Tracking continental emergence and weathering on the early earth : seawater Hf-Nd isotope constraints from precambrian iron formations

• Main Author: Foster, Ian
• Other Authors: Laboratoire Géosciences Océan (LGO), Université de Bretagne Sud (UBS)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS), Université de Bretagne occidentale - Brest, Stefan Lalonde
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2019
• Subjects: Continental emergence; Hafnium; Lutetium; Isotopes; Banded iron formations; Émergence continentale; Lutétium; Formations de fer en bandes; [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph]; Greenland
• Online Access: https://theses.hal.science/tel-03640207; https://theses.hal.science/tel-03640207/document; https://theses.hal.science/tel-03640207/file/These-2019-SML-Geosciences_marines-FOSTER_Ian.pdf

The emergence of continental crust abov sea level in the early Precambrian would have created the first terrestrial habitats, and initiated atmosphere driven weathering of the continents, yet the history of continental emergence is largely unknown. Precambrian chemical sediments, specifically Banded Iron Formation (BIF) previously studied, appear to have sampled the Hf-Nd isotope composition of ancient seawater, and maypreserve a historical record of the emergence of continental landmasses via Lu/Hf fractionation induced by subaerial differential weathering. However, paired Hf-Nd isotope data are available for only a few localities todate, but indicate appreciable emerged continental landmass ca. 2.7 Ga. Our work seeks to confirm andextend this record from the Neoproterozoic into the Paleo- and Eo-Archean using samples of 750 Ma BIFfrom the Braemar formation in central Australia, 3.2 Ga BIF from the Moodies Group, S. Africa, and 3.8 Ga BIFfrom Isua, Greenland, as well as several samples from Nuvvuagittuq Quebec, and Nulliak Labrador, Canada.Eoarchean samples appear to have been altered by amphibolite-grade metamorphism, however Moodies Groupsamples appear primary, having experienced significantly lower metamorphic grades. Moodies appear to retain their primary seawater signatures with !Hf values comparable to those observed in modern oceanic detrital and chemical deposits. Results from the Braemar formation are also interpreted as retaining primary !Hf values signatures and are found to have strikingly similar values to those recently reported for glacial and non-glacial river waters in Greenland. Together our studies suggest that emerged land masses have been a feature of the Earth since at least the Paleoarchean, despite theoretical models that suggest emergence occurred sometime after 3 Ga ago, and that the interpretation of !Hf and !Nd data from BIF is not as straightforward as previouslysuggested. A large range of !Hf values can be recorded in BIF that may represent not only a record of ocean water Hf and ...