Isochron Dating as a Current Scientific Clock

By Calvin Krogman

Radioactive decay has become one of the most useful methods for determining the age of formation of rocks. However, in the very principal of radiometric dating there are several vital assumptions that have to be made in order for the age to be considered valid. These assumptions include: 1) the initial amount of the daughter isotope is known, 2) neither parent or daughter product has migrated into, or out of, the closed rock system, and 3) decay has occurred at a constant rate over time.

But what if one or some combination of these assumptions is incorrect? Then the computed age based on the accumulation of daughter products will be incorrect (Stasson 1998). In order to use the valuable information provided by radiometric dating, a new method had to be created that would determine an accurate date and validate the assumptions of radiometric dating. For this purpose, isochron dating was developed, a process "that solves both of these problems (accurate date, assumptions) at once" (Stasson 1992).

A natural clock must meet four requirements. 1) The process must be irreversible. Isotope dating satisfies this requirement, as daughter products do not decay back to the original parent element. 2) The process must occur at a relatively uniform rate. It has been established through extensive experimentation that radioactive decay occurs at a constant rate. 3) The initial condition must be known. In this case, the initial condition is the amount of daughter isotope in the rock when it was formed. This amount is often unknown and is one of the downfalls of conventional radiometric dating. However, isochron dating bypasses this assumption, as explained below. 4) The final condition must be known. The final condition is the number of atoms of parent and daughter isotopes remaining in the rock and can easily be measured in a lab.

Isochron dating bypasses the necessity of knowing the quantity of initial daughter product in the rock by not using that value in the computation. Instead of using the initial quantity of daughter isotope, the ratio of daughter isotope compared to another isotope of the same element (which is not the product of any decay process) is used as the comparison for isochron dating. The plot of the ratios of the number of atoms of the parent isotope to the number of atoms in the non-daughter isotope compared to the number of atoms of the daughter isotope to the non-daughter isotope should result in a straight line that intersects the vertical y-axis (which is the ratio of daughter to non-daughter isotopes). This point of intersection gives the initial ratio of daughter to non-daughter isotopes, which would also be the ratio in a mineral that crystallized without any parent isotope present.

According to Brent Dalrymple (2004:68-69), "the trick to the isochron diagram is the normalization of both parent and daughter isotope to a third isotope." This third isotope is the non-decay product isotope of the same element as the daughter element. In the initial state, the graph of daughter isotope to the third isotope versus parent isotope to the third isotope should result in a straight, horizontal line.

The process of evaluating the daughter product as a ratio against another isotope of the same element is a valid method because, when a mineral or rock forms from a homogenous state, the elements that are assimilated into crystalline formation are very restricted. The key to the formation of crystals in the rock is that the process is selective between elements, but is indifferent to isotopes of the same element. Thus, the daughter product and any other isotopes of the same element will be incorporated into the minerals of a rock with the same ratio. This initial ratio allows the non-daughter product isotope to be representative of the initial amount of the daughter product (Stassen 1998).

anatomy of an isochron

To view an animation of how an isochron changes over time as decay occurs, see the following website: http://www.talkorigins.org/faqs/isochron-dating/AnimatedIsochron.html

As time progresses and decay occurs, the number of atoms of the parent isotope decreases, and the number of atoms of the daughter isotope increases accordingly. The amount of non-decay isotope in the sample does not change. Thus, as decay occurs, the parent ratio decreases and the daughter ratio increases. On an isochron diagram, this change in ratios shifts each measurement from the sample up and to the left at a one-to-one rate. As time progresses, the line connecting the measurements within the sample moves counter-clockwise around a point intersecting the y-axis, a point that represents the initial ratios (Dalrymple 2005:71).

Once the ratios are plotted, the age of the rock being dated can be determined based on the slope of the line. The steeper the slope of the line, the more decay has occurred in a sample and the older the sample is (Dalrymple 2005:71).

The features of the isochron method provide a way do reduce doubt and speculation about an age that is computed using these methods. Based on the assumptions of basic radioactive dating, the problem of an unknown initial amount of daughter isotope is eliminated by the definition of the isochron itself. The problem of contamination is "self-checking". If contamination has occurred within a sample, the ratios from the sample shouldn't fall on a line. Instead, the points would be in a scatter on the graph. Points that do not fall on a straight line suggest contamination, and this invalidates the results. However, by this same principle, points falling relatively close to a best fit line should provide an accurate date for the age of the rock being dated (Stasser 1992).

