Therefore, one can assume that the entire lead content of the zircon is radiogenic , i. Thus the current ratio of lead to uranium in the mineral can be used to determine its age. The method relies on two separate decay chains , the uranium series from U to Pb, with a half-life of 4. The above uranium to lead decay routes occur via a series of alpha and beta decays, in which U with daughter nuclides undergo total eight alpha and six beta decays whereas U with daughters only experience seven alpha and four beta decays.
The existence of two 'parallel' uranium—lead decay routes U to Pb and U to Pb leads to multiple dating techniques within the overall U—Pb system. The term U—Pb dating normally implies the coupled use of both decay schemes in the 'concordia diagram' see below. However, use of a single decay scheme usually U to Pb leads to the U—Pb isochron dating method, analogous to the rubidium—strontium dating method.
Finally, ages can also be determined from the U—Pb system by analysis of Pb isotope ratios alone. This is termed the lead—lead dating method.
Radiometric Dating
Clair Cameron Patterson , an American geochemist who pioneered studies of uranium—lead radiometric dating methods, is famous for having used it to obtain one of the earliest estimates of the age of the Earth. Although zircon ZrSiO 4 is most commonly used, other minerals such as monazite see: Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied.
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These types of minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record. During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay. This damage is most concentrated around the parent isotope U and Th , expelling the daughter isotope Pb from its original position in the zircon lattice. In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form a network of radiation damaged areas.
These fission tracks inevitably act as conduits deep within the crystal, thereby providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal. Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming exponential decay of Uranium.
Historical Geology/U-Pb, Pb-Pb, and fission track dating
As each dating method was developed, tested, and improved, mainly since , a vast body of knowledge about the behaviour of different isotopic systems under different geologic conditions has evolved. It is now clear that with recent advances the uranium—lead method is superior…. In uranium—lead dating, minerals virtually free of initial lead can be isolated and corrections made for the trivial amounts present. In whole rock isochron methods that make use of the rubidium—strontium or samarium—neodymium decay schemes see below , a series of rocks or minerals are chosen that….
For other systems we have to proceed further. The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. We can see how do deal with this if we take a particular case.
The neutron emits an electron to become a proton. We still don't know 87 Sr 0 , the amount of 87 Sr daughter element initially present.
Thus, 86 Sr is a stable isotope, and the amount of 86 Sr does not change through time. So, applying this simplification,.
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The reason for this is that Rb has become distributed unequally through the Earth over time. For example the amount of Rb in mantle rocks is generally low, i. Thus we could tell whether the rock was derived from the mantle or crust be determining its initial Sr isotopic ratio as we discussed previously in the section on igneous rocks. Two isotopes of Uranium and one isotope of Th are radioactive and decay to produce various isotopes of Pb. The decay schemes are as follows. Note that the present ratio of. If these two independent dates are the same, we say they are concordant.
We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates. The Concordia curve can be calculated by defining the following:. Zircon has a high hardness 7.
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Zircon can also survive metamorphism. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced. Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene. Discordant dates will not fall on the Concordia curve. Sometimes, however, numerous discordant dates from the same rock will plot along a line representing a chord on the Concordia diagram. Such a chord is called a discordia.
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We can also define what are called Pb-Pb Isochrons by combining the two isochron equations 7 and 8. Since we know that the , and assuming that the Pb and Pb dates are the same, then equation 11 is the equation for a family of lines that have a slope. The answer is about 6 billion years.
This argument tells when the elements were formed that make up the Earth, but does not really give us the age of the Earth. It does, however, give a maximum age of the Earth. Is this the age of the Earth? Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites.
Modern Oceanic Pb - i. Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon. Thus, our best estimate of the age of the Earth is 4. The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. Since K is one of the 10 most abundant elements in the Earth's crust, the decay of 40 K is important in dating rocks.
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But this scheme is not used because 40 Ca can be present as both radiogenic and non-radiogenic Ca. Since Ar is a noble gas, it can escape from a magma or liquid easily, and it is thus assumed that no 40 Ar is present initially. Note that this is not always true. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped. If this happens, then the date obtained will be older than the date at which the magma erupted. For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar.