Tracing Earth's Oldest Rocks
By Allan Fraser
How old is planet earth? Our Solar System formed from a vast cloud of gas and dust 4.65 billion years ago. The age of 4.65 billion years is well established from the decay rates of radioactive elements found in meteorites and rocks from the Moon as well an abundance of evidence from chemistry and physics. As Earth is a dynamic planet in which rocks are continuously being recycled by plate tectonics much of the primordial material from the time of the formation of the Earth is no longer around. If there are any of Earth's primordial rocks left in their original state, they have not yet been found. The oldest rocks found to-date on Earth are those of the Nuvvuagittuq greenstone belt and these give an age of 4.3 billion years. It was not until relatively recently that it has been possible to measure the age of rocks. The early ideas of the age of the Earth date to the ancient Greeks and Romans and it was not until the late 1700s that scientists begun to realise that the Earth was indeed ancient. However, it was not until the discovery of radioactivity and the invention of the mass spectrometer that the quantification of isotopes of various radiometric decay schemes could be performed. In recent decades improvements in detector technology and electronics in mass spectrometers has resulted in an improvement in the precision of analytical data translating to an increased confidence in the radiometric ages. The use of hyphenated analytical techniques such as laser ablation - mass spectrometry has also allowed the analysis of small sample sizes and individual crystals.
In the Beginning
The ancient Greeks and Romans realised that long time spans were required to lay down the thick layers of sediments observed and from this they estimated that the Earth was thousands of years old. But it wasn't until the late 1700s that scientific interest in geological age began when Scottish geologist James Hutton (1726-1797), who observed that sediments built up on landscapes were indeed indicative of an old Earth (Dalrymple, 1991). Before then, the Bible had provided the only estimate for the age of the world. Bishop James Ussher (1581 – 1656) established the time of “creation” to 6000 years. Using the book of Genesis as a history book, Ussher meticulously examined the genealogy of the Bible and concluded that the date of the creation as the night of Sunday, 23 October 4004 BC (McSween, 1997). Today some biblical scholars, as well as a number of literalist evangelical Christians, believe in a literal interpretation of the Bible calling for a 6000-year-old Earth (Barr, 1984). In 1785 Hutton published ‘Theory of the Earth’ in which he concluded that “slow” processes shape the Earth, mountains arise continuously as a balance against erosion and weathering and the physical and chemical laws that govern nature are uniform. Most geological processes are extremely slow, and evidence for slow change was everywhere; rivers eroded rock and rain inexorably wore away the tops of mountains and the slow movement of glaciers carved out entire valleys. Hutton and other contemporary scientists of the time concluded that the single most important factor why the Earth looks like the way is does was due to time, and lots of it. With this Hutton established the ‘Doctrine of Uniformitarianism: "Present is key to the past" (Ward, 1995). Hutton used fossils to establish relative ages of rocks. There was however a need to determine the absolute age of the Earth. In the late 19th Century this question was first addressed by William Thompson (Lord Kelvin, 1824 - 1907). Kelvin assumed that the Earth was originally molten and calculated a date for the age of the Earth using the then young science of thermodynamics. His calculation was based on the cooling of the Earth through conduction and radiation of heat. Kelvin’s age of Earth was calculated to be about 24-40 million years (Ward 1995). The problem with this view is that the Earth has an internal heat source from radioactive decay – a fact not known to Lord Kelvin at the time of his estimation of the Earths age. At around the same time, John Joly (1857 – 1933) calculated the rate of transfer of salt to the ocean as a means to determine the age of the Earth. The age of Earth by this method was calculated to be 90-100 million years. The main problem with this approach was there was no means to account for recycled salt, salt incorporated into clay minerals and salt deposits. Later work by other scientists used the thickness of total sedimentary record, to determine an age of 500 million years. It was not until the discovery of radioactivity by Henri Becquerel in 1896 that geologists had a tool for determining the age of rocks and ultimately the age of the Earth (Dalrymple, 1991).
