Periodic Reporting for period 2 - GOforISOBIF (Probing the Geochemistry of the First Oceans with Novel ISOtope Proxies in Banded Iron Formations (BIFs): New Perspectives into Early Continental Weathering, Ocean Oxygenation and Mantle Geodynamics)
Période du rapport: 2021-09-01 au 2023-02-28
The oceans act as an interface between the Earth’s atmosphere, the continents and the deep mantle via surface processes, erosion, and hydrothermalism. The aim of the EU-funded GOforISOBIF project is to investigate the composition of early seawater recorded in Precambrian banded iron formations to provide unique insight into the processes that have affected the Earth’s major reservoirs at the beginning of our planet history. Using new and innovative geochemical tools, the project will focus on three first-order questions: 1) the origin, composition and size of the first continents; 2) the detection of whiffs of oxygen in the early oceans before the great oxidation event (GOE); 3) the relationship between hydrothermal activity and global mantle geodynamics.
The proposed project promotes the application of state-of-the-art isotopic measurements, with an emphasis on further methodological development to set-up new isotope proxies. The samples that will be analyzed in this project are amongst the most ancient remnants of sedimentary rocks discovered on Earth so far. They have never been subjected to such a detailed level of isotopic analyses and will open up new perspectives in our understanding of the Precambrian environment and geodynamics.
The proposed project promotes the application of state-of-the-art isotopic measurements, with an emphasis on further methodological development to set-up new isotope proxies. The samples that will be analyzed in this project are amongst the most ancient remnants of sedimentary rocks discovered on Earth so far. They have never been subjected to such a detailed level of isotopic analyses and will open up new perspectives in our understanding of the Precambrian environment and geodynamics.
The first half of the project was mainly focused on analytical development and improvement of existing methodologies that were necessary to achieve the objectives of the proposed in the project. In this context, we developed new methodologies and improved existing ones to measure isotopic compositions with higher precisions than previously done. We have worked on a method to precisely analyze the trace element contents of samples with high-Fe matrices such as banded iron formations (BIFs) using Q-ICP-MS and QQQ-ICP-MS. We have successfully calibrated a series of spike that will be useful to date the BIFs and determine their initial isotopic composition with high precision. We have made significant improvements on existing ion chromatography procedures to isolate rare earth elements from Fe- and Mn-rich matrices. Finally, we tested several mass bias correction techniques to get the highest possible precision for MC-ICP-MS isotopic analyses.
In parallel, we have worked on a large compilation of literature data and developed a new model to re-interpret the existing Sm-Nd isotopic record of ancient detrital sedimentary rocks. This work provides new constraints on the composition and formation of continents through time. It shows that the continents were felsic very early on and that they did not form continuously through Earth’s history but grew by pulses every 500 Ma. The results of this work add new piece of information to the big debate of continental crust formation and constitute a step forward in our understanding of continent evolution. These new constraints must now be considered in future studies to determine what kind of geodynamical models can be responsible for the episodic growth of felsic continental crust through Earth history. This work led to the publication of a paper entitled “Episodic growth of felsic continents in the past 3.7 Ga” in Science Advances.
In parallel, we have worked on a large compilation of literature data and developed a new model to re-interpret the existing Sm-Nd isotopic record of ancient detrital sedimentary rocks. This work provides new constraints on the composition and formation of continents through time. It shows that the continents were felsic very early on and that they did not form continuously through Earth’s history but grew by pulses every 500 Ma. The results of this work add new piece of information to the big debate of continental crust formation and constitute a step forward in our understanding of continent evolution. These new constraints must now be considered in future studies to determine what kind of geodynamical models can be responsible for the episodic growth of felsic continental crust through Earth history. This work led to the publication of a paper entitled “Episodic growth of felsic continents in the past 3.7 Ga” in Science Advances.
The analytical developments that we performed during the first years of the project, both on the chemical protocols and by optimizing the instrument settings, allowed us to reach higher precision than previously published for several isotopic ratios of rare earth element (REE), typically few tens of ppm (external reproducibility, 2SD).
We expect to measure significant REE isotopic fractionations in BIFs to study the nature of the Precambrian continental crust as well as redox conditions in the Precambrian oceans. The amplitude of REE isotopic fractionation will be particularly interesting when comparing samples deposited before and after 3 Ga because several studies suggested that it was the pivotal time during which the continental crust changed its composition from mafic to felsic. Comparisons between samples deposited before and after the GOE will also be critical to study changes in redox conditions in shallow- and deep-water environments.
We expect to measure significant REE isotopic fractionations in BIFs to study the nature of the Precambrian continental crust as well as redox conditions in the Precambrian oceans. The amplitude of REE isotopic fractionation will be particularly interesting when comparing samples deposited before and after 3 Ga because several studies suggested that it was the pivotal time during which the continental crust changed its composition from mafic to felsic. Comparisons between samples deposited before and after the GOE will also be critical to study changes in redox conditions in shallow- and deep-water environments.