Torch, Summer 1993

Scientists who believe that the earth is extremely old often base their conclusion on the observation of certain natural processes such as radioactive decay. For example, rubidium-87 (B7Rb) is a common radioactive isotope which decays into stable strontium-87 (87Sr), called radiogenic strontium when it is formed by radioactive decay. Since both rubidium-87 and strontium-87 are often found in igneous rocks (rocks which were once molten) along with normal strontium-86 and nonradiogenic strontium-87, and since the rate of radioactive decay is known, many "old earth" scientists believe that the age of igneous rocks can be calculated from the present concentrations of these isotopes. This is called a radiometric dating method. However, this calculation involves certain rather unlikely assumptions . about the initial concentrations of isotopes. Furthermore, these assumptions cannot be verified, and therefore may not even be valid in many cases. To reduce the likelihood of calculating ages using invalid assumptions, or even invalid data, these scientists sometimes use a graphical method, called the isochron method, to calculate ages. (See DEMONSTRATION sidebar.) If the data forms a straight line (an isochron), it is believed to be valid. The slope of the line is then used to calculate the age of the rock. If the data is scattered, it is assumed to be invalid due to leaching of elements into or out of the rock since its formation, and is therefore ignored. However, even this method often yields results which are obviously not valid. Our demonstration shows how four contradictory results can be obtained for the same rock! All four cannot be valid. How, then, do scientists determine which result to accept and which to ignore? The answer is not determined by the data or the method of analysis. It is determined primarily by their preconceived ideas. If the result seems to fit what they believe to be true, it is considered valid. If it doesn't, it is considered invalid and ignored. 10 Torch In spite of the serious limitations of this method, it appears convincing and is now accepted by most scientists without question. Yet there is another simple explanation of the data which is nearly always ignored because it produces nothing interesting or publishable about a rock, such as the age. However, it can account for all the data, not just part of it, and therefore might be considered to be a better explanation of the data. It is known as the mixing model. The mixing model assumes that small rocks containing different concentrations of elements and isotopes were once partially mixed while in a molten state to form one larger igneous rock. This assumption is consistent with the worldwide observation of large scale catastrophic events, such as volcanoes, which produce molten rock. If mixing has indeed occurred in a large percentage of igneous rocks, then we would expect to obtain each of the four different types of results occasionally, depending upon which combination of data happened to be obtained and plotted. This is exactly what is found. Therefore, scattered data is not necessarily the result of leaching. It may also be the result of mixing and therefore would still be considered valid. More significantly, it is possible to obtain isochrons due to incomplete mixing of molten rocks, but these isochrons have nothing to do with age. In fact, calculation of an age from the slope of the isochron would clearly be a misinterpretation of the data and would be very misleading! In this case, the data simply represent various ratios of isotopes in the rock and have nothing to do with the age of the rock. Now, is it possible to determine which of the two possible explanations for formation of isochrons, aging or mixing, is correct? Fortunately, yes! Through a simple mathematical analysis, it is possible to determine in most cases if mixing has indeed occurred. Examination of 18 isochrons published in the scientific literature indicates that 72% appear to be the