ISUA,
GREENLAND BIF
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).