Radioisotope Age

By R.H. Brown Ph.D. (Retired)

Reprinted from Geoscience Reports, Spring 1996, Geoscience Research Institute, Loma Linda University. Used by Permission. See many additional reports at Geoscience Research Institute

Introduction

"Lucy, we can now confidently say, lived 3.18 million years ago, plus or minus 10,000 years." Claims that humanoids existed on Earth 3.18 +/- 0.01 million years ago are based on radioisotope dating. The assignment of Lucy's age was made from potassium-argon dating of the rock in which her skeleton was found. The conflicting testimony of the Bible concerning the amount of time since Creation Week requires a critical evaluation of radioisotope dating.

What is a Radioisotope Age?

The radioisotope age of a specimen is obtained from a calculation of the time that would be required for unstable parent atoms [P] to spontaneously convert to daughter atoms [D] in sufficient amount to account for the present D/P ratio in the specimen. For the dating of Lucy, P was the unstable isotope 40 of potassium [⁴⁰K] and D was the stable isotope 40 of argon [⁴⁰Ar].

Conflicting Data

Lava from the AD 1901 eruption of Hualalei in Hawaii has potassium-argon (K-Ar) ages as great as 1.1 billion years. Historic eruptions of Mt. Kilauea in Hawaii (Figure 1) have produced submarine lavas with K-Ar ages as great as 43 million years. Obviously these K-Ar ages do not represent the time of eruption, or the age of the lava flows. The ages must reflect other characteristics of the lava.

Figure 1. 1986 Kilauean eruption. (Photo courtesy of Clyde Webster)

K-Ar ages from a volcanic deposit at Katmai, Alaska suggest volcanic activity four million years ago. Historical records establish that the eruption which produced these deposits occurred in AD 1912.

A dominant feature of the topography in the Auckland, New Zealand area is Mt. Rongitoto. During the time this volcano was active, a nearby forest was buried and fossilized in material having a K-Ar age of 485 thousand years. However, the carbon-14 (¹⁴C) content of these fossil trees indicates burial less than 300 years ago! (The trees contain over 96% as much radioactive carbon-14 as found in living trees. The amount of ¹⁴C in living material will diminish to 50% 5715 years after death.)

These examples adequately establish that a radioisotope age does not necessarily have real-time significance. The relationship of a radioisotope age with real-time must be based on an interpretation. A discussion of rubidium-strontium ages in the Isotope Geoscience Section of the journal, Chemical Geology, specifically states that a radioisotope age determination "does not certainly define a valid age information for a geological system. Any interpretation will reflect the interpreters’ presuppositions (bias).

One Explanation

From a biblical perspective, radioisotope mineral ages associated with fossils are characteristics of the minerals in which the organic material was buried, and provide no information concerning the time of burial. The dates of human interment in cemeteries are determined from the historic information on the grave markers, not from radioisotope age data for the headstones, rock, and soil associated with the burial sites. Similarly, there is a reasonable and scientifically sound basis for estimating fossil ages on the basis of the chronological data in the Bible, rather than on the radioisotope ages of the minerals and rock layers now associated with these fossils.

History of the Radioisotope based Geologic Time Scale

Before the discovery of radioactivity in the late nineteenth century, a geological time scale had been developed on the basis of estimates for the rates of geological processes such as erosion and sedimentation, with the assumption that these rates had always been essentially uniform. On the basis of being unacceptably old, many geologists of the time rejected these early twentieth century determinations of rock age from the ratio of daughter to radioactive parent (large). By 1925, increased confidence in radioisotope dating techniques and the demands of evolution theory for vast amounts of time led to the establishment of an expanded geological time scale. With the K-Ar dating techniques developed after World War II, this time scale was refined to the standard Geologic Time Scale adopted in 1964.

The construction of this time scale was based on about 380 radioisotope ages that were selected because of their agreement with the presumed fossil and geological sequences found in the rocks. Radioisotope ages that did not meet these requirements were rejected on the basis of presumed chemical and/or physical modifications that made the "ages" unreliable indicators of real time. About 85% of the selections were K-Ar dates, 8% rubidium-strontium dates, and 4% uranium-lead dates. Igneous rocks are particularly suited to K-Ar dating. The crucial determiners are therefore volcanic (extrusive igneous) rocks that are interbedded with sediments, and intrusive igneous rocks that penetrate sediments.

Processes Affecting K-Ar Ages

Since the geologic time scale (Table 1) is largely based on K-Ar ages for selected samples of igneous material, consideration must be given to the possibility that any K-Ar age may merely reflect a characteristic of the material, rather than indicate real time. The examples of anomalous K-Ar ages previously cited in this article strongly support this possibility and justify further examination of those characteristics, and of the processes affecting K-Ar ages.

K-Ar ages for successive flows or ash deposits on the flanks of volcanoes generally increase (as expected) with reverse order of flow, i.e., with depth, even when the real time lapse between eruptions is not equal to the K-Ar age difference. This feature has been identified as indicative of two factors: zonation in the supporting magma (molten rock) chamber, and progressive heating of the magma conduit.

Since argon is an inert gas, it is readily understandable that the argon concentration may increase from the lower to the upper portions of a magma chamber within Earth's crust. In a series of eruptions to the surface, or intrusions below the surface, the argon concentration may decrease progressively. Since K-Ar age is proportional to the ratio of [daughter] ⁴⁰Ar to [parent] ⁴⁰K, successive eruptions or intrusions may have decreasing K-Ar ages, none of which specifies the real time at which the event occurred.

As magma forces passage through surface rock, the passage conduit is heated, with corresponding cooling of the magma; and melting of the conduit walls dilutes some magma.

Consequently, in a volcanic event or closely spaced sequence of events, the ejecta is progressively hotter. The higher the eruption temperature, the more dissolved argon will escape as the ejecta cools, and the lower or younger the K-Ar "age" will be with respect to that which characterizes the eruption source. Accordingly, there are two factors that cause K-Ar ages to increase with depth, yet do not necessarily indicate real time intervals.

Table 1. The Geologic Column

Conclusion

From a creationist's perspective, the geological evidence indicates that there was massive volcanic and intrusive activity throughout the world associated with the Flood. The expression "fountains of the deep" (Gen. 7:11) might indicate magma as well as water. Due to 1) the variation of argon concentration and other elements with depth in the magma chambers of Earth's crust, and 2) the nature of the magmatic activity associated with the Flood, sequential geologic formations world-wide may often be marked by sequential radioisotope ages from "older" to "younger" upward.

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