早期地球的热管构造:来自木卫一的启示

构造体制极大地制约着地球和其他太阳系类地天体（类地行星、岩石质卫星和小行星等）的地表散热、内部温度和物质演化。现有的少量地质记录表明，地球在板块构造启动之前就存在非常活跃的"前板块构造"运动并可能对其早期壳幔分异产生了重要的影响，在这些构造体制下，物质和能量循环的规模和速率可能是后续的板块运动无法比拟的。但受限于早期地质记录的稀缺以及研究手段不成熟等因素，对前板块构造运动的研究一直被学界所忽视，人们对其的认识主要局限于停滞盖层（stagnant-lid tectonics）等。长期以来的空间探测和地基观测表明，木星系统的木卫一存在大规模的火山活动，随之形成了极高的地表热流和地表更新速率以及活跃的造山作用。这些观测事实不同寻常，颠覆了人们对类地天体构造演化模式的一些固有认识，需要新的构造模式——"热管构造"（heat-pipe tectonics）予以解释，其涵义为：类似木卫一上的大规模火山作用可使类地天体的软流圈-岩石圈-地表之间发生快速的物质和能量循环，该循环以岩浆的形成-上升-喷发-冷却和沉降-折返为主要形式，可将天体内部的热散快速散发到外太空。上述过程涉及类地天体内、外部之间物质的大规模、快速迁移和相变，其导热原理与热管相同，因而被称为"热管构造"，其散热效率远高于现今大多数类地天体单纯依赖岩石圈进行内外热传导的停滞盖层构造，以及地球上以板块的形成和俯冲过程主导内部散热的板块构造体制。尽管早期地球与木卫一在内生热机制等方面存在显著差异，但二者的内部温度和内生热率较高，导致其岩浆作用总体均较为活跃，这些关键动力学特征的相似性暗示其构造体制可能类似。因此，研究木卫一的热管构造体制对揭示地球的前板块构造的性质和演化有重要的启示意义。本文综述了近40年来人类对木卫一的主要探测成果，论述了热管构造提出的必要性和依据，总结了该构造体制的特征和发生条件，讨论了早期地球发生热管构造的可能性。早期地球可能经历了热管构造阶段，期间地球通过大规模火山作用散发了内部热量、促进了壳幔分异，并在地球内生热作用减弱、热管构造不能继续维持时被板块构造等取代。由于热管构造的垂向物质循环较为强烈，不利于保留TTG等低密度的壳幔分异产物，我们依据TTG大规模形成的时间上限推测：地球发生热管构造时间可能限于冥古宙-始太古代时期（约38亿年以前）。由于前板块构造时期地球自身的地质记录十分有限，对其热管构造体制的性质和确切的形成条件等很大程度上需要从木卫一获得答案。

Possible heat-pipe tectonics of the early Earth: Insights from Jupiter's moon Io

The tectonic regimes of the Earth and other terrestrial bodies (e.g., terrestrial planets, rocky moons and asteroids) dominates their rate of surface cooling, internal and compositional evolution. The sporadic geological records of the Earth show clues of the presence of very active "pre-plate tectonics" before the onset of plate tectonics, which resulted in rapid early crust-mantle differentiation. However, our understanding of the pre-plate tectonics confines to the conventional models, such as stagnant-lid regime, due to scarcity of geological records and incomplete means of researches when it comes to this "dark age" of the Earth. From a view of comparative planetology, however, results of over 40 years'' space explorations challenge our understanding of how terrestrial bodies evolves, e.g., by discoveries of large-scale volcanism, high surface heat flow and resurfacing rate on Jupiter''s moon Io. These exotic observations requires a new tectonic model, heat-pipe tectonics, to provide better explanations. The heat-pipe tectonics features rapid, vertical migration of energy and materials between the interior and the surface of a terrestrial body via large-scale volcanism, specifically through swift advection and phase changes of materials during generation, ascent and eruption of the mantle-derived magma/melts, as well as their subsequent return to the deep mantle after cooling, stacking and subsidence at the surface. As a result, the volcanism-dominated heat-pipe tectonics is capable of achieving much higher efficiency of heat transfer between the interior and the surface of Io than the conduction-dominating stagnant-lid tectonics or the plate tectonics cooling Earth''s interior via formation and subduction of plates. Despite the significant differences in term of internal heating mechanism between the early Earth and Io, they share much in common in the activeness of volcanic activities, intense internal heating and high internal temperature. Therefore, Io is of great reference value for understanding the pre-plate tectonic dynamics of the early Earth. In this paper, we reviewed the key discoveries of Io in the past 40 years, as well as the birth of the concept of heat-pipe tectonics on the basis of these observations. The possibility of heat-pipe tectonics on the early Earth was discussed. The distinctive vertical recycle of crust-mantle materials during operation of the heat-pipe tectonics tends to impede the formation and accumulation of the mantle-derived felsic components. Therefore, the upper limit age of large scale occurrence of TTG confines the possible heat-pipe tectonics to the Hadean-Eoarchean Earth (before ~3.8Gyr ago) and was replaced by other tectonic regimes in response to the decrease of its internal heating rate. To some extent, our further understanding of Earth''s possible heat-pipe tectonics depends on the knowledge of the Io''s on-going heat-pipe tectonics that is more liable to be acquired, due primarily to the scarcity of geological observations of early Earth itself.