Unravelling the evolution of continents using detrital zircons from modern rivers




Zhu, Ziyi

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Detrital zircons collected from Earth's modern rivers provide the most representative samples to study the evolution of the continental crust on a global scale. Most of the eroded material in modern rivers has experienced long-time sediment-sediment recycling, and thus they contain detrital zircons from source rocks of diverse origins, including the crustal segments that are currently inaccessible by surface geology. Zircon is a common accessory mineral that retains the isotopic composition of the host magmas from which it crystallized through multiple cycles of erosion, transport and sedimentation. A range of trace elements and isotopes in zircon can be analysed by ICP-MS and ion microprobe, which provide crucial information about history of the Earth and the evolution of its continents. The U-Pb isotopic system can be used to determine the crystallization age of the zircon, O isotope is indicative of the sedimentary contribution to the zircon parental magma, and Lu-Hf isotopes are used to infer the time elapsed since the crustal source region, which melted to form the zircon-hosting magma, was separated from the mantle. This study consists of three projects. The first is to use phosphorous and rare earth element compositions to distinguish between detrital zircons from igneous (I-type) and sedimentary (S-type) sources, in a global compilation of 6911 detrital zircons from 52 major rivers. These data give a large-scale insight into the generation and composition of felsic magmas on a global scale. The relative abundances of I-type and S-type zircons are used to assess the varying contributions of I- and S-type magmas to the continents over geological time. The second project uses the Lu content of zircons to identify those that come from the high pressure to ultra-high pressure roots that underlie high, Himalayan-type mountains. Two periods of extensive high mountain (supermountain) formation are identified. The first at 2.0-1.8 billion years ago (Ga) is associated with the formation of the Nuna supercontinent, which coincides with the emergence of the first macroscopic fossils at approximately 1.9 Ga. The second occurred 650-500 million years ago (Ma), and is associated with the formation of the Gondwana supercontinent. This event is coeval with the emergence of animal-like organisms at 575 Ma and the Cambrian Explosion at 540 Ma. We argue that these evolutionary steps were advanced by the transport of abundant bio-essential nutrients (e.g. P, Fe) into the ocean, which resulted from rapid erosion of supermountains. The third project uses the combined U-Pb, Lu-Hf and O isotopic data of detrital zircons from European rivers to identify periods of crustal growth and to calculate the growth rate of the preserved European continental crust. This is the first attempt to study the growth history of the whole European continent. The bootstrap method is introduced for calculating model age uncertainties. The results show that the growth of the preserved European continental crust started at ca. 3.5 Ga, and that about 50% of the present crustal volume had formed by the late Paleoproterozoic.






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