Isua gneiss

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The Isua Greenstone Belt is a rock formation of Archean in the Canadian Shield north-east of Nuuk on the edge of the Greenland ice sheet . With the Isua greenstone belt, the rock formation contains rocks around 3.8 billion years old, which for a long time were considered to be the oldest preserved rocks on earth. There are indications in them that the geological processes already then, around 700 million years after the formation of the earth , resembled those of today. The existence of traces of organic processes in the Isua rocks as evidence of early life is still controversial.

Location and geological framework

The rock formation of Isua- gneisses belongs geologically to Nain craton , part of the North Atlantic Craton in eastern Canada and Greenland. The North Atlantic craton is continued in the Karelian craton in Finland , from which it has been separated since the Jurassic when the Atlantic was formed .

The Isua rocks are located in the Itsâq gneiss complex, which stretches from Nuuk to Isua over a length of about 150 km. It is possible that the rocks of the Aasivik Terran, located almost 100 km to the north, also belong to the Itsâq gneiss complex, but a possible connection is hidden under the ice of Greenland.

In meso to neoarchaischen Isua- greenstone belt early-archaic rock complexes are as small enclaves in the so-called Amîtsoq-gneiss. The largest and best preserved of them is that of Isua, which is known for its great age and the occurrence of sediments formed on the surface (supracrustal) . It is a rock body about 30 kilometers long and 1 to 5 kilometers wide, relatively little deformed mica schist , which stretches in a large, almost circular arc from the edge of the inland ice to the west.

The deposit is located almost 20 km southeast of the locality of the same name, directly north of the Ataneq mountain and about 10 km north of the Qallunaatsiaat glacier ; it is divided into an eastern and a western deposit by a lake. The next inhabited places - Neriunaq and Kapisillit - are about 60 km further south and southwest and are small settlements with few inhabitants.

In terms of plate tectonics, this could be the oldest ophiolite ( oceanic crust that is pushed onto continents ) on earth.

History of exploration

Towards the end of the 1960s, during a geophysical survey of an area in western central Greenland from an airplane, a major magnetic anomaly was discovered at the edge of the ice sheet. Subsequent investigations revealed a large deposit of iron ore in a belt of highly metamorphic rocks that was later named the Isua Greenstone Belt after a location nearby . In the 1970s and early 1980s the area was extensively surveyed and mapped in detail by scientists from the Geological Survey of Denmark and Greenland (GEUS) . In 1988 the first geological map of the area was published.

As early as the early 1970s, the geologist Vic McGregor pointed out the similarity between the Amîtsoq gneiss near Nuuk and the recently found Isua rocks. In the Amîtsoq gneisses he had found small areas of older rocks that could be traced back to a formation on the earth's surface. He assumed that these rocks , which he called the Akilia series , were older than the Amîtsoq gneisses, and compared them with the rocks near Isua that were much better exposed. First age dating had shown with 3.6 to 3.7 billion years old age for Amîtsoq gneisses. So if the Akilia sequence was equated with the Isua rocks, they must have been older. This was confirmed on the basis of subsequent dating which determined the Isua rocks to be 3.7-3.8 Ga in age.

Since then, they have been investigated by several research groups under various questions, and from 1998 to 2001 they were the subject of the Isua Multidisciplinary Research Project (IMRP).

Geological situation and rocks

The Isua greenstone belt can be divided into five structural units, one of which in the east and one in the west consists of little deformed rocks, in which supracrustal rocks are relatively well preserved.

The south-east-north-west stretched western deposit is divided into two outer mica schist units by geological faults , which surround a central core of chloritic amphibole mica schists with remains of volcanic pillow lava structures, metamorphic silica schists , conglomerates and ribbon ores . This central core is relatively little deformed compared to the accompanying mica schists. The western deposit is stretched in a north-south direction and contains, among other things, ribbon ores, carbonate rocks with magnetite and siderite ribbons and clastic sediments .

Geochemical analyzes of the rocks showed a distribution of the rare earths that is compatible with the formation of acidic volcanic rocks . The first dating was carried out on individual zircons and showed an age of 3.761 to 3.782 Ga (billion years).

The original rocks of the gneiss and mica slate were subject to metamorphosis at 450–600 ° C after their deposition and were intensely folded and scaled , so that a real stratigraphic sequence has probably not been preserved anywhere . The archaic rocks in younger, granitic to tonalitic gneisses have been largely spared from intensive deformation and metamorphosis as countless small and kilometer-sized clods, so that in some areas of lower stress sedimentary and volcanic features such as graded stratification , ignimbrite stratification or pillow Lava have been preserved.

