Does Radiometric Dating Prove the Earth Is Old?
Earth Materials Tulane University Prof. Nelson Radiometric Dating Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Although we dooes recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists.
Then, inradioactivity was discovered. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer. It provided a means by which the age of the Earth could be determined independently. Principles of Radiometric Dating. Radioactive decay is described in whst of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus.
The energies involved are so large, and the nucleus is so small that physical conditions in radiomeric Earth i. T and P cannot affect the rate of decay. The rate datint decay or rate of change of the number N of particles is proportional to the what does radiometric dating allows us to determine present at any time, i. The proportionality constant is lthe decay constant.
So, we can write. Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0. After the passage of two half-lives only 0. This can only be done for 14 C, since we know N 0 from the atmospheric ratio, assumed to be constant through time. 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 0the 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. In nature, however, each mineral in the rock is likely to have a different amount of 87 Radiometrix.
Thus, once the rock has cooled to the point where diffusion of elements does not occur, the 87 Rb in each mineral will decay to 87 Sr, and each mineral will have a different 87 Rb and 87 Sr after passage of time. So, applying this simplification. The reason for this is that Rb des 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 allws 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 determnie various isotopes of Pb. The decay schemes are as follows. Note that the present ratio of. 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:. The Concordia is particularly useful in dating of the mineral Zircon ZrSiO 4. Zircon has a high hardness 7.
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 what does radiometric dating allows us to determine Concordia diagram. Such a chord is called a discordia. We can also define what are called Pb-Pb Isochrons by combining the two isochron equations 7 and 8.
Since we know that theand assuming that the Pb and Pb dates are the same, then equation 11 what does radiometric dating allows us to determine the equation for a family whah 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 what does radiometric dating allows us to determine 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. But this scheme is not used because 40 Ca can whaf 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 What does radiometric dating allows us to determine, 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. Such trapped Ar is not problematical when the age of the rock is in hundreds what does radiometric dating allows us to determine millions of years. The dating equation used for K-Ar is: Some of the problems associated with K-Ar dating are Excess argon.
This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. Minerals should not contain any excess Ar because Ar should not enter the crystal structure allpws a mineral when it crystallizes. Thus, it always better to date minerals that have high K contents, such as sanidine or biotite. If these are not present, Plagioclase or hornblende. If none of these are present, then the only alternative is to date whole rocks.