The oldest rocks

Rocks older than about 3.8 Ga are extremely rare and are known in outcrop from only a few localities worldwide. Here we describe three localities, from which much of our knowledge of the pre-3.8 Ga Earth has been gained. These localities are marked in Fig. 1.4. 

1 The Gothabsfjord region of west Greenland - the most extensive area of ancient rocks 

One of the most important areas of ancient rocks is the Isua Greenstone Belt (or supracrustal belt, as it is often called) (Appel et al., 1998), located in the northern part of the Isukasia terrane (Fig. 1.9). The Isua Greenstone Belt comprises a sequence of metabasalts, ultramafic rocks, clastic, and chemical sediments, with ages between 3.7 and 3.8 Ga (Nutman et al., 1996). The most ancient materials so far recorded from Isua are mineral 

2 The Acasta Gneisses, northwestern Canada - the most ancient rocks 

Whilst the great age of the Acasta Gneisses is not in doubt, they have been the subject of some controversy. Bowring and Housh (1995) calculated initial eNd values for a range of samples from this area and found that they are extremely heterogeneous. This observation, coupled with the ancient zircon ages for these samples, has profound implications for the 

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Superposition is fundamental to the study of layered rocks. This means that in a normal layered (sedimentary) rock sequence, the oldest rocks were deposited first and are at the bottom of the sequence. The younger rocks were deposited last and are at the top of the sequence. 

If the amount of a radioactive nuclide in a rock sample is N, the sum of this amount plus the amount of its product nuclide is A/q. For argon dating, Nq is the sum of potassium-40 and argon-40 present in a sample of rock. Assuming that Ar gas escapes from molten rock but is trapped when the rock cools and solidifies, the lifetime obtained by substituting these values into Equation is the time since the rock solidified. Such analyses show that the oldest rock samples on Earth are 3.8 X 10 years old. 

All the models of the chemical composition of the atmosphere of primeval Earth are hypothetical. Samples from this period of development of the Earth are not available And the oldest rocks give us only a limited amount of information. 

The oldest rock formations on Earth are mainly found in three regions ... 

Meteorites present an opportunity to look at geological time or the time told by radionucleotides within rocks. The oldest rocks found on Earth are not as old as the age of the Earth due to continual reprocessing of the Earth s surface. The oldest discovered rocks so far are the Acasta gneisses from Northwestern Canada, which are 4.03 Gyr, but these are young compared with the CAIs found in the Allende meteorite, which are 4.566 0.002 Gyr or 4.556 billion years. The ages of these species are derived from the relative abundances of radioisotopes and their daughter species, as seen in Table 6.3. 

Among the oldest rocks on Earth are those on Isua, an island off the coast of Greenland they are 3.8 Gyr old, formed some 0.7 Gyr after accretion of the Earth. The rocks mark the beginning of the Archean period of geological time. The Isua rocks suggest that there was an extensive hydrosphere at this time, with erosion, transportation and deposition of minerals from water solution. The oldest lunar rocks, however, record an earlier high-temperature event - the Earth-Moon capture event. 

Uranium-238 has a half-life of 4.468 billion years over which time it decays into stable lead-206. This process can be used to date ancient rocks by comparing the ratio of the isotope lead-206, the last isotope in the uranium decay series, to the level of uranium-238 in the sample of rock to determine its age. This system has been used to date the oldest rocks on Earth as being about 4.5 billion years old, which is about the time of the formation of our planet. 

A popular method used to date rocks is the potassium-argon method. Potassium is abundant in rocks such as feldspars, hornblendes, and micas. The K-Ar method has been used to date the Earth and its geologic formations. It has also been applied to determine magnetic reversals that have taken place throughout the Earth s history. Another method used in geologic dating is the rubidium-strontium, Rb-Sr, method. Some of the oldest rocks on Earth have been dated with this method, providing evidence that the Earth is approximately 5 billion years old. The method has also been used to date moon rocks and meteorites. 

The decay of °Th leads to radioisotopes of other elements, ultimately concluding with the stable isotope lead-206. Happily, some of the oldest rocks on Earth, called zircons, contain no lead when they are formed. This means that the amount of lead they accumulate over time from uranium decay reflects their age. Until the rocks crystallized, uranium atoms could move freely through the molten magma from which they formed, and decayed uranium could be replenished. Solidification of a zircon does for uranium what an organism s death does for radiocarbon it stops the influx of fresh radioactive material, and the decay clock starts ticking. Because of U s long half-life, zircons can be dated back to the Earth s earliest days. 

One conclusion drawn from radioactive measurements is that the pre-Cambrian history of the earth s crust extends beyond 2,700 million years, The pegmatites that have been found to be this old are located in North America and Australia, and they probably exist oil all the continents. The oldest rocks in the United States that have been measured are on the south rim of the ftridger Mountains near the Wind River Canyon in Wyoming. These ancient pegmatites intrude geologic formations of sedimentary and volcanic rocks that, themselves, are the result of even more ancient processes than those in which they were formed. Thus, a period of the order of 3,000 million years or more is available for geologic processes that have formed the crust seen today. 

Chondrites or stony meteorites are the most common type of meteorite and are some of the oldest rocks in the solar system. They are made of materials similar to rocks in the crusts of the terrestrial planets, such as Earth and Mars. 

According to the data of Perry and Tan (1973), most of the carbonates from the oldest rocks (from 3 10 to 1 10" yr) of South America and the Canadian shield have S C within 2%o. Determinations by Hoering (1967) also showed that the carbon isotopic composition of Precambrian carbonates falls within the values established for normal Phanerozoic marine sediments. 

Stages of the evolution of the earth are listed in Table 15.3. The evolution of the sun and the planets from solar nebula began about 4.6 10 y ago. Materials of this age are not found on the earth, because most primordial solids on the earth went through one or several metamorphoses. However, the material of meteorites which have been formed simultaneously with the earth make it possible to date the age of the earth at 4.5 10 y. The oldest minerals on the earth have an age of the order of 4.3 10 y and suffered metamorphoses about 3.8 10 y ago. The age of the oldest rock for-... 

In the first part of this chapter, some of the primary lines of evidence for long-term chemical changes in the Earth s mantle were summarized. In this section, the processes that can modify mantle chemical compositions are considered. The focus here is on consideration of potential mechanisms that could have created the chemical signatures preserved in the oldest rocks, and the subsequent processes that result in the patterns of chemical evolution present in the mantle. 

So when does the first evidence of improbable, information-containing, metabolic replication occur in the fossil record The Earth is 4,500 million years old, as judged by several corroborating radionuclide studies of the oldest rocks on the planet show. Meteoric bombardment of the proto-Earth continued heavily until 4,000 MY A, probably precluding life during this period. The majority of the oldest rocks on Earth are 3,500 million years old, and the earliest microfossils are from 3,000+ MYA, hence we only have a window of about 500 million years from the end of the meteoric bombardment to the first signs of microbial life. This means we are either very lucky, or life is a high-on certainty ... 

FIGURE 1.4 The distribution of Archaean rocks worldwide, showing the principal cratons and regions where the oldest rocks are preserved. Not all these areas contain exposed Archaean rocks, for some cratons are now partially covered by younger sediment, or have been reworked during later orogenic events. Not shown is the Archaean Enderby Land Craton in Antarctica. The dashed lines indicate the rocks of the North Atlantic Craton now separated by the creation of the Atlantic Ocean. 

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