中国科学院海洋研究所机构知识库

地球是太阳系中已知的唯一存在板块构造的行星，板块构造的演化对大陆地壳生长、造山作用过程、地幔的热状态变化、地球内部物质循环、以及地表环境等都有非常深远的影响。地球早期的上地幔温度很高，难以形成坚固稳定的大洋板块来维持持续性的俯冲作用。随着地幔温度的逐渐降低，地球的板块构造样式在新元古代晚期发展为以全球范围持续性的深俯冲、冷俯冲为特征的现代板块构造。

地幔温度是控制岩石圈物理性质和化学组分、软流圈深度，以及板块构造样式的关键因素。前人研究表明，地球的地幔温度从中太古代(约40-25亿年前)开始逐渐降低，但在此之后地幔温度的详细变化过程，目前还鲜有研究。本文统计了全球陆内玄武岩的地球化学成分，以玄武岩的碱性指数[A.I. = (Na₂O + K₂O)²/(SiO₂ – 38)]来反映玄武质岩浆形成时的温度和压力条件，从而限定了新元古代-显生宙这10亿年来地幔潜在温度(T_P)的演化规律。统计结果显示，T_P在新元古代早期稳定在约1450 °C左右；而在雪球地球事件(约720-635百万年前)开始以后，T_P在约180百万年的时间内迅速下降了约50 °C。T_P在这一时期的突然降低很可能是现代样式的板块构造在全球范围启动的结果。雪球地球事件期间，大量的沉积物被运移堆积于海沟，对板块俯冲界面起到了润滑作用，从而加速了俯冲过程，大量的冷的大洋岩石圈被输送至地幔，导致地幔迅速降温，触发了现代板块构造的开始。

现代板块构造的开始对碳酸盐俯冲和深部碳循环具有深远的影响，而这方面的研究目前还相对薄弱。本文基于全球基性-超基性岩浆岩的统计学分析，发现埃迪卡拉纪结束后出现了大量的霞石质岩浆活动。霞石类岩石是硅不饱和的高碱性岩石，通常形成于含碳酸盐地幔的低程度部分熔融。本文将霞石质岩石全球范围内的喷发归因于地幔的加速降温和俯冲碳酸盐通量的增加。晚元古代末期，现代板块构造开始后，大量冷的大洋和大陆物质俯冲进入地幔造成地幔温度迅速降低，减弱了俯冲碳酸盐在浅部的脱碳作用，使更大通量的碳酸盐俯冲至深部地幔，从而导致大量霞石类岩石的形成。由此产生的霞石类岩石和其它高碱性岩石具有较高的氧逸度，因此现代板块构造开始后，地幔温度的迅速降低和深部碳循环的增强可能会对显生宙以来地表较高的氧气含量有所贡献。

综上所述，本文的研究结果表明，现代板块构造在新元古代末期的启动造成了地幔温度的快速降低和地球深部碳循环作用的增强。

Earth is the only known planet in the solar system where plate tectonics exists, though plate tectonics was not born with it. The evolution of plate tectonics is closely related to the formation of continental crust, the orogenesis, the evolution of thermal history and the material recycle between Earth’s surface and interior. The mantle temperature in the early Earth was relatively too high to allow the steady subduction of the oceanic slab. With the cooling of the mantle, the regime of Earth’s tectonics has transited from the early plate tectonics to modern plate tectonics, which is characterized by the deep and cold subduction network.

The temperature of the convecting mantle exerts a first-order control on the rheology, composition, thickness of Earth’s lithosphere, and consequently, tectonic regime of Earth. Although the mantle has likely been cooling since the Archaean eon (4.0-2.5 billion years ago), how mantle temperature has evolved thereafter is poorly understood. Here, we apply a statistical analysis to secular changes in the alkali index [A.I. = whole-rock (Na₂O + K₂O)²/(SiO₂ – 38) as weight%] of global sodic intra-continental basalts, a proxy for the pressure and temperature of magma generation, to constrain the evolution of mantle potential temperature (T_P) over the past billion years. Our results show that, during the early Neoproterozoic, T_P remained relatively constant at ca. 1450 °C until the Cryogenian (720-635 million years ago), when mantle temperature dropped by ca. 50 °C over less than 180 million years. This remarkable episode of cooling records the onset of modern-style plate tectonics characterized by continuous deep subduction of the lithosphere, consistent with the widespread appearance of blueschists in the metamorphic rock record. The emergence of modern plate tectonics is suggested to have been triggered by a huge increase in the supply of sediments to lubricate trenches during the thawing of the Snowball Earth, which rapidly enhanced mantle cooling due to subduction of much larger volumes of cold oceanic lithosphere than previously.

The onset of modern plate tectonics and its influence on the subduction of carbonates and deep carbon cycle has not been fully understood. Here we apply statistical analysis on a continental ultramafic-mafic igneous rock database and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates recycled into the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The decreased upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.

In summary, the establishment of modern plate tectonics since the late Neoproterozoic contributed to enhanced mantle cooling and deep carbon cycle.