The entire geological sequence of events in the formation of the Isua greenstone belt is as follows:

Both the granitic gneisses and the mica schist are penetrated by a swarm of meta- doleritic dykes , which emerged from around 3.5 Ga as the conclusion of the geological formation. Since then, the Isua gneisses have remained essentially unchanged.

meaning

In contrast to the igneous rocks of the up to 4.03 Ga old Acasta gneiss in northern Canada, but similar to the possibly up to 4.3 Ga old rocks of the Nuvvuagittuq greenstone belt on Hudson Bay , the Isua rocks were originally on deposited on the earth's surface. Although the exact deciphering of their geological history is not easy, they allow important conclusions to be drawn about the still young earth. For example, the interpretation of scree rocks as a conglomerate is difficult due to the strong deformation, but is now recognized by many geologists. Such rocks, like pillow lavas, require the existence of a hydrosphere to form . Their occurrence is an indication that geological processes took place similar to today.

This also applies to plate tectonic processes. The mapping and dating has shown that the supracrustal rocks have been pushed together in stacks of originally disjointed units and may represent a duplex structure . The stacking of rocks that do not belong together is a typical feature of younger orrogens , which were created by plate tectonic processes. The study of fluid inclusions in minerals also allows conclusions to be drawn that hydrothermal systems already existed then, such as those that exist today near the mid-ocean ridges .

In addition, the Isua rocks give clues to the early phase of the earth's formation. The end of the Great bombardment (Late Heavy Bombardment, LHB), a time for which a heavy bombardment of the earth by meteorites is believed to dating could in lunar rocks are estimated at approximately 3.8 to 3.83 Ga. The Isua gneisses are therefore only about 30 to 60 million years younger, and so far no indications of the effects of the LHB have been found, in particular not on the destruction of an existing atmosphere or water cover. Since the original deposit date of the Isua gneiss parent rocks is not known exactly, it could be that they were deposited during the end of this event.

Early life signs?

Since the late 1970s, some of the researchers were of the opinion that there were signs of carbon of organic origin in some rocks of the Isua gneiss , and that life could have existed as early as this time. This assumption was based on unusually low values ​​for δ 13 C in graphites , which occur mainly in carbonate-rich rocks. This depletion of 13 C is also typical for material produced in biological processes, since photosynthesis leads to an accumulation of 12 C isotopes .

After it became clear that some of the rocks were metasomatically formed, Mark A. van Zuilen and his colleagues showed in a study published in 2003 that at least part of the carbon probably originated from the thermal transformation of siderite . Further investigations did not rule out an organic origin through methane production and photosynthesis for other deposits, especially in the conglomerate units in the west and the ribbon ores in the east .

literature

Individual evidence

  1. ↑ McDonald 2005, p. 197. Map of the Isua green stone belt
  2. ↑ Harald Furnes, Minik Rosing , Yildirim Dilek, Maarten de Wit: Isua supracrustal belt (Greenland) - A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology , Lithos, Volume 113, Issues 1–2, November 2009 , Pages 115-132, ISSN  0024-4937 , doi : 10.1016 / j.lithos.2009.03.043 . ( http://www.sciencedirect.com/science/article/pii/S0024493709001170 )
  3. ↑ a b Douglas Page: The Isua Rocks. ( Memento of the original from May 15, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (English) @1@ 2Template: Webachiv / IABot / home.earthlink.net
  4. ↑ a b c d e f Peter WU Appel, Christopher M. Fedo, Stephen Moorbath and John S. Myers: Early Archaean Isua supracrustal belt, West Greenland: pilot study of the Isua Multidisciplinary Research Project. ( Memento of the original from December 7, 2008 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Geology of Greenland Survey Bulletin, Vol. 180, pp. 94–99, 1998 (PDF; 388 kB) @1@ 2Template: Webachiv / IABot / www.geus.dk
  5. ↑ Hugh Rollonson et al .: A 3.75 GA Sea-floor hydrothermal system from the Isua Greenstone Belt, West Greenland. Earth System Processes - Global Meeting (June 24-28, 2001)
  6. ^ Mark A. van Zuilen, Aivo Lepland, Jane Teranes, John Finarelli, Martin Wahlen, Gustaf Arrhenius: Graphite and carbonates in the 3.8 Ga old Isua Supracrustal Belt, southern West Greenland. ( Memento of the original from June 26, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Precambrian Research, Vol. 126, pp. 331–348, 2003 (PDF; 916 kB) @1@ 2Template: Webachiv / IABot / www.arrhenius.ucsd.edu
  7. ↑ McDonald 2005, p. 202

Web links

  • Douglas Page: The Isua Rocks. (English)
  • Geological maps of Greenland 1: 500,000. Map sheet no. 2, Frederikshåb Isblink -Søndre Strømfjord, JH Allaart, 1982. Descriptive text by F. Kalsbeek and AA Garde, 1989. Geological map of the Geological Survey of Denmark and Greenland (GEUS)

Coordinates: 65 ° 5 ′  N , 50 ° 2 ′  W