Figure 1: William Thomson, (Lord Kelvin) (1824 – 1907). Kelvin assumed that the Earth was originally molten and calculated a date for the age of the Earth using the science of thermodynamics. Source: Wikipedia.org
Figure 2: James Hutton (1726-1797). A Scottish farmer and naturalist, is known as the founder of modern geology. Source: Wikipedia.org
Figure 3: Ernest Rutherford (1871-1937). A New Zealand-born British chemist and physicist who became known as the father of nuclear physics In early work he discovered the concept of radioactive half-life, proved that radioactivity involved the transmutation of one chemical element to another. Source: Wikipedia.org
Nuclear Changes in Nature
The solid rock of the Earth’s lithosphere formed from molten material that cooled and hardened. And in this process a “clock” that gives the age of the rock was going. Within the molten rock there are trace amounts of uranium-238, a radioactive element. Once the rocks had cooled this element was firmly locked in the rock. The atoms of uranium-238 however, decay at a constant rate to form atoms of lead-206 which would also be sealed within the solid rock. With the passing of time the rock would have less and less atoms of uranium-238 and more atoms of lead-206. Therefore, the rock contains some of the original amount of uranium-238 and the decay product, lead-206. Since we know the rate at which uranium-238 decays to lead-206, and the amounts of each of these atoms remaining we can calculate the age of the rock (Brandwein, 1968).
In 1905, Ernst Rutherford and Bertram Boltwood used radioactive decay to measure the age of rocks and minerals. And in 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. He then published the age of a sample of urananite at 1.64 billion years which was based on Uranium-Lead dating (Ward 1995). Scientists of the time began to realise that our planet was indeed ancient and may exceed 2 billion years in age and the search for older and older rocks was on. The invention of the Mass Spectrometer in 1918 allowed isotopes of different atoms to be separated and quantified including four isotopes of lead and two isotopes of uranium (McSween, 1997). Many radioactive elements can be used as geological clocks. Each element decays at its own constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.
There is no doubt of the Earth’s antiquity. Abundant and conclusive evidence of this is found in the rock record. However, fragments of Earth’s early primordial crust are extremely rare as most of it has been melted and recycled numerous times by plate tectonics since the Earth formed (Dalrymple, 1991). If there are any of Earth's primordial rocks left in their original state, they have not yet been found. Meteorites formed at the same time as the rest of the material in the solar system. Therefore by dating meteorites, we also get the age of the Earth, Mars, the Sun and everything else in the solar system. The best age for the Earth is 4.54 billion years (4.54 Ga)which is based on radiometric dating of iron meteorites, specifically the Canyon Diablo meteorite (Ward 1995). The Moon is better preserved that the Earth because it has not been disturbed by plate tectonics or erosion and therefore its more ancient rocks are more abundant than Earth’s ancient rocks. Rocks returned to Earth by the Apollo missions show that the oldest moon rocks have ages between 4.4 and 4.5 Ga. This is an important data as it provides a minimum age for the formation of the Moon (Dalrymple, 1991). However, remnants of ancient rocks exceeding 3.5 billion years (3.5 Ga) in age are found on all of Earth's continents. In 2008 a research group from McGill University discovered an amphibolite in Northern Quebec in an area known as the Nuvvuagittuq greenstone belt which has been radiometrically dated to 4.3 Ga (ref 8.), making them the oldest rocks discovered so far on Earth. Before the McGill study, the oldest dated rocks were from a body of rock known as the Acasta Gneiss in the Northwest Territories of Canada, which are 4.03 billion years old (ref.8). Other rocks that have been studied are nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 Ga), in Swaziland (3.4-3.5 Ga), and in Western Australia (3.4-3.6 Ga) (Barton et al, 1978). Southern Africa also has a host of rocks dating to more than 3 Ga, such as the Sand River Gneisses in the Limpopo Valley of South Africa, have been dated at 3.79 billion years (Barton et al, 1978).