In most cases, the slope of the line generated by the isochron method gives an age for a rock sample of millions, or even billions of years. In general, these ages are supported by the science community, who declare that the Earth is about 4.5 billion years old. However, young-Earth creationists believe in an Earth that was created only 6,000 years ago. The old age provided by isochron dating methods obviously conflicts with the young age of only 6,000 years held by these creationists.

While isochron dates have been used by both old-Earth and young-Earth proponents to promote their respective viewpoints, attacks on isochron dating have also been made by young-Earth creationists, such as William Overn. These creationists challenge the assumptions made by the isochron dating method itself. The first of these assumptions, that all rocks and minerals that formed from the same homogenous mixture have the same age, is not disputed (Overn 2005). The second assumption of isochrons is that the initial ratios of the daughter isotope to the non-decay product isotope of the same element are uniform throughout the sample. This assumes that the two isotopes were incorporated in the same ratio in each mineral as the rock formed. While this should occur in an ideal, homogenized liquid state of rock, Overn (2005) states that "this enabling assumption must fail in the absence of an initial homogenized melt." He also states that field data suggests that each sample has its own, independent ratio. This can happen, but it causes the points on the isochron plot to be scattered, so it is easy to recognize.

One final assumption of the isochron method is that mixing, or re-homogenization, has not occurred. In that case, the ratios may become altered when the minerals re-crystallize. The problem with the isochron, then, is that the date being calculated is not the date that the rock was initially formed, but the date that it re-homoginized and re-crystallized to its current state. The age being dated, then, is the age when the mineral was re-crystallized, not when it originally formed. This problem is undetectable even within the isochron's "self-checking" methods and can result in error when computing a given age for a rock. However, if the rocks and minerals are only partially re-homogenized, then not all ratios of isotopes in the rock may be altered. For example, one part of a rock might be heated enough to cause re-homogenization, while another part might not be heated at all. This partial re-homogenization should result in the ratios, when plotted on the isochron, not falling on the same line. Because the isochron wouldn't form a straight line, the results are considered invalid.

According to Overn (2005), violation of any of the assumptions above should produce a scatter of points rather than a line. In general, a violation of the assumptions of the isochron method does result in the points of the isochron not falling in a straight line. The main exception to this is when a rock has been completely re-homogenized; in which case the date recorded from the isochron method should be the correct date of the re-crystallization of the rock or mineral. It should be noted, however, that if too few minerals are being dated, there is an increased chance that the points would fall on a straight line by chance (for example, any two points can fit a straight line). As the number of mineral samples that are used in the isochron increases, the more confident we can be that the assumptions of isochron dating are valid, and that the date being reporded is accurate.

Recently, there was a creationist research team that set out to explore some of the assumptions of radiometric dating. The Radioisotope and the Age of the Earth (RATE) team explored different techniques used by scientists to obtain ages for rocks. In his book Thousands.Not Billions, Don De Young (2005) summarized the findings of the RATE team's research. Chapter 7 (p. 110-121) deals with the RATE team's exploration of isochron dating methods. As part of their research, the RATE team does not dispute that isochron dating is a valid method for dating the ages of rocks, nor do they dispute that the dates of millions or billions of years of age are accurate based on the usual assumptions. Instead, the RATE team challenges the assumption that decay rates have been constant over time. They propose that decay rates have been accelerated on several occasions, so that the isochron date given is correct for the amount of decay that has occurred, but the time that has elapsed is not the same as the age given.

Although these assumptions of the isochron method have been challenged by young-Earth proponents, isochron dating methods have been used by both young-Earth and old-Earth scientists to make claims about the age of the Earth based on the rocks they have dated. Both sides support isochron dating as a valid method, and both sides acknowledge that isochron dating is likely a more reliable source of dating rocks than simple accumulation radioactive decay clocks.

Sources:
  1. Dalrymple, Brent G. 2004. Ancient Earth, Ancient Skies. Stanford University Press, 247p.
  2. De Young, Don. 2005. Thousands.Not Billions: Challenging an Icon of Evolution, Questioning the Age of the Earth. Master Books, Forest Green, Arkansas, 190 p.
  3. Fleming, Jon. 2002. "An Animated Isochron Diagram". http://www.talkorigins.org/faqs/isochron-dating/AnimatedIsochron.html. Accessed December 3, 2005.
  4. Overn, William. 2005. "Isochron Dating is Fatally Flawed". http://www.tccsa.tc/articles/isochrons2.html. Accessed December 3, 2005.
  5. Stassen, Chris. 1998. "Isochron Dating". http://www.talkorigins.org/faqs/isochron-dating.html. Accessed December 1, 2005.
  6. Stassen, Chris. 1992. "FAQ: Isochron Dating". http://www.skepticfiles.org/evo2/isochron.htm. Accessed December 1, 2005.