The Acasta gneiss, oldest-known rock on Earth

--- survivor from early Archean time, in the Northwest Territories

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Figures 1-2. Two slices of a sample of the Acasta gneiss. The smaller sample was cut from the polished face seen at right. The Canadian flag pin is 13 mm (half an inch) in width. This metamorphic rock is a textbook gneiss, segregated into lighter (quartz- and feldspar-rich, trace pyrite) and darker bands, and flecked with red mm-scale porphyroblasts of garnet. The rock has a strong directional banding (foliation). It has appreciable heft, but it is not appreciably magnetic.

The mineralogy appears to be a mixture of quartz, plagioclase feldspar and dark greenish-black hornblende (amphibole), with minor red almandine garnet, traces of fine-grained pyrite, plus (probably) some iron-titanium oxide such as ilmenite, and a film of biotite mica on late fractures cutting the foliation. Traces of secondary limonite occur around pyrite. The magnetic susceptibility is about 0.25-0.28x10-3 SI units. The larger slice weighs 443.94 grams, 116x80x20 mm, the smaller end piece is 205.69 grams, 100x60x32 mm.


"Rock of the Month #212, posted for February 2019" ---

Acasta gneiss, oldest-known rock on Earth:

is found in a small area of the Slave craton, a western province of the Precambrian shield of Canada. Rock that is approximately 4 billion years (4000 Ma, that is, million years) old is known over an area of about 40 km2, about 150 km south of the Arctic circle (Bowring and Williams, 1999; Zimmer,1999). The area of these oldest rocks lies at circa 65°10'N, 115°30'W, roughly halfway north from Yellowknife, on Great Slave Lake, to Kugluktuk, on the Coronation Gulf of the Arctic Ocean. These ancient rocks lie just east of a younger belt, the Wopmay orogen (detailed regional maps and description in Ootes et al., 2016). The world's oldest known rocks, though not especially colourful nor gemmy, are now increasingly known also to rockhounds and visitors to Yellowknife. Bastedo (2001) notes that early visitors to the region (explorers, prospectors) were rock-obsessed to the point that the local Dogrib people referred to them as "Kwet'i", or "rock people"!

The Story Develops

Careful applications of the science of geochronology, the determination of the ages of rocks, in the past 30 years affirm that the Acasta gneiss outcrops include rocks as old as 4000-4030 Ma. Bowring et al. (1989) analysed the isotopes of lead and uranium in zircon crystals from the region, using an ion microprobe, a mass spectrometer capable of high spatial resolution, to examine the chemistry of individual crystals, and of growth bands (rather like tree rings) within individual crystals. The rock samples had crystallized as tonalite or granite, igneous rocks that might not hold the imagination of non-specialists for very long. The mineralogy of early Archean rocks is summarized by Papineau (2010). Within these igneous rocks, now metamorphosed, recrystallized and deformed into gneiss, zircon crystals indicated that the original magmas had crystallized some 3962 Ma. Further studies on the uranium-lead, samarium-neodymium and lutetium-hafnium isotope systems added further detail to the story (Bowring and Housh, 1995; Amelin et al., 1999; Sano et al., 1999; Amelin et al., 2000; Patchett and Samson, 2005). Zircons as old 4030 Ma were found (Stern and Bleeker, 1998) and data hint at rocks as old as 4060 Ma (Bowring and Williams, 1999).

The Big Picture

Table 1 indicates the span of some of the isotopically-determined ages of some of the oldest rock, crystal and meteorite samples, as well as the rise of "modern" life. Earth has a long history! Source: MINLIB database and references therein. Note that there are many primitive meteorites that "set the clock" for the formation of the solar system, including Sun, planets, asteroids and comets. Dating to the late 1970s, there are some reports of apparent ages that predate the solar system. It is to be expected that such dates may, in principle, be possible, since there are about a dozen species of minerals found as presolar grains in meteorites. These quaintly-termed ur-minerals, such as diamond, graphite, corundum, moissanite (silicon carbide), forsterite and enstatite condensed in the atmospheres of large stars that predated the Sun, and thus ARE older than our solar system, though they tend to be tiny, and not amenable to direct dating. Starting from a simple menu of elements, planetary evolution has led to ever-larger numbers of minerals (Hazen, 2015; Hazen et al., 2015), of which we have to date (late 2018) some 5,400 individual species that are considered valid (clearly and uniquely identified)! At this time, the oldest terrestrial material we can recognize is discrete crystals of minerals such as zircon, that preserve isotope systems storing chronological data.

