据悉，由于岩石样本的稀缺性，40亿年前(Ga)的冥古宙(Hadean Era)对于理解对生命进化至关重要的陆地风化和板块构造等地质过程提出了挑战。西澳大利亚的杰克山锆石是目前现存最早的冥古宙锆石样本，它通过微量元素特征为岩浆强度和构造成因提供了有价值的见解。但是，尚未就这些证据达成一致的结果。
为了解决这个问题，研究人员开发了一种能够破译锆石的地球化学指纹的机器学习分类器。这使得能够识别出最古老的碎屑锆石，这些锆石来自于沉积形成的“S型”花岗岩。结果表明，早在4.24Ga就存在S型花岗岩，从冥古宙一直持续到太古宙。研究地球历史上的全球碎屑锆石揭示了从现在到冥古宙持续的类似超大陆的循环。这些发现表明，大量的冥古宙大陆地壳暴露出来，风化成沉积物，并融入杰克山锆石的岩浆源中。只有早期的陆上风化作用和板块俯冲作用，才能解释研究所观察到的S型花岗岩的普遍存在。
此外，S型花岗岩比例的周期性演化意味着，俯冲驱动的构造旋回至少在4.2Ga的冥古宙附近非常活跃。证据表明，早期地球活跃的构造和可居住的地表条件方面与现代地球相似。这表明生命可能起源于温暖的池塘环境，而不是极端的热液环境。
附：英文原文
Title: Sediment subduction in Hadean revealed by machine learning
Author: Jiang, Jilian, Zou, Xinyu, Mitchell, Ross N., Zhang, Yigang, Zhao, Yong, Yin, Qing-Zhu, Yang, Wei, Zhou, Xiqiang, Wang, Hao, Spencer, Christopher J., Shan, Xiaocai, Wu, Shitou, Li, Guangming, Qin, Kezhang, Li, Xian-Hua
Issue&Volume: 2024-7-8
Abstract: Due to the scarcity of rock samples, the Hadean Era predating 4 billion years ago (Ga) poses challenges in understanding geological processes like subaerial weathering and plate tectonics that are critical for the evolution of life. The Jack Hills zircon from Western Australia, the primary Hadean samples available, offer valuable insights into magma sources and tectonic genesis through trace element signatures. However, a consensus on these signatures has not been reached. To address this, we developed a machine learning classifier capable of deciphering the geochemical fingerprints of zircon. This allowed us to identify the oldest detrital zircon originating from sedimentary-derived “S-type” granites. Our results indicate the presence of S-type granites as early as 4.24 Ga, persisting throughout the Hadean into the Archean. Examining global detrital zircon across Earth’s history reveals consistent supercontinent-like cycles from the present back to the Hadean. These findings suggest that a significant amount of Hadean continental crust was exposed, weathered into sediments, and incorporated into the magma sources of Jack Hills zircon. Only the early operation of both subaerial weathering and plate subduction can account for the prevalence of S-type granites we observe. Additionally, the periodic evolution of S-type granite proportions implies that subduction-driven tectonic cycles were active during the Hadean, at least around 4.2 Ga. The evidence thus points toward an early Earth resembling the modern Earth in terms of active tectonics and habitable surface conditions. This suggests the potential for life to originate in environments like warm ponds rather than extreme hydrothermal settings.
DOI: 10.1073/pnas.2405160121
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2405160121