Radioactive dating method ‘under fire’
by Andrew A. Snelling
In most people’s minds today, the radioactive dating of the earth’s
rocks by geologists has supposedly proved that the earth is billions of years old.
Yet most people really don’t know much about these radioactive dating methods.
So slick and convincing are the presentations of results, particularly in glossy
media and museum propaganda, that no one even bothers to question how these dating
methods work, what assumptions are involved, and how reliable they are.
Such questions, however, are highly relevant. The answers are not only instructive,
but demolish the evolutionary geologist’s case for a 4.5-billion-year old
earth. This in turn allows the evidence for a young earth and universe1 to ‘speak’ more loudly in support of the
scriptural chronology of a 6,000–7,000 year age, which of course leaves no
time for any ‘big bang’ and ‘molecules-to-man’ evolutionary
scenarios.
Recently, the radioactive dating method which geologists (and physicists) have considered
to be perhaps the most reliable has come under heavy ‘fire’. The big
surprise is that the attack has come from an evolutionary geologist and has been
published in a secular scientific journal! But more of that in a moment. First,
let’ s find out how radioactive dating methods are supposed to work.
Radioactive dating explained
Some types (technically known as ‘isotopes’) of ‘parent’
elements such as uranium, thorium, potassium and rubidium are said to be radioactive
because the nuclei of the atoms are unstable, resulting in readjustments between
the ‘particles’ (primarily neutrons and protons) in the nuclei with
time. To achieve stability, some ‘particles’ are ejected from the atoms,
and these moving ‘particles’ constitute the radioactivity measured by
Geiger counters and the like. The end result is stable atoms of the ‘daughter’
elements lead, argon, and strontium respectively.
Thus the first step in the radioactive dating technique is to measure the amounts
of the parent and daughter elements (isotopes) in a rock sample via chemical analyses.
This is done in specially equipped laboratories with sophisticated instruments capable
of very good precision and accuracy, so in general there is no quarrel with the
resulting chemical analyses.
However, it is with the interpretation of the chemical analyses of the radioactive
parents and resultant daughters that the problems with radioactive dating of rocks
begin. In order to interpret these chemical analyses, geochronologists must make
three vital assumptions, otherwise the radioactive ‘clock’ cannot be
made to ‘read’ the ‘age’ of the rocks. These assumptions
are:
- the initial conditions are known;
- the system has been closed; and
- the radioactive decay rate has remained constant.
So that these assumptions are easily understood, they are best explained in the
context of the hourglass analogy (see Figure 1). Grains of fine sand fall at a steady
rate from the top glass bowl to the bottom. At time t = 0, the hourglass is turned
upside-down so that all the sand starts in the top bowl. By time t = 1 hour, all
the sand is supposed to have fallen into the bottom glass bowl.
Now this ‘clock’ works because the initial conditions are known—that
is, all the sand grains are in the top glass bowl and none are in the bottom one.
If there is already some sand in the bottom glass bowl, then unless this amount
is known the hourglass ‘clock’ cannot ‘tell’ the time. Similarly,
if the system has not remained closed (for example, if sand were somehow added or
subtracted), then the calculation of the elapsed time, based on comparing the amounts
of sand in the two glass bowls, will again lead to an incorrect conclusion. And
finally, if the rate at which the sand grains fall from the top glass bowl to the
bottom one varies (for example, moisture causes some clogging of the sand in the
constriction between the two glass bowls), then again the hourglass ‘clock’
will be inaccurate.
Unproven assumptions
The radioactive decay of ‘parent’ isotopes of uranium, thorium, potassium,
and rubidium to ‘daughter’ isotopes of lead, argon and strontium respectively
is analogous to our hourglass ‘clock’, including these three assumptions.
However, in the case of these radioactive ‘clocks’ these three assumptions
can be shown to be not only unprovable, but invalid, rendering these ‘clocks’
virtually useless.
In the case of the initial conditions, no scientist can ever be sure as to what
they were, because no scientist was present here on the earth at its origin. Thus
the amount of daughter isotope that has actually been derived from the parent isotope
by radioactive decay is unknown, since some of the daughter isotope might have been
present with its respective parent isotope at the time of the earth’s origin.
So geochronologists have assumed that the uranium, thorium and lead isotopic composition
of particular meteorites is equivalent to the initial composition of these isotopes
when the earth came into existence. This is assumed because it is supposed that
these meteorites represent fragments from another planet in the solar system similar
to our earth that disintegrated very early in the history of the solar system. However,
not all meteorites have the same uraniumthorium-lead isotopic composition, so why
should the isotopic composition of these particular meteorites be considered to
be the ‘correct’ composition for the earth at its origin rather than
some other composition found in other meteorites?
An hourglass ‘clock’ tells us the elapsed time by comparing the amount
of sand in the top bowl (‘parent’) with the amount in the bottom bowl
(‘daughter’).