York (1993) wrote a popular account of some of these oldest rocks and the means by which their ages are estimated. More recently, an encyclopaedic review comes from Van Kranendonk et al. (2007), and more concise summaries are provided by Sankaran (2000) and Kamber et al. (2001). The oldest rocks, 4000 Ma and older, are now termed Hadean, an allusion to a distant, stark and uncomfortably hot world! An earlier term for the period without any surviving rocks, after the Earth formed, is the Priscoan. The table picks out 20-plus dates in our universe, mostly on Earth, that range from the Big Bang to the most distant origin of our own species. We have thus far persisted, at best, for a mere 0.015% of Earth's history. To use a well-known metaphor, a 24-hour clock, to represent the time elapsed since our planet formed, then if the story of Earth started at midnight, and stretches through the entire day, Homo sapiens, so-called, appeared in the last 15 seconds of the last minute of that day!

Further, if you can imagine the Earth as being a whole year old, not just one day, then an early evidence of our planet-modifying powers, the Great Wall of China, was built in stages starting at 15 seconds to midnight on 31st December of that year! Back to the "one-day world", and saving a last word for the Acasta gneiss, it would have formed at 2:49 in the morning, a terrestrial early-riser.....

Table 1. Chronology of our World

Table 1. Chronology of Our World
Oldest date (for event / formation / region) Date (Ma)Notes
Big Bang _13820 Cosmological beginning
Allende- anomalous Ar-Ar dates on white inclusions __5100-4800 Presolar grains or (?) - meaning unclear
Oldest meteorites, formation of solar system __4567 Refractory inclusions and chondrules in Allende
Formation of the Moon by impact on Earth __4510 Dating the lunar magma ocean
Mount Narryer / Jack Hills zircons, Australia __4280 Detrital crystals in younger sediment
Acasta gneiss __4030 Oldest known terrestrial rocks
Hadean era ends __4000 Definition of the end of the oldest Precambrian epoch
Greenland __3807 Isua gneisses
Africa __3540 Barberton komatiites, South Africa
Minnesota, U.S.A. __3520 Morton gneiss, ancient Superior craton
Brazil __3420 Gaviao block - oldest rock in S.America
India __3400 Rocks in Anmodghat, Goa, west India
China __3200 Oldest rocks in Yangtze craton (?)
Ontario __2980 Agutua Arm assemblage, Sachigo subprovince
Archean-Proterozoic boundary __2500 Official boundary
"Great Oxidation Event" __2400 Approximate time of rise of the oxygen content of the atmosphere
Vredefort impact event, South Africa __2023 Largest impact in geological record
Sudbury impact event, Ontario __1850 2nd largest impact in geological record
Midcontinent Rift magmatism, N.America __1107 Suite of ultramafic intrusions, Lake Superior region
Ediacaran era begins ___635 The radiation of diverse life forms
Base of the Cambrian era ___541 Veritable explosion of new life!
Siberian flood basalts, Noril'sk-Putorana, Russia ___252 Permian-Triassic extinction event
Base of Jurassic period ___201 Age of Dinosaurs begins
Rise of angiosperms ___130 Flowering plants, Liaoning, China
Chicxulub impact event, Deccan volcanism in India ____66 KT boundary, demise of the dinosaurs (3rd largest impact...)
Oldest hominid _____6-4 Ardipithecus (et al.) diverge from primates
Rise of Homo sapiens _____0.7 Gradual divergence from Homo neanderthalensis

You can download the impressively detailed official "International Chronostratigraphic Chart" for yourself, in various languages. Many of the finer subdivisions have changed since I was in college! And even in 2018!

Acknowledgements

This special rock was kindly provided by the office of the Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, in Thunder Bay *. Special thanks to Greg Paju for his generous and dedicated help with sample acquisition and preparation. Great job! The Acasta gneiss is one of a small number of samples, and the only one from outside Ontario, in preparation for a museum display. This meets a request from the Museu Joias da Natureza in Santos, the port of the great city of Sao Paulo, Brazil. Next month's "Rock" will be another sample from this project, from the Thunder Bay area.

* It is important also to thank the original supplier of the material, the Northwest Territories Geological Survey.

References (n=18)

Bowring,SA, Williams,IS and Compston,W (1989) 3.96 G.a. gneisses from the Slave province, Northwest Territories, Canada. Geology 17, 971-975, November.

York,D (1993) The earliest history of the Earth. Scientific American 268 no.1, 90-96, January.

Bowring,SA and Housh,T (1995) The Earth's early evolution. Science 269, 1535-1540, 15 September.

Stern,RA and Bleeker,W (1998) Age of the world's oldest rocks refined using Canada's SHRIMP: the Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada 25, 27-31.

Amelin,Y, Lee,D-C, Halliday,AN and Pidgeon,RT (1999) Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons. Nature 399, 252-255, 20 May.

Bowring,SA and Williams,IS (1999) Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contrib.Mineral.Petrol. 134, 3-16.

Sano,Y, Terada,K, Hidaka,H, Yokoyama,K and Nutman,AP (1999) Palaeoproterozoic thermal events recorded in the ≈4.0 Ga Acasta gneiss, Canada: evidence from SHRIMP U-Pb dating of apatite and zircon. Geochim. Cosmochim. Acta 63, 899-905.

Zimmer,C (1999) Ancient continent opens window on the early Earth. Science 286, 2254-2256, 17 December.

Amelin,Y, Lee,D-C and Halliday,AN (2000) Early-middle Archaean crustal evolution deduced from Lu-Hf and U-Pb isotopic studies of single zircon grains. Geochim. Cosmochim. Acta 64, 4205-4225.

Sankaran,AV (2000) The quest for Earth's oldest crust. Current Science 79 no.7, 935-937, 10 October.

Bastedo,J (2001) Have a gneiss day. Up Here 17 no.7, 38-40, October.

Kamber,BS, Moorbath,S and Whitehouse,MJ (2001) The oldest rocks on Earth: time constraints and geological controversies. In `The Age of the Earth: from 4004 BC to AD 2002' (Lewis,CLE and Knell,SJ editors), Geol.Soc. Spec.Publ. 190, 177-203.

Patchett,PJ and Samson,SD (2005) Ages and growth of the continental crust from radiogenic isotopes. In `The Crust' (Rudnick,RL editor). Treatise on Geochemistry volume 3 (Holland,HD and Turekian,KK editors), Elsevier-Pergamon, Oxford, 683pp., 321-348.

Van Kranendonk,MJ, Smithies,RH and Bennett,VC (editors) (2007) Earth's Oldest Rocks. Elsevier, Developments in Precambrian Geology 15, 1307pp.

Papineau,D (2010) Mineral environments on the earliest Earth. Elements 6 no.1, 25-30, February.

Hazen,RM (2015) Mineral evolution: the great oxidation event, and the rise of colorful minerals. Mineral.Record 46, 805-812,834.

Hazen,RM, Grew,ES, Downs,RT, Goldem,J and Hystad,G (2015) Mineral ecology: chance and necessity in the mineral diversity of terrestrial planets. Can.Mineral. 53, 295-324.

Ootes,L, Jackson,VA, Davis,WJ, Bennett,V, Smar,L and Cousens,BL (2017) Parentage of Archean basement within a Paleoproterozoic orogen and implications for on-craton diamond preservation: Slave craton and Wopmay orogen, northwest Canada. Can.J.Earth Sci. 54, 203-232.

Graham Wilson, 14-19,22,28,30 November 2018, last amended 06 December 2018

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