摘要
冥古宙是地球历史的最初阶段,虽然仅持续不到6亿年,但在此期间发生巨大的物质运移和重新分布,地球的地核、地幔、地壳和大气层等主要圈层框架已基本形成,为地球后续漫长的物质和能量演变奠定了基础.由于关键地质记录缺失、研究手段匮乏等原因,当前地球科学界针对冥古宙地球演化的研究仍十分有限,使其成为地球科学最为薄弱的一环.鉴于冥古宙地球存在地质记录匮乏这一“痛点”,以及太阳系类地天体(泛指类地行星、矮行星、卫星和小行星等岩石质天体)在原始物质组成、内部结构、热演化和构造体制演变等方面的相似性,借助比较行星学手段,从太阳系内各类地天体(现今或早期)保存完好的地质记录研究类地天体演化的一般规律,以此弥补早期地球演化认识的空白.这是一种重要且可行的研究手段,也是突破现有地学理论框架(如板块构造理论)、开辟新的地学研究格局的重要契机.本文对早期地球的热演化、内部分异和构造体制等关键领域的研究进展及其存在的问题进行了梳理,同时探讨了未来可能的突破点.
The Hadean Eon,an elusive“dark age”of the Earth,is the earliest period in Earth’s history spanning less than 600 Ma,during which Earth’s magma ocean may have formed and solidified,the primordial core,mantle,crust and atmosphere may have formed,laying the groundwork for the Earth’s subsequent evolution.Among others,the thermal evolution might have massive impact on Hadean Earth’s evolution but remains poorly understood.This is even more true since there are intricate interactions among the secular cooling,evolution of tectonic regimes and crust-mantle differentiations of the early Earth.The types of the tectonics on a global scale(e.g.,the plate tectonics and stagnant-lid tectonics),which essentially involve ways of Earth’s cooling and dominate the vigor of interplays between the rigid lithosphere and mantle convection,are strongly temperature-dependent.On the other hand,the tectonic regimes in turn have large impacts on the Earth’s heat output,since the former could determine the mechanisms of heat transfer(i.e.,conduction,convection or radiation)between Earth’s surface and the interior,and the resultant cooling rates of these mechanisms could differ by orders of magnitudes.Similarly,the degree of differentiation of Earth’s interior directly relates to its temperature for a given composition and pressure,while the formation of the continents enriches the incompatible elements(esp.the radioactive heat production elements U,Th and K),and lower the heat input in the mantle.Consequently,only if the cooling history of the early stage of Earth is well constrained will the nature of Hadean tectonic regimes and ways of differentiation of the hot early mantle be revealed.Nevertheless,our understanding of Hadean Earth’s thermal evolution has been sluggish due to the scarcity of geological records of Earth itself.A remedy is to extrapolate the thermal evolution trend of the Hadean mantle from the available data of younger igneous rocks(with ages<3.5 Ga)by means of petrological geothermometers,the results of which show the presence of a counterintuitively cool early state in Earth’s interior before~3.5 Ga.Furthermore,the conventional theoretical approaches based on the stagnant-lid regime tend to exaggerate the cooling rate of early Earth’s and yield an unrealistically high temperature during the Hadean(i.e.,the so-called“thermal catastrophe”).It is likely that we have been misled either by the over-extrapolated results of thermal evolution or by the problematic theoretical estimations based on the improper scaling laws among the geotherm,the rigor of mantle convection and the convective cooling rate.The issues could be overcome by acquiring the ancient geological records or by observing the ongoing geological activities on other terrestrial bodies(i.e.,the rocky bodies such as terrestrial planets,dwarf planets,moons,and asteroids)in the Solar System.The well-preserved records accessible on these terrestrial bodies,such as the presence of 4.4 Ga-old anorthosite of the Moon that is suggestive possibly of its formation in a terrestrial magma ocean common in the early solar system,or the violent volcanism that indicates the prevalence of the heat-pipe tectonics on Jupiter’s moon Io,could provide crucial clues for the composition,heat budget and tectonics during Earth’s“dark age”,and boost our understanding of its early history.The method is feasible since there are clear similarities among Earth and these terrestrial bodies in their primitive compositions,internal structures,and significant generalities in their thermo-tectonic-chemical evolutions at least at certain stages in their geological history.In this paper,several key questions and their progresses in the thermal evolution,internal differentiation and tectonics of the early Earth are reviewed,with the remaining issues and the potential directions of breakthroughs in the future being discussed.
作者
刘耘
章清文
Yun Liu;Qingwen Zhang(State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China;International Center for Planetary Science,College of Earth Sciences,Chengdu University of Technology,Chengdu 610059,China)
出处
《科学通报》
EI
CAS
CSCD
北大核心
2023年第18期2284-2295,共12页
Chinese Science Bulletin
基金
中国科学院战略性先导科技专项(XDB18010100,XDB41000000)
国家自然科学基金(41804092,42130114)资助
关键词
冥古宙地球
热演化
化学分异
构造体制
比较行星学
Hadean Earth
thermal evolution
chemical differentiation
tectonic regime
comparative planetology
作者简介
联系人:刘耘,E-mail:liuyun@vip.gyig.ac.cn
