K+ and Mg2+concentrations produce relatively small uncertaintiesin our calculated values for the carbonic acid system. However, larger uncertainties result from reasonableranges for Ca2+ concentrations and the saturationstate of Hadean seawater with respect to calcite.Our calculations support the hypothesis that acarbonate chemistry of seawater roughly similar tothat of modern oceans could have been acquired veryearly in Earth history. If seawater composition werebuffered by reactions involving carbonates andsilicates, then the composition of late Hadean-earlyArchean seawater was not vastly different from that oftoday. Thus, by the conclusion of the Hadean Eon, ifnot before, environmental conditions at the Earth's surface, including temperature and seawatercomposition, were sufficiently equable for theevolution of life, including the Archaebacteria: theextreme halophiles and thermophiles and methanogens.Contrary to the hypothesis of an early Na-bicarbonateocean, our calculations suggest the possibility thatthe early oceans of Earth were a NaCl-dominatedaqueous solution, with somewhat higher DIC andalkalinity concentrations, higher saturation state,and the possibility of lower calcium concentrations.The time course of approach of Hadean seawater to acarbonate composition closer to that of today isdifficult to predict. It is distinctly possible thatthe concentration of calcium in seawater did not reachlevels like that of modern seawater until the latePrecambrian and thus constrained the timing of the"Big Bang" of organic evolution, the emergence of theshelled invertebrates at the beginning of thePhanerozoic.  相似文献   

8.
本文概括性地阐述我国前寒武纪冥古宙、太古宙、元古宙三大地史阶段的重大地质事件,粗略勾绘前寒武纪地球演化的轨迹,期望了解我国与全球变化的异同,进一步突出我国前寒武纪三大地史阶段中新太古代超级地质事件及元古宙时期中国大陆块体对哥伦比亚及罗迪尼亚两个超大陆形成与破裂的地质响应。冥古宙是地球最早期的地史阶段,从太阳系形成的4 567 Ma至地球上最老的4 030 Ma的Acasta片麻杂岩。碎屑锆石保存最好的地点是西澳的Mt. Narryer和Jack Hills。目前在中国大陆至少有7个地点发现具有罕见的约4.0 Ga的碎屑锆石,这些地点并不位于克拉通区,而是赋存于造山系新元古代至古生代以碎屑岩为主的地层中。太古宙(4 030~2 420 Ma)定义为从最古老的岩石出现(4 030 Ma Acasta片麻岩)至冰碛层首次广泛分布的寒冷期之间的一段地史。最古老的岩石为英云闪长片麻岩,构成加拿大西北斯拉夫克拉通4.03~3.94 Ga Acasta片麻岩的一部分。西南格陵兰Isua带保存全球有最老的表壳岩,形成于3 810 Ma。太古宙最重大的地质事件莫过于2 780~2 420 Ma时期的新太古代超级事件。值得指出的是华北克拉通最古老、也是中国最古老的岩石出露在中国辽宁鞍山地区,约3.80 Ga英云闪长岩奧长花岗质片麻岩和3.30 Ga的表壳岩已被识别。华北克拉通太古宙有与世界各地太古宙相似的演化历史和特点,包括花岗岩绿岩带及高级变质片麻岩带、广泛的英云闪长岩奧长花岗岩花岗闪长岩(TTG)片麻岩、古陆壳的出露(略老于3.8 Ga)、广泛分布的BIF等。我国太古宙花岗岩绿岩带虽然在华北克拉通分布较广,但与南非、格陵兰、加拿大、西澳等地经典的花岗岩绿岩带相比,时代偏新,仅以新太古代为主,规模偏小,缺少大面积分布的科马提岩,且变质程度偏高,主要为角闪岩相麻粒岩相变质。演化到元古宙(2 420~541 Ma),则进入成熟的、较冷的、刚性程度较高的地球,以现代样式板块构造、超大陆旋回和更复杂的疑源类(eukaryotic)生命的发育为特征。这种变化大致出现在2 420 Ma左右,与哈默斯利型BIF的消失及地史中首次广泛出现的冰川沉积物年代相近。古元古代早期十分重要的“休伦冰川事件”、指示大氧化事件的古老红层在我国尚未被发现,与Lomagundi Jatuli (LJE) δ13C的同位素漂移有关的关门山组古元古代沉积地层的同位素年代学依据不足;古元古代磷矿和具有巨大石油潜力的2.01 Ga Shunga事件也未能鉴别。但中国最大特色是发育了与哥伦比亚和罗迪尼亚超大陆汇聚与裂解有关的良好地质记录,特别是华北克拉通保存了古元古代与哥伦比亚超大陆汇聚有关的超高温、高压麻粒岩等变质及岩浆事件,1 780 Ma以后的中元古代又保存了与哥伦比亚超大陆裂解有关的裂谷沉积及岩浆活动;而在扬子和塔里木陆块区则保存了与新元古代早期与罗迪尼亚超大陆汇聚有关的蛇绿岩、混杂岩、洋内弧、俯冲增生杂岩及大陆边缘弧,在约800 Ma以后则发育了与罗迪尼亚超大陆裂解有关的沉积及岩浆活动的地质记录,为中国和全球地质学者研究这一时期地球系统变化和成矿作用提供了客观的野外实验室和良好的范例。  相似文献   

