Here's one of the oldest rocks on Earth.  The Isua Supracrustal Belt in southwestern Greenland has been the subject of much geologic research.  Some geologists have concluded that the carbon isotopic composition of graphite found in some Isua rocks is consistent with a biogenic origin.  In other words, some Isua rocks have been claimed to contain chemofossils.  If so, they would be the oldest evidence for life on Earth, being 3.8 billion years old - the oldest known undisputed fossils are 3.5 billion years old stromatolites and bacterial body fossils from western Australia and southern Africa.


Below is an Isua rock, specifically a weakly metamorphosed banded iron formation (BIF).  The light-colored layers are rich in finely-crystalline quartz.  The darker layers are rich in iron oxide.  This rock, and other Isua rocks, contain varying amounts of the mineral graphite (C).  Most Isua researchers have concluded that the graphite in rocks of the Isua Supracrustal Belt is inorganic in origin, and formed by various metamorphic chemical reactions (for example, metamorphic decomposition of siderite, FeCO3, results in the formation of graphite).


Isua Supracrustal Belt banded iron formation (BIF) that dates to 3.8 b.y. (Eoarchean) (FMNH Li 9223, Field Museum of Natural History, Chicago, Illinois, USA).




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