Radiometric Accumulation as a Current Scientific Clock

By Brian Giedd

The age of the earth is a highly debated topic, and there are several methods that people have employed to calculate an age. Some proposed natural clocks have been used to support a young earth and some to support an old earth. There are even some clocks that have been used to support both a young and an old earth depending upon the assumptions and biases of the people using them. One category of natural clocks that can be used in a versatile manner is the radiometric accumulation clocks. In this paper, I will focus on explaining how radioactive elements decay and how daughter isotopes accumulate. The discussion will begin by looking at the requirements for a natural clock and how radiometric accumulation fits into this category. It will also include the arguments for and against the accuracy of the dates for the age of the earth that are given by radioactive decay. This will include a discussion of other methods that have been used in dating the earth.

What information is needed for a proper natural clock? First of all we must look at the initial condition of the system that is to be measured. In the case of radioactive decay of parent to daughter isotopes, we are looking for the initial concentrations of isotopes that are present. The initial daughter concentration is more important to note than the parent isotope concentration. This is because the daughter products are the ones that we are measuring in terms of accumulation, and any atoms that were present to begin with could skew the results. The next factor that is needed for a natural clock to work successfully is the rate at which it proceeds. In the case of radioactive decay, we are looking at the rate of decay from parent to daughter. When looking at the rate of decay, we use the half life, which is the amount of time it takes for half of the atoms of an element to undergo radioactive decay to daughter products. The final aspect of a natural clock that must be known is its final condition. In the case of radioactive decay, this is found by measuring the concentrations of parent and daughter isotopes that are present when the sample is studied.

Now that we have seen how radiometric accumulation dating can fit into the criteria of being a natural clock, we should look more at the method of the dating itself. Radiometric dating is based upon the idea that certain isotopes present in rocks are not stable. These unstable parent isotopes undergo radioactive decay to stable daughter isotopes. The parent and daughter isotope pairs can be used to determine geologic time based on the ratios and the half lives of the parent isotopes. The method by which these parent and daughter pairs are used to determine geological time is referred to as radiometric dating, and it can be very useful in determining the amount of time that has passed since the minerals in question were formed.

The rate of radioactive decay of the parent elements is determined experimentally. This decay process has been found to follow statistical probability. A very useful animated diagram of a radioactive decay curve is present at Each parent nucleus has the same probability of decaying within a given amount of time. This rate of decay is determined by a decay constant that ranges from 0-1. An element with a decay constant of 0 or close to it is stable and will not decay. An element with a decay constant of 1 or close to 1 is very unstable and will decay almost instantly. Due to the statistical nature of radioactive decay, it is impossible to tell when a particular atom will decay. This is why it is important to have a large enough sample of atoms to reduce the effect of this randomness.

half-life progressionSeveral different pairs of parent and daughter isotopes are commonly used in radiometric dating. Many different pairs are used because some pairs work better in different situations than others. All of the pairs used for radiometric dating have at least two things in common: the isotopes occur in measurable quantities in common rocks and minerals, and all of the parent isotopes have sufficiently long half lives that they are useful in measuring time in millions and billions of years. The reason that all of the elements that are used have such long half lives is that most of the elements with shorter half lives are no longer present. They have either decayed to their daughter isotopes, or they have been formed through continuous processes that make them less useful in dating. Some of the most widely used pairs include potassium-40/argon-40, which has a half life of 1.25 billion years; rubidium-87/strontium-87, which has a half life of 48.8 billion years; and uranium-238/lead-206, which has a half life of 4.47 billion years (Dalrymple 1991:93-102).

The half life of an isotope is a logarithmic function, so the concentration of daughter isotopes increases logarithmically, and the concentration of the parent isotope decreases logarithmically. Parent and daughter isotopes are generally measured using a mass spectrometer. The atoms are introduced into one end of the spectrometer, where they are changed into ions by a particle beam that strips one or more of their electrons. They are then influenced by electrical and magnetic forces as they pass down a tube in which they are separated according to mass. The heavier elements fall off sooner than the lighter elements. The amount of each ion is then measured using electrical signaling of various types.