9.
《Comptes Rendus Geoscience》2007,339(14-15):928-936
Recent geochemical evidence based on the 146Sm–142Nd system and Hadean zircons shows that the Earth's mantle experienced depletion approximately 100 Ma after the formation of the solar system, and possibly even before (earlier than 30 Ma), due to the extraction of a crust enriched in incompatible elements. Depending on the model 142Nd abundance assumed for the Bulk Earth, the early crust may have been stored in the deep mantle, or may have been remixed in the mantle with a timescale of ∼1 Ga. If the Earth is considered to have a 142Nd composition identical to that of ordinary chondrites, then it implies that the early crust (or the enriched reservoir) is now present at the core–mantle boundary and has remained isolated from the rest of the Earth for the past 4.5 Ga. If the primordial crust had a basaltic composition, then it is unlikely that this enriched reservoir remained isolated for more than 4.5 Ga due to entrainment; yet, there is no signature of this reservoir in hotspot lavas that should sample this enriched reservoir. In contrast, if the Bulk Earth has an Nd isotopic composition slightly distinct from that of chondrites, then there is no need to invoke a hidden reservoir and the early crust must have been remixed by mantle convection prior to the formation of the modern continents.  相似文献   

10.
http://dx.doi.org/10.1016/j.gsf.2016.06.008   总被引：1，自引：1，他引：0  
The mineral schreibersite, (Fe,Ni)3P, provides a reactive source of phosphorus capable of forming phosphorylated molecules. These molecules may have been an important component of prebiotic chemistry, allowing their build-up and eventual commencement of autopoiesis. Discussed here are potential geochemical routes to providing schreibersite, as a potentially important prebiotic mineral, to the Hadean Earth. Two routes are identified: delivery of phosphides by meteoritic material and the reduction of phosphates to phosphides by high-temperature, low-redox conditions. About 1–10% of all crustal phosphorus is estimated to have been in schreibersite during the Hadean, making the long-term reaction of this mineral with organic-laden water plausible for many years. Ultimately, such conditions would have been conducive to the formation of life as we know it today.  相似文献   

11.
The granite-greenstone terranes nested in Archean cratonic nuclei of continents over the world are composed of variably metamorphosed igneous and sedimentary remnants of ancient ocean basins.These rocks preserve distinct geological and geochemical imprints of mantle evolution and differentiation of primordial crust,varying  相似文献   

12.
《China Geology》2018,1(1):109-136
The mainland of China is composed of the North China Craton, the South China Craton, the Tarim Craton and other young orogenic belts. Amongst the three cratons, the North China Craton has been studied most and noted for its widely-distributed Archean basement rocks. In this paper, we assess and compare the geology, rock types, formation age and geochemical composition features of the Archean basements of the three cratons. They have some common characteristics, including the fact that the crustal rocks prior to the Paleoarchean and the supracrustal rocks of the Neoarchean were preserved, and Tonalite-Trondhjemtite-Granodiorite (TTG) magmatism and tectono-magmatism occurred at about 2.7 Ga and about 2.5 Ga respectively. The Tarim Craton and the North China Craton show more similarities in their early Precambrian crustal evolution. Significant findings on the Archean basement of the North China Craton are concluded to be: (1) the tectonic regime in the early stage (>3.1 Ga) is distinct from modern plate tectonics; (2) the continental crust accretion occurred mostly from the late Mesoarchean to the early Neoarchean period; (3) a huge linear tectonic belt already existed in the late Neoarchean period, suggesting the beginning of plate tectonics; and (4) the preliminary cratonization had already been completed by about 2.5 Ga. Hadean detrital zircons were found at a total of nine locations within China. Most of them show clear oscillatory zoning, sharing similar textures with magmatic zircons from intermediate-felsic magmatic rocks. This indicates that a fair quantity of continental material had already developed on Earth at that time.  相似文献   

