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Hf-Nd Isotopes in Archean Marine Chemical Sediments: Implications for the Geodynamical History of Early Earth and Its Impact on Earliest Marine Habitats

Department of Geodynamics and Sedimentology, University of Vienna, Althanstraße 14, Vienna 1090, Austria
Geosciences 2018, 8(7), 263; https://doi.org/10.3390/geosciences8070263
Received: 30 May 2018 / Revised: 5 July 2018 / Accepted: 9 July 2018 / Published: 16 July 2018
(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)
The Hf-Nd isotope systems are coupled in magmatic systems, but incongruent Hf weathering (‘zircon effect’) of the continental crust leads to a decoupling of the Hf-Nd isotope systems in low-temperature environments during weathering and erosion processes. The Hf-Nd isotope record was recently dated back from the Cenozoic oceans until the Archean, showing that both isotope systems were already decoupled in seawater 2.7 Ga ago and potentially 3.4 Ga and 3.7 Ga ago. While there might have existed a hydrothermal pathway for Hf into Archean seawater, incongruent Hf weathering of more evolved, zircon-bearing uppermost continental crust that was emerged and available for subaerial weathering accounts for a significant decoupling of Hf-Nd isotopes in the dissolved (<0.2 µm) and suspended (>0.2 µm) fractions of Early Earth’s seawater. These findings contradict the consensus that uppermost Archean continental crust was (ultra)mafic in composition and predominantly submerged. Hence, Hf-Nd isotopes in Archean marine chemical sediments provide the unique potential for future research to trace the emergence of evolved continental crust, which in turn has major implications for the geodynamical evolution of Early Earth and the nutrient flux into the earliest marine habitats on Earth. View Full-Text
Keywords: Hf-Nd isotopes; Archean; seawater; zircon effect; Hf-Nd decoupling; incongruent weathering; emergence of continents; early life Hf-Nd isotopes; Archean; seawater; zircon effect; Hf-Nd decoupling; incongruent weathering; emergence of continents; early life
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Viehmann, S. Hf-Nd Isotopes in Archean Marine Chemical Sediments: Implications for the Geodynamical History of Early Earth and Its Impact on Earliest Marine Habitats. Geosciences 2018, 8, 263.

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