Metasomatic origin of quartz-pyroxene rock, Akilia, Greenland, and implications for Earth's earliest life.
A quartz-pyroxene rock interpreted as a banded iron formation (BIF) from the island of Akilia, southwest Greenland, contains (13)C-depleted graphite that has been claimed as evidence for the oldest (>3850 million years ago) life on Earth. Field relationships on Akilia document multiple intense deformation events that have resulted in parallel transposition of Early Archean rocks and significant boudinage, the tails of which commonly form the banding in the quartz-pyroxene rock. Geochemical data possess distinct characteristics consistent with an ultramafic igneous, not BIF, protolith for this lithology and the adjacent schists. Later metasomatic silica and iron introduction have merely resulted in a rock that superficially resembles a BIF. An ultramafic igneous origin invalidates claims that the carbon isotopic composition of graphite inclusions represents evidence for life at the time of crystallization.
Fedo
Christopher M
CM
Department of Earth and Environmental Sciences, George Washington University, Washington, DC 20052, USA. cfedo@gwu.edu
Whitehouse
Martin J
MJ
D002247:Carbon Isotopes; D003460:Crystallization; D018534:Earth, Planet; D004602:Elements; D005580:Fossils; D019015:Geologic Sediments / Q000737:chemistry; D006108:Graphite / Q000032:analysis; D006116:Greenland; D007501:Iron / Q000032:analysis; D019369:Life; D008903:Minerals / Q000032:analysis; D011791:Quartz / Q000032:analysis; D013995:Time; D014131:Trace Elements / Q000032:analysis
D002247:Carbon Isotopes; D004602:Elements; D008903:Minerals; D014131:Trace Elements; C092478:pyroxene; D011791:Quartz; D006108:Graphite; D007501:Iron