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Tracking heavy δ18O signatures in 3.7-3.6 Ga crust: a zircon and garnet record in Isua clastic metasedimentary rocks

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Gauthiez-Putallaz Laure, Nutman A P, Bennett V C, Rubatto Daniela,
Project Tracing the invisible path of fluids in the crust with microscale oxygen isotope measurements in key metamorphic minerals
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Original article (peer-reviewed)

Journal Chemical Geology
Volume (Issue) 537
Page(s) 119474
Title of proceedings Chemical Geology
DOI 10.1016/j.chemgeo.2020.119474

Open Access

Type of Open Access Repository (Green Open Access)


Elevated δ18O is used as a marker for the presence of continents and surficial alteration in the Eoarchean and Hadean. This study establishes a timeline for δ18O enrichment in Eoarchean metasedimentary rocks of the Isua supracrustal belt in Greenland. The source-rocks for the protolith of these metasedimentary rocks are mafic to intermediate magmatic rocks of ≥3709 ± 4 Ma, based on the age of zircons found in volcanogenic layers. The δ18O of +5.4 ± 0.4‰ of the zircon crystals indicate that the sources had a primary mantle-derived signature. However, garnet in two metasediments yields higher δ18O values of +8.7 to +9.7‰, in equilibrium with a whole-rock of +11 to +12‰ at 500–600 °C. This requires that the mafic protolith was weathered at surficial conditions, in agreement with previous conclusions based on major element geochemistry. The garnet grains with high δ18O record four growth zones, assigned to I) arc-building thermal metamorphism, II–III) terrane assembly at medium to high-pressure conditions, estimated to occur at 3660–3690 Ma and IV) late-Archaean overprint likely at ca. 2690 Ma. This shows that material with originally mantle-like δ18O was altered at low temperature (near-surface) to generate elevated oxygen isotope signatures and then recycled to middle-crustal conditions within 10–50 million years of crystallization in the Eoarchean. We propose that melting of such rocks could produce the zircon crystals with high δ18O that are found in the detrital and magmatic record in the Archean.