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Decoding Ancient Deep-Earth Geodynamics Using High-Mg Basalts From Central South China

Published by Xuan-Ce Wang

The School of Earth Science and Resources, Chang’an University, and Department of Applied Geology, Curtin University

These findings are described in the article entitled The 825 Ma Yiyang high–MgO basalts of central South China: Insights from Os–Hf–Nd data, recently published in the journal Chemical Geology (Chemical Geology 502 (2018) 107-121). This work was conducted by Tao Wu from Zhejiang University and Curtin University, Xuan-Ce Wang from Chang’an University and Curtin University, Wu-Xian Li and Jie Li from the Chinese Academy of Sciences, Simon A. Wilde from Chang’an University, Liyan Tian from the Chinese Academy of Sciences and Qingdao National Laboratory for Marine Science and Technology, and Chong-Jin Pang from the Guilin University of Technology.

Deeply-sourced lavas carry key information about our planet’s evolution – information that is crucial for understanding how it became a habitable one. Consequently, such deeply-sourced lavas are a valuable encyclopedia that can enable us to read the Earth’s history. But how precisely to read such a book is a very important question for geologists.

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In a case study, recently published with Associate Professor Xuan-Ce Wang and his team from Chang’An University and Curtin University, researchers reveal how to decode the ancient rock record. Here they show that the ca. The 825 million-year-old Yiyang basalts in central South China are deeply-sourced high-MgO lavas (called basaltic komatiites) and are direct products of Earth’s deep geodynamics and, thus, provide information on the thermochemical state of Earth’s interior. The team has presented strong evidence to show that some of ca. 825-810 million-year-old high-MgO basalts from South China were most likely produced under a relatively dry and hot environment.

Comprehensive new radiogenic isotopic data show that these high-MgO basalts have distinctive geochemical features when compared to those of high-MgO hydrous magmas (called boninite) that result from water fluxed melting at convergent plate margins. They are in fact much more similar to typical anhydrous, hot lavas, called komatiites. This conclusion is also supported by mineral textures of the Yiyang suite. Therefore, the authors consider that the ca.825 million-year-old Yiyang basalts were most likely produced by hot upwelling mantle (a plume) rather than by subduction. By integrating mineral analysis with an in-depth chemical and isotopic investigation it has been possible to show that some of the previous arguments in favor of a subduction origin resulted from using incorrect petrological and geochemical conceptions.

The most prominent feature of our planet is the continued material circulation between its surface and its interior. This is the first-order driving force of our planet that has continually reshaped it from the earliest of times and has to be taken into account when considering the question of what the Earth was like and how it worked in the past, as well as how it affected later processes and events. Addressing these issues is essential for establishing the thermal conditions of the dynamic systems of the Earth’s interior, the volatile content of the planet, and the origin of the continents.

Studies such as this, “The 825 Ma Yiyang high–MgO basalts of central South China: Insights from Os–Hf–Nd data” recently published in Chemical Geology, will improve our understanding of how the Earth evolved to become a habitable planet favorable for life. Since Earth is the only planet currently known to maintain life, it is essential to identify the key ingredients that help make this blue orb in space a unique ecosystem in our solar system.

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