The iron isotope, trace and major element compositions of Eoarchean supracrustal rocks from southern West Greenland (Isua Supracrustal Belt, the islands of Akilia and Innersuartuut) were analyzed in order to identify protoliths and characterize the imprints of metamorphism and metasomatism. Banded iron formations (BIFs) from the Isua Supracrustal Belt (ISB) have trace element characteristics that are consistent with seawater derivation, including high Y/Ho ratios, positive Eu/Eu* anomalies, positive La/La* anomalies, and concave upward REE patterns. These rocks also have heavy Fe isotopic compositions relative to surrounding igneous rocks (∼+0.4‰/amu). The most likely interpretation is that this signature was inherited from partial oxidation in a marine setting of Fe emanating from a source similar to modern mid-ocean ridge hydrothermal vents (∼-0.15‰/amu). Banded quartz-rich rocks from the island of Akilia with high Fe/Ti ratios share many similarities with bona fide BIFs from Isua (heavy Fe isotopic compositions up to +0.4‰/amu, elevated Y/Ho ratios compared to igneous rocks, sometimes positive Eu/Eu* anomalies) suggesting a chemical sedimentary origin. Iron-poor metacarbonates from the southwestern part of the ISB have light Fe isotopic compositions (∼-0.4‰/amu). This is consistent with derivation of these rocks by fluid flow through surrounding ultramafic rocks and deposition as metasomatic carbonates. Iron-rich metacarbonates from the northwest and northeast parts of the ISB have Fe isotopic compositions (from +0.1 to +0.4‰/amu) and trace element patterns (high Y/Ho ratios, positive Eu/Eu* and La/La* anomalies, and concave upward REE) similar to associated BIFs. The most likely interpretation is that these iron-rich metacarbonates were derived from mobilization of Fe in BIFs by metasomatic fluids.
ASJC Scopus subject areas
- Geochemistry and Petrology