It would appear to be rather easy to determine an age for a rock or mineral based upon the ratios of parent and daughter isotopes that are present, but it is certainly much more difficult in practice. For most of the parent-daughter pairs used in radiometric dating, it is difficult to tell if the total amount of daughter isotopes came from radioactive decay, or if some of it had been present when the rock or mineral was formed. The only exception to this problem is the potassium-argon pair. In this case, the daughter isotope is an inert gas. This means that it will not combine with anything after it has been formed through decay. There is also usually no problem with initial daughter concentration due to the fact that as the molten rock is cooling, any argon that was present will have escaped before it hardened to form solid crystals (Dalrymple 2004:63). A few exceptions have been found, but generally it can be said that there is no initial argon. The main problem that does occur with using the potassium-argon method, however, is that if the rock was heated to a high enough temperature at any point in its history, the argon could escape, and the clock would be reset.

The uranium/lead method of radiometric dating is also used with much confidence by the scientific community. The dates that are obtained by using uranium and lead usually show little variance in comparison to the dates obtained using other isotope pairs. The reason that these elements work so well as a pair is that there are different forms of lead that are present in a sample. Along with the lead-206 that results from the decay of uranium-238, we also find lead-204 that was not induced by radioactive decay (Dalrymple 2004:65). This information allows us to determine the initial lead-206 concentration with a high amount of certainty. The initial ratios of all isotopes of lead are known, and we can determine the initial condition by looking at the form of lead that has not been produced by decay and has, therefore, not increased over time.

That is a basic summary of how radiometric dating is used to date old objects. It is not, however, the only method that scientists have attempted to use in dating the earth. Two other methods that have been used in the past are sediment accumulation and heat loss from the earth's core. Sediment accumulation was based upon looking at how layers of rock had accumulated in layers over time. Estimates for the ages of these layers were made by measuring their thickness. This method did not prove to be very successful. There were far too many assumptions that needed to be made in order to use it. The main problem was the assumption that the sediment was deposited at a uniform rate. Sedimentation rates differ from place to place, and they could have been altered in certain areas by cataclysmic events throughout the earth's history.

The measurement of heat loss to date the earth was also highly futile. A measurement could be made by calculating the amount of heat that escaped from the earth, but there were once again too many factors that needed to be taken into consideration. There were two main problems that arose in measuring heat loss. First of all, the conduction of heat through the mantle and the crust of the earth is not uniform due in part to the movement of the continental plates. The other problem is that heat is not simply lost without being regenerated in the earth's core. These are problems that could not have been foreseen when the method was being applied, but we can look upon it now and see that it was a futile attempt.

The scientific community generally accepts that radiometric dating provides accurate dates for the ages of rocks. There are people who would argue against the validity of dates that have been obtained by using radioactive decay. These people are usually individuals who believe in a young earth. They have made several attempts to discredit that validity of radiometric dating by showing examples where the rates of decay have not appeared to remain constant. There are also some radiometric dates that have been found to be somewhat misplaced. For example, there are some rocks in the Grand Canyon that should have relatively young dates due to their positioning, but they show rather old radiometric dates.

Radiometric dating can be used to determine the ages of very old rocks and minerals on earth. The accumulation of daughter isotopes fits the criteria for a natural clock. The rate of decay gives a rate by which to keep time, and the final condition of the clock is available to us when we measure the concentrations of parent and daughter isotopes that are currently present. In some cases, as with the potassium-argon method, the initial condition may also be known. This gives us all of the information that we need for a useful natural clock. Radiometric dating, like most other dating methods, does have its limitations. For most of the parent-daughter isotope pairs, the exact initial condition can be difficult to determine for certain due to daughter atoms that were present prior to decay. Radiometric dating is certainly not the first scientific effort to find an age for the earth, but it does appear to be the most scientifically accurate method that is currently available. This does not mean that it is perfect. Like every other method that has been used to show an old earth, there are many opponents to it, and they have found ways that, at least in their eyes, prove that radiometric dating cannot be trusted to give us an accurate age for the earth. Radiometric accumulation does, however, remain a method that is still looked at with considerable scientific certainty.

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