Furthermore, even if today’s scientists believe they have the methods, for
example graphical and mathematical, for determining how much of the daughter isotope
might have been present either at the origin of the earth or the origin of the rock
being dated, no one can ever be sure that these ‘answers’ are correct,
because there was no scientist present at the beginning to observe those initial
conditions, even though the scientists’ calculations may be extremely logical.
Similarly, there is no way that it can be proved that these radioactive systems
have been closed through all the supposed millions of years of decay of parent isotopes
into daughter isotopes. Again, the main reason for this is because no scientist
has been present to observe everywhere these radioactive systems and so report that
they have been closed through all their history. Indeed, the evidence indicates
the very opposite, that is, that these systems have been open to all sorts of external
influences.
For example, it is known that uranium is generally a mobile element in the natural
environment, particularly in groundwaters near the earth’s surface. Thus,
if a rock sample is analysed at or near the earth’s surface for its uranium
and lead isotopes, it would be incorrect to assume that all the uranium and lead
in the sample were there only because of the amounts placed in the rock at its origin
and because of undisturbed radioactive decay from uranium into lead. Some of the
uranium might have been leached out of the rock sample, hence making the rock appear
older than it really is according to this radioactive ‘clock’. Or, some
uranium might have been deposited by groundwaters into the sample, thus making it
appear younger than what it really is.
Indeed, geochronologists often plot the chemical analyses of the isotopes, expressed
as isotope ratios, on graphs, and these often show that the parent-daughter systems
have not been closed, but open. Furthermore, by interpretation of these graphs they
often claim to be able to quantify the loss or gain and thus overcome this difficulty
to still ‘read’ the radioactive ‘clock’. However, once again
this interpretation to overcome this problem of the invalidated closed-system assumption
cannot be proved, but is merely assumed to be correct because it makes the radioactive
‘clock’ work.
The final assumption is, of course, that the radioactive decay rates have remained
constant. However, once again, this assumption can in no way be proved, because
there were no human observers present right throughout the earth’s history
to be constantly measuring the radioactive decay rates and to have recorded them.
It is special pleading on the part of geochronologists and physicists to say that
the radioactive decay rates have been carefully measured in laboratories for the
past 80 or 90 years and that no significant variation of these rates has been measured.
The ‘bottom line’ is really that 80 or 90 years of measurements are
being extrapolated backwards in time to the origin of the earth, believed by evolutionists
to be 4.5 billion years ago. That is an enormous extrapolation. In any other field
of scientific research, if scientists or mathematicians were to extrapolate results
over that many orders of magnitude, thereby assuming continuity of results over
such enormous spans of unobserved time, they would be literally ‘laughed out
of court’ by fellow scientists and mathematicians. Yet geochronologists are
allowed to do this with impunity, primarily because it gives the desired millions
and billions of years that evolutionists require, and because it makes these radioactive
‘clocks’ work!
So we have seen that none of these three basic assumptions which are foundational
to all the radioactive dating techniques can be proved. Indeed, we have also seen
that each of these three assumptions is invalid, not only because no scientist has
been present from the origin of the earth to see what it was like then and to report
as an eyewitness all that has happened everywhere since, but because we know of
observations contrary to these assumptions.
The isochron dating method
Apart from the initial conditions, the major problem facing geochronologists is
that geological systems are invariably open to external influences. Thus, analyses
of radioisotopes often produce results that reflect loss, or sometimes gain, of
either parent or daughter isotope, rendering single radioactive age determinations
suspect. Thus geochronologists tackle the problem by performing a number of radioactive
age determinations on a group of samples from the rock under investigation, hoping
to pin-point a pattern that will enable the calculation of the desired ‘true’
age.
If these multiple isotopic analyses of various rock samples, and minerals within
those rock samples, are from the same geological unit, then geochronologists can
also use what is known as the isochron age determination method. This method is
supposed to allow some of the more uncertain assumptions of the normal age calculating
method to be circumvented and so permit a higher degree of confidence in the resulting
‘age’ estimate. Consequently, geochronologists favour this isochron
method and so it has become very popular, particularly with rubidium-strontium,
samarium-neodymium and uranium-lead isotopic systems.
The isochron method works as follows. If a number of rock samples from a single
geological unit are carefully collected, then it is claimed that it is reasonable
to assume that each rock sample from that geological unit formed at the same time,
and therefore ultimately has the same age. However, from experience it is known
that each rock sample differs in the amounts of both daughter and parent isotopes
contained.
A graph is then constructed so as to plot the amount of daughter isotope against
the amount of parent isotope, so the isotopic analysis of each rock sample will
then be represented as a single point on this graph. Often these data points, plotted
on the graph of daughter isotope composition against parent isotope composition,
form a linear array through which a sloping line can usually be drawn with a high
degree of fit of the data points to the line, as shown in Figure 2. This is because
those samples with larger amounts of parent isotope have correspondingly larger
amounts of daughter isotope, and those samples with smaller amounts of parent isotope
have correspondingly smaller amounts of daughter isotope, assuming of course that
all the daughter isotope has been produced by radioactive decay from the parent
isotope.
