EGU2020-6942, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-6942
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The lithologic composition of Earth’s emerged lands reconstructed from the chemistry of terrigenous sediments

Nicolas D. Greber1, Nicolas Dauphas2, and Matous P. Ptàček2
Nicolas D. Greber et al.
  • 1University of Bern, Institute of Geological Sciences, Bern, Switzerland (greber@geo.unibe.ch)
  • 2Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, USA

Despite its importance for the biological and atmospheric evolution of our planet, the question how the lithological composition of Earth’s landmasses evolved from around 3.5 Ga to present is still a matter of considerable debate. Furthermore, the type of rocks that build the continents are an expression of the prevailing rock forming mechanisms and thus the geodynamic regime that was at work. Therefore, a good understanding of the lithological and chemical composition of Archean continents is crucial to gain a comprehensive picture of Earth’s evolution.

Lately, the view that Earth’s continents were dominated by basaltic rocks until around 3.0 Ga became increasingly popular. The subsequent rapid transformation from mafic to felsic continents has been used to argue for the onset of plate tectonics at 3.0 Ga and to suggest that the change in the chemical composition of the emerged continents initiated the Great Oxidation Event. Here we present a summary of our work over the past three years (Greber et al., 2017, Greber and Dauphas 2019, Ptáček et al., accepted) that challenges this view. Reconstructing the composition of past continents is difficult because erosion and crustal reworking may have modified the geologic record in deep time, so direct examination of the nature of igneous rocks could provide a biased perspective on the nature of the continents through time. A less biased record is provided by terrigenous sediments that average the composition of rocks exposed to weathering on emerged lands. We use the Ti isotopic, major and trace element composition of fine grained terrigenous sediments (shales) as a proxy for the average composition of the emerged continents in the past. Our model shows that since 3.5 Ga, the landmasses that were subjected to erosion were dominated by felsic rocks. Furthermore, our reconstructed relative abundance of felsic, mafic and komatiitic rocks in the Archean is close to that currently observed in Archean terrains. The combination of Ti isotopes and element abundances also indicates that the rocks exposed to weathering in the Archean resemble that of modern type calc-alkaline rocks and that tholeiitic rocks (e.g. Icelandites) were of subordinate importance.

To summarize, the lithological composition of the Paleoarchean continents should no longer be used as argument against the existence of subduction zones at that time. Instead, their nature rather supports that some form of subduction process was already operating since the early Archean.

References:  Greber N.D., et al (2017), Science 357, 1271–1274; Greber N.D. and Dauphas N. (2019), GCA 255, 247–264; Ptáček M.P., Dauphas N. and Greber N.D. (accepted), EPSL.

How to cite: Greber, N. D., Dauphas, N., and Ptàček, M. P.: The lithologic composition of Earth’s emerged lands reconstructed from the chemistry of terrigenous sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6942, https://doi.org/10.5194/egusphere-egu2020-6942, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-6942, Els Ufkes, 04 May 2020

    Dear Nicholas,

     

    Thank you for your presentation.

    This has huge consequences for the current textbook models on continent formation.

    Apparently large felsic continents were already present during the (Early) Archean. No formation of felsic continents is needed via welding of island arcs in combination of inclusion of marine sediments in subducted ocean floor and subsequent magmaformation.

    However, where did this felsic material originates from, specific pools of magma due to ...??

    Thank you for your response.

     

    With kind regards,

    Els Ufkes

    • AC1: Reply to CC1, Nicolas Greber, 07 May 2020

      Dear Els Ufkes

      Thank you for your interest in our work. The question in what setting you can produce significant amounts of felsic magma is formed is hotly debated. Comparing to modern settings, plume-related magmatism like in Iceland can produce around 10% felsic melt. Subduction settings on the other hand are more efficient in producing felsic rocks. Some more recent models suggest, that in early Archean times plume related magmatism could have produced more felsic rocks than nowadays, but some other also say it is not possible.

      Another idea is, that the firs felsic rocks were produced in an Iceland type setting, producing stable felsic cratonic nuclei (~4.0 Ga). These nuclei then allowed for subduction zones to initiate, leading to felsic continents by 3.5 Ga (see e.g. the work by Jesse Reimink et al., 2019, Geochem. Perspect. Lett., v10).

       

      Best wishes

      Nicolas