Modern Mass Spectrometry methods are used with laser technology and this has made it possible to analyse very small samples such as zircon crystals down to single grains and achieve very high accuracy and precision (Kruger et al, 2000). An improvement in the precision of analytical measurement allows a reduction in the uncertainty of measurement of an individual zircon crystal or a population of zircons from a particular rock deposit (Allen, 1999). The knowledge of the uncertainty implies increased confidence in the analytical determination and does not imply doubt about the validity of a measurement (Fraser, 2010).
There is abundant evidence that our planet is indeed ancient. This has made us think in terms of deep time, which has profoundly affected the way we the way we see ourselves in the world.
A small Collection of some of Earth’s oldest Rocks
Figure 4: A 5 cm specimen of gneiss from an outcrop of the Acasta gneiss in northern Canada. This gneiss outcrop is dated at 4.02 billion years which is considered to be from one of the oldest outcrop of rocks on Earth. Specimen and photograph: A. Fraser.
Figure 5: 3.6 billion year old Morton Gneiss, Minnesota, USA. 9 cm. Specimen and photograph: A. Fraser
Figure 6: 3.2 billion year old granite specimen (8 cm) from the Klein Jukskei River, Johannesburg. Specimen and photograph: A. Fraser
Figure 7: A polished section of Mary Ellen Jasper (7 cm) from Minnesota USA, dated at 2.5 billion years. Specimen and photograph: A.Fraser
Figure 8: Greenstone schist (8 cm), Walter Sisulu Botanical Gardens, JCI Trail.
Specimen and photograph: A. Fraser
Tracing Earth's Oldest Rocks
Figure 9: Barberton Greenstone (4 cm) dated at 3.3 Ga. Specimen and photograph; A.Fraser
1. Allen L.A., Georgitis S.J., (1999) “Technical Brief - High Precision Isotope Ratio Measurements by the LECO Renaissance™ TOF-ICP-MS.
2. Barr. J. (1984). "Why the World Was Created in 4004 BC: Archbishop Ussher and Biblical Chronology", Bulletin of the John Rylands University Library of Manchester 67:575–608
3. Barton, J. M., Jr., B. Ryan, & R. E. P. Fripp. (1978) “The relationship between Rb-Sr and U-Th-Pb whole-rock and zircon systems in the 3790 m.y. old Sand River gneisses, Limpopo mobile belt, Southern Africa”. In R. E. Zartman, ed. Short papers of the fourth international conference, geochronology, cosmochronology, isotope geology. U.S. Geol. Survey Open-File Report 78-701. Page 476.
4. Brandwein P.F., Stollberg R., Burnett R.W., (1968), “Matter, it’s forms and changes” Harcourt, Brace & World, Inc. Page 182
5. Dalrymple G.B., (1991). “The Age of the Earth” Stanford University Press.
6. Fraser. A.W., (2010). “Statistical Method Validation in Analytical Chemistry – a practical approach”. Training course for Samancor.
7. Kruger, F.J., Allen, L., Fraser, A.W., (2000) “Combined electron probe and LA-ICP-TOF-MS analysis of Major and trace elements in garnet, apatite and zircon” Geoanalysis 2000.
8. McGill University (2008, September 26). Oldest Known Rocks On Earth Discovered: 4.28 Billion Years Old. ScienceDaily. (Accessed April 19 2011).
9. McSween. H.Y., (1997) “Fanfare for Earth – the origin of our planet and life”. Pages 160-161.
10. Ward, P., 1995 “The End of Evolution” . Phoenix Grant Science ISBN 1-85799-368-3. Page 133.
Allan Fraser is a consulting analytical chemist and a registered Professional Natural Scientist with the South African Council for Natural Scientific Professions. His area of interest is in the minerals of the Kalahari manganese field, the Phalaborwa Carbonatite and Peru. Allan’s other areas of interest are rocks of Archean and Hadean age, meteorite impacts and their relation to extinction events (the Cretaceous-Tertiary extinction event in particular) and the geology of Mars and Earth’s moon.
PO Box 369