This line is then interpreted as an effect produced by radioactive decay to give
an ‘age’ interpretation. Since all of these rock samples are supposed
to have been formed at the same time because they come from the same geological
unit, this line is called an ‘isochron’ (from the Greek isos
equal, and chronos time) or line of equal age. Furthermore, it can be shown
mathematically that the slope of the line can then be used to calculate the isochron
‘age’ of the geological unit from which the rock samples came.
This method has become popular because no knowledge or assumptions about the initial
conditions of parent and daughter isotopes need be made. Furthermore, since the
analytical equipment determines isotopic ratios, not absolute abundances of isotopes,
parent and daughter isotopes are usually expressed as ratios relative to a reference
isotope whose abundance is not affected by radioactive decay, thus providing easy
application of the method and more confidence in its results.
While the assumptions of constant decay rate and a closed system are again necessary,
the isochron method also has two other critical assumptions—the rock samples
must represent the one unit that formed at the same time geologically, and the daughter
isotope was uniformly distributed through all the samples when the rock unit formed.
Because of the apparent ‘success’ of this isochron method, it has become
in recent years the cornerstone of radioactive dating in geology.
Isochron dating questioned
However, it is this isochron dating method that has recently come ‘under fire’.
Writing in the international journal Chemical Geology,2 Y.F. Zheng of the Geochemical Institute at the University
of Gottingen in Germany says:
‘The Rb-Sr isochron method has been one of the most important approaches in
isotopic geochronology. But some of the basic assumptions of the method are being
questioned at the present time. As first developed the method assumed a system to
have: (1) the same age; (2) the same initial 87Sr/86Sr ratio;
and (3) acted as a closed system. Meanwhile, the goodness of fit of experimental
data points in a plot of 87Sr/86Sr vs. 87Rb/86Sr
served as a check of these assumptions. However, as the method was gradually applied
to a large range of geological problems, it soon became apparent that a linear relationship
between 87Sr/86Sr and 87Rb/86Sr ratios
could sometimes yield an anomalous isochron which had no distinct geological meaning.
A number of anomalous isochrons have been reported in the literature and various
terms have been invented, such as apparent isochron (Baadsgaard et al., 1976), mantle
isochron and pseudoisochron (Brooks et al., 1976a, b), secondary isochron (Field
and Ra- Heim, 1980). inherited isochron (Roddick and Compston, 1977), source isochron
(Compston and Chappell, 1979), erupted isochron (Betton, 1979; Munksgaard, 1984),
mixing line (Bell and Powell, 1969; Faure, 1977; Christoph, 1986) and mixing isochron
(Zheng, 1986; Qin, 1988). Even a suite of samples which do not have identical ages
and initial 87Sr/86Sr ratios can be fitted to isochrons, such
as aerial isochrons (Kohler and Muller-Sohnius, 1980; Haack et al., 1982).’3
He went on to say:
‘Evidently, the theoretical basis of the classical Rb-Sr isochron is being
challenged and some limitations of its basic assumptions are being revealed….
Some of what this paper contains is not new to isotopic geochronologists, but it
is drawn together here for the first time and is placed in a context within unifying
general models for Rb-Sr dating.’4
However, Zheng’s paper really isn’t the first time that these problems
with the isochron dating method have been comprehensively highlighted and treated
mathematically. It was in fact creation scientists who first comprehensively pointed
to the problems with the isochron dating method. In a series of short articles published
in the Bible-Science Newsletter in 1981, Dr. Russell Arndts, Professor of Chemistry
at St Cloud State University in Minnesota, and Dr. William Overn, a former engineer
and physicist with the National Aeronautics and Space Administration (NASA), showed
how isochrons were in fact often a result of the mixing of the radioisotopes from
different sources.’5 They also
illustrated this with various examples from the geological literature. They concluded:
‘It is clear that mixing of pre-existent materials will yield a linear array
of isotopic ratios. We need not assume that the isotopes, assumed to be daughter
isotopes, were in fact produced in the rock by radioactive decay. Thus the assumption
of immense ages has not been proven. The straight lines, which seem to make radiometric
data meaningful, are easily assumed to be the result of simple mixing.’(their
emphasis)6
They go on to suggest that the concept of mixing a material from wide ranges seems
to suggest that the earth has undergone widespread stirring. Such processes do not
of course always involve the actual physical movement of rock, rock-forming components
such as mineral grains, or molten materials, but more often involve the mixing of
chemical components via fluxes of fluids, principally water, through the rocks.
Zheng concurs with this in his paper when he speaks of geological processes such
as hydrothermal (hot water) alteration, metasomatism, and metamorphism, the latter
two involving changