Paper No. 1
Presentation Time: 1:35 PM
CRITICAL ASSESSMENT OF THE GEOLOGIC AND ISOTOPIC EVIDENCE CONCERNING EARTH'S OLDEST ENVIRONMENTS AND CLAIMS FOR LIFE ON AKILIA, SW GREENLAND
At > 3.7 Ga, Earth's oldest known supracrustal rocks, comprised dominantly of mafic igneous with less common sedimentary units including banded iron formation (BIF), are exposed in southwest Greenland. After regional intruson by younger tonalites, both were intensely dynamothermally metamorphosed to granulite facies in the Archean then subsequently at lower grades until about 1500 Ma. Claims for the first preserved life on Earth have been based on the occurrence of > 3.83 Ga isotopically light C occurring as graphite inclusions within apatite crystals from a purported 5 m thick BIF on the island of Akilia. Geologic mapping and observations indicate the banding, first claimed to be depositional, is clearly deformational in origin. Furthermore, the mineralogy of the supposed BIF, being dominated by pyroxene, amphibole and quartz, is unlike well known BIF from the Isua Greenstone Belt, with which it had been compared. Instead, the Akilia lithology resembles enclosing mafic and ultramafic igneous rocks modified by metasomatism and repeated metamorphic recrystallization. Such a scenario parsimoniously links the geology, whole-rock geochemistry, 2.7 Ga single crystal zircon ages in the unit, an approximately 1500 Ma age for apatites that lack any graphite, non-MIF sulfur isotopes in the unit, and an inconclusive Fe isotope signature. Any claims for life on Akilia no longer have any geologic, geochemical, or isotopic context that is coherent, and so can be rejected. More fruitful targets still exist in the nearby Isua Greenstone Belt, where continued study could yield much more information about Earth's earliest surface envrionments.