13.
在地球各大陆上工程地质条件水平和垂直上的变化反映出它们的区域性,它受控于该地区的热量、湿度及其比率。这就是在类似地质结构的地区中,不同的纬度和海拔高度形成了不同的工程地质条件的原因。引入工程地质区这个概念的目的在于说明工程地质条件的区域性。工程地质区一词的意义是指一个大的工程地质地区或省,在其中主要由水的形态决定的岩土状态是均一的并从工程地质观点来看具有区域稳定性。已经确定了以下工程地质区类型:永久性的多年冻土区域、多年冻土和融化沉积共存区域,以及融化和不冻岩石区域,它们再细分为亚区。工程地质条件区域性概念要考虑的主要问题有:分区原则、工程地质区域的类型和种类、地球各大陆上工程地质区域的空间分布规律性、“理想”大陆上的分布模型。  相似文献   

14.
Geologists investigate the evolution of the atmosphere, crust, and mantle through time by direct study of the rock record. However, the Hadean eon (>3.85 Ga) has been traditionally viewed as inaccessible due to the absence of preserved rocks. The discovery of >4.0 Ga detrital zircons from Western Australia in the 1980s — coupled with the development of new micro-analytical capabilities — made possible new avenues of early Earth research. The prevailing view that emerged is that the early Earth may have contained a stable hydrosphere, water-saturated or (near watersaturated) granitic magmas, and volcanic emanations dominated by neutral gas species (e.g., CO2, H2O, and SO2). The Hadean Earth may have been capable of supporting life ～200 Ma after accretion and perhaps earlier. Many of these models are formulated — or have been subsequently supported — by laboratory experiments of zircon. Important petrological variables such as temperature, pressure, oxygen fugacity, and component activities (e.g., SiO2/TiO2-activities) can be controlled. These experiments are fundamental for extrapolation to ‘deep time’ because they provide a means to understand primary chemistry preserved in ancient zircons. This review paper specifically focuses on zircon experimental studies (oxygen isotope fractionations, Ti-thermometry, and redox sensitive element incorporation into zircon), which have influenced our view of the very early Earth.  相似文献   

15.
W.G. Ernst   《Gondwana Research》2007,11(1-2):38
In the early Earth, accretionary impact heating, including collision with a large, Mars-sized object, decay of short-lived radioisotopes, and (after an initial thermal run-up) continuous segregation of the liquid Fe–Ni core resulted in extensive the melting of the silicate mantle and in the formation of a near-surface magma mush ocean. Progressive, continuous degassing and chemical–gravitational differentiation of the crust–mantle system accompanied this Hadean stage, and has gradually lessened during the subsequent cooling of the planet. Mantle and core overturn was vigorous in the Hadean Earth, reflecting deep-seated chemical heterogeneities and concentrations of primordial heat. Hot, bottom-up mantle convection, including voluminous plume ascent, efficiently rid the planet of much thermal energy, but gradually decreased in importance with the passage of time. Formation of lithospheric scum began when planetary surface temperatures fell below those of basalt and peridotite solidi. Thickening and broadening of lithospheric plates are inferred from the post-Hadean rock record. Developmental stages of mantle circulation included: (a) 4.5–4.4 Ga, early, chaotic magma ocean circulation involving an incipient or pre-plate regime; (b) 4.4–2.7 Ga, growth of small micro-oceanic and microcontinental platelets, all returned to the mantle prior to 4.0 Ga, but increasing in size and progressively suturing sialic crust-capped lithospheric amalgams at and near the surface over time; (c) 2.7–1.0 Ga, assembly of cratons surmounting larger, supercontinental plates; and (d) 1.0 Ga–present, modern, laminar-flowing asthenospheric cells capped by gigantic, Wilson-cycle lithospheric plates. Restriction of komatiitic lavas to the Archean, and of ophiolite complexes ± alkaline igneous rocks, high-pressure and ultrahigh-pressure metamorphic terranes to progressively younger Proterozoic–Phanerozoic orogenic belts supports the idea that planetary thermal relaxation promoted the increasingly negative buoyancy of cooler oceanic lithosphere. The Thickening of oceanic plates enhanced the gravitational instability and the consequent overturn of the outer Earth as cold, top-down oceanic mantle convection. The scales and dynamics of deep-seated asthenospheric circulation, and of lithospheric foundering + shallow asthenospheric return flow evidently have evolved gradually over geologic time in response to the progressive cooling of the Earth.  相似文献   

