Origins of high δ18O in 3.7–3.6 Ga crust: A zircon and garnet record in Isua clastic metasedimentary rocks ...

• Main Authors: Gauthiez Putallaz, Laure, Nutman, Allen, Bennett, Vickie, Rubatto, Daniela
• Format: Text
• Language: English
• Published: Elsevier
• Subjects: 550 Earth sciences & geology; Greenland
• Online Access: https://dx.doi.org/10.7892/boris.145542; https://boris.unibe.ch/145542/

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, ...