More Bad News for Radiometric Dating
The isochron technique of radiometric dating is often presented as overcoming problems with earlier methods. However, it is easy to imagine scenarios that. an isochron have been described, we will discuss the potential problems of the "simple" dating. There is a refinement of the radiometric dating method known as isochron dating. claimed to overcome other problems with the assumptions involved in all radiometric dating techniques.6 .. Hayes proposed two solutions.
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Let x, y, and z refer to their concentrations. Since Y and Z are isotopes, we would assume they have similar chemical properties. Let's assume that initially, the ratio of y and z is constant, and then X begins decaying to Y. If these ratios are observed to obey such a linear relationship in a series of rocks, then an age can be computed from them.
However, we can imagine situations in which such a linear relationship could be produced without indicating a true age.
Suppose A is very old or appears very old and B is very young. Suppose A and B become thoroughly mixed.
Their perceived radiometric age would then be between that of A and B. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination. In uranium—lead datingthe concordia diagram is used which also decreases the problem of nuclide loss.
Problems with Isochrons
Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established.
On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.
As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes.
This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis. The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value N.
The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature.
This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition. Modern dating methods Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth.
In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s. It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization.
On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.
Uranium—lead dating method A concordia diagram as used in uranium—lead datingwith data from the Pfunze Belt, Zimbabwe. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.
Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.
This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample. Samarium—neodymium dating method This involves the alpha decay of Sm to Nd with a half-life of 1. An additional nice feature of isochron ages is that an "uncertainty" in the age is automatically computed from the fit of the data to a line.
A routine statistical operation on the set of data yields both a slope of the best-fit line an age and a variance in the slope an uncertainty in the age. The better the fit of the data to the line, the lower the uncertainty. For further information on fitting of lines to data also known as regression analysissee: Yorka short technical overview of a technique specially designed for assessing isochron fits.
Note that the methods used by isotope geologists as described by York are much more complicated than those described by Gonick. This will be discussed in more detail in the section on Gill's paper below. The "generic" method described by Gonick is easier to understand, but it does not handle such necessities as: Unfortunately, one must wade through some hefty math in order to understand the procedures used to fit isochron lines to data.
General comments on "dating assumptions" All radiometric dating methods require, in order to produce accurate ages, certain initial conditions and lack of contamination over time. The wonderful property of isochron methods is: This topic will be discussed in much more detail below. Where the simple methods will produce an incorrect age, isochron methods will generally indicate the unsuitability of the object for dating.
Avoidance of generic dating's problems Now that the mechanics of plotting an isochron have been described, we will discuss the potential problems of the "simple" dating method with respect to isochron methods. Initial daughter product The amount of initial D is not required or assumed to be zero.
The greater the initial D-to-Di ratio, the further the initial horizontal line sits above the X-axis. But the computed age is not affected.
If one of the samples happened to contain no P it would plot where the isochron line intercepts the Y-axisthen its quantity of D wouldn't change over time -- because it would have no parent atoms to produce daughter atoms. Whether there's a data point on the Y-axis or not, the Y-intercept of the line doesn't change as the slope of the isochron line does as shown in Figure 5. Therefore, the Y-intercept of the isochron line gives the initial global ratio of D to Di.
For each sample, it would be possible to measure the amount of the Di, and using the ratio identified by the Y-intercept of the isochron plot calculate the amount of D that was present when the sample formed. That quantity of D could be subtracted out of each sample, and it would then be possible to derive a simple age by the equation introduced in the first section of this document for each sample. Each such age would match the result given by the isochron.
Contamination - parent isotope Gain or loss of P changes the X-values of the data points: Gain or loss of P. In order to make the figures easy to read and quick to drawthe examples in this paper include few data points. While isochrons are performed with that few data points, the best ones include a larger quantity of data. If the isochron line has a distinctly non-zero slope, and a fairly large number of data points, the nearly inevitable result of contamination failure of the system to remain closed will be that the fit of the data to a line will be destroyed.
For example, consider an event which removes P. The data points will tend to move varying distances, for the different minerals will have varying resistance to loss of P, as well as varying levels of Di: Loss of P in all samples The end result is that the data are nearly certain not to remain colinear: Loss of P destroys the fit to a line. Even in our simple four-data-point example isochron, a change to two of the samples Migration of parent in two data points.
Specific loss of P required to yield a different colinear plot. The two samples must each change by the indicated amount -- no more and no less -- if the data are to remain colinear. In the special case where the isochron line has a zero slope indicating zero agethen gain or loss of P may move the data points, but they will all still fall on the same horizontal line.
In other words, random gain or loss of P does not affect a zero-age isochron. This is an important point. If the Earth were as young as young-Earth creationists insist, then the "contamination" which they suggest to invalidate dating methods would have no noticeable effect on the results.
Moreover, the daughter atoms produced by decay in a mineral are isotopes of different elements and have different ionic charges and radii compared with their parents.
The energy released during the decay may produce dislocations or even destroy the crystal lattice locally, thus making it all the more easy for the radiogenic daughters to escape. This will change the vertical position of the data points: Gain or loss of D. As with gain or loss of P, in the general case it is highly unlikely that the result will be an isochron with colinear data points: Exceptions for loss of daughter There are two exceptions, where it is possible for migration of D to result in an isochron with reasonably colinear data points: If the D is completely homogenized, then the isochron age is reset to zero.
When this happens, any later dating attempt will yield the age of that metamorphic event rather than the original time of crystallization: Complete homogenization of radiogenic daughter resets the isochron age to zero.