16.
As we continue searching for exoplanets,we wonder if life and technological species capable of communicating with us exists on any of them.As geoscientists,we can also wonder how important is the presence or absence of plate tectonics for the evolution of technological species.This essay considers this question,focusing on tectonically active rocky(silicate) planets,like Earth,Venus,and Mars.The development of technological species on Earth provides key insights for understanding evolution on exoplanets,including the likely role that plate tectonics may play.An Earth-sized silicate planet is likely to experience several tectonic styles over its lifetime,as it cools and its lithosphere thickens,strengthens,and becomes denser.These include magma ocean,various styles of stagnant lid,and perhaps plate tectonics.Abundant liquid water favors both life and plate tectonics.Ocean is required for early evolution of diverse single-celled organisms,then colonies of cells which specialized further to form guts,appendages,and sensory organisms up to the complexity of fish(central nervous system,appendages,eyes).Large expanses of dry land also begin in the ocean,today produced above subduction zones in juvenile arcs and by their coalescence to form continents,although it is not clear that plate tectonics was required to create continental crust on Earth.Dry land of continents is required for further evolution of technological species,where modification of appendages for grasping and manipulating,and improvement of eyes and central nervous system could be perfected.These bioassets allowed intelligent creatures to examine the night sky and wonder,the beginning of abstract thinking,including religion and science.Technology arises from the exigencies of daily living such as tool-making,agriculture,clothing,and weapons,but the pace of innovation accelerates once it is allied with science.Finally,the importance of plate tectonics for developing a technological species is examined via a thought experiment using two otherwise identical planets:one with plate tectonics and the other without.A planet with oceans,continents,and plate tectonics maximizes opportunities for speciation and natural selection,whereas a similar planet without plate tectonics provides fewer such opportunities.Plate tectonics exerts environmental pressures that drive evolution without being capable of extinguishing all life.Plate tectonic processes such as the redistribution of continents,growth of mountain ranges,formation of land bridges,and opening and closing of oceans provide a continuous but moderate environmental pressure that stimulates populations to adapt and evolve.Plate tectonics may not be needed in order for life to begin,but evolution of technological species is favored on planets with oceans,continents,plate tectonics,and intermittently clear night sky.  相似文献   

17.
The majority of continental crust formed during the hotter Archean was composed of Tonalite-Trondhjemite-Granodiorite (TTG) rocks. In contrast to the present-day loci of crust formation around subduction zones and intra-plate tectonic settings, TTGs are formed when hydrated basalt melts at garnet-amphibolite, granulite or eclogite facies conditions. Generating continental crust requires a two step differentiation process. Basaltic magma is extracted from the pyrolytic mantle, is hydrated, and then partially melts to form continental crust. Here, we parameterise the melt production and melt extraction processes and show self-consistent generation of primordial continental crust using evolutionary thermochemical mantle convection models. To study the growth of TTG and the geodynamic regime of early Earth, we systematically vary the ratio of intrusive (plutonic) and eruptive (volcanic) magmatism, initial core temperature, and internal friction coefficient. As the amount of TTG that can be extracted from the basalt (or basalt-to-TTG production efficiency) is not known, we also test two different values in our simulations, thereby limiting TTG mass to 10% or 50% of basalt mass. For simulations with lower basalt-to-TTG production efficiency, the volume of TTG crust produced is in agreement with net crustal growth models but overall crustal (basaltic and TTG) composition stays more mafic than expected from geochemical data. With higher production efficiency, abundant TTG crust is produced, with a production rate far exceeding typical net crustal growth models but the felsic to mafic crustal ratio follows the expected trend. These modelling results indicate that (i) early Earth exhibited a “plutonic squishy lid” or vertical-tectonics geodynamic regime, (ii) present-day slab-driven subduction was not necessary for the production of early continental crust, and (iii) the Archean Earth was dominated by intrusive magmatism as opposed to “heat-pipe” eruptive magmatism.  相似文献   

18.
The paper is focused on the fundamental problem of influence of extraterrestrial factors on the Earth’s geologic and tectonic evolution. Extraterrestrial factors played a decisive role in the Earth’s genesis, the formation of the first Hadean continental crust, and the beginning of the Archean era. Their significant influence persisted in the later epochs: Even in the Phanerozoic, extraterrestrial factors might have had a considerable influence on the environment. The sialic cores of protocontinental crust (4.4-3.9 Ga) with first-generation greenstone zones (3.8-3.2 Ga) and the global system of granite-greenstone belts (3.1-2.7 Ga) formed in the rotation-plume regime, mainly in the subequatorial hot belt. The formation of these global structures was, to a large extent, influenced by asteroid impacts, which caused the impact-triggered genesis of mantle plumes. Dramatic changes in the subsequent geologic history began at 2.7-2.0 Ga; at 2.0 Ga they terminated with the Moon’s transition to an orbit similar to the present-day one (50 ± 3 Earth’s radii), accompanied by the abrupt slowdown of the Earth’s axial rotation, the termination of formation of the layer D", and the start of recent plate tectonics, which is accompanied by the plume tectonics.  相似文献   

19.
梅冥相 《古地理学报》2016,18(4):513-524
在现行的前寒武纪地质年代表中,由于太古宙底界没有得到很好的定义,只是被粗略地置于大约4000,Ma,因此也造成了一个没有得到较好定义的“冥古宙”。2个重要的发现促使学者们对冥古宙的地层学属性进行修订:(1)在西澳大利亚Jack山脉太古宙砾岩中发现了真正古老的锆石晶体,其不仅可将地层时代延伸到4404,Ma,而且包含了有关地球早期环境条件较为丰富的信息; (2)在加拿大北部发现了大约4030,Ma的Acasta片麻岩。根据这些发现,并结合月球和陨石的测年数据,就产生了大量有关太阳系和地球早期历史的新知识,包括太阳系与地球的形成、初生地球时期的重要变化及其物质记录和生命的起源及早期进化,这成为修订冥古宙地层学属性的重要基础。修订后的冥古宙代表了地球演变历史的最早时段,即从太阳系和地球在T0=4567,Ma的形成,一直延续到地球上最古老岩石的出现(4030,Ma)。冥古宙还可被进一步划分为2个代:(1)“混沌代”(4567,Ma—4404±8,Ma);(2)“杰克山代”或“锆石代”(4404±8,Ma—4030,Ma)。由于没有保存相应的地层记录,因此冥古宙顶界(4030,Ma)与底界(4567,Ma)的年龄值,仅为一个计时性的年代界限。上述这个被赋予了明确地层学属性的冥古宙,不仅代表了前寒武纪地层学研究的一个重要进展,而且也为深入了解地球早期演变历史提供了许多重要的理念。  相似文献   

20.
The paper discusses a possible model of the ancient (Hadean-Archean) Earth’s geodynamic evolution. We believe that the early Earth was characterized by a stagnant lid regime and whole-mantle convection suggesting cells that convect through the whole mantle (from the core-mantle boundary to the lithosphere base). The lid tectonics was perturbed by asteroid-meteorite bombardments that destroyed the primary terrestrial partly granitoid crust. The destroyed crust together with the residual enriched mantle reservoirs sank into the lower mantle. In addition to the crust destruction, the bombardments led to emplacement of a huge proportion of basalt-komatiitic melts, which can be interpreted as mantle overturn events. In the Hadean, the Earth survived frequent large-scale asteroid-meteorite bombardments, which resulted in almost a complete destruction of the primary terrestrial crust. In the Early Archean, the Earth still experienced the same tectonic processes, as in the Hadean; however, meteorite impact was small-scale and the bombardments influenced only a limited area of a common, as it seems to us, subequatorial supercontinent. Those bombardments led to the sagduction of the Archean basalt-komatiiic terrestrial crust, which sank into the mantle, transforming into amphibolite-eclogite rocks giving rise to a tonalite-troondhjemite-granodiorite suite. As preserved in the zircon record, the formation of the Archean mantle-derived magmas occurred as pulses at 4.5, 4.2-4.3, 3.8-3.9, and 3.3-3.4 Ga. These peaks, most likely, correspond to the Hadean-Archean meteorite bombardments. There is evidence of formation of the subcontinental lithospheric mantle (SCLM) beneath the cratons between 3.3 and 3.5 Ga. This SCLM was markedly different from peridotites of modern ophiolites. However, the existence of ophiolitic peridotites indicates that modern style plate tectonic processes were in operation at that time, as we will discuss below. The transition from the early Earth (Hadean-Archean) tectonic style to the recent tectonics occurred between 3.4 (2.7?) and 2.0 Ga.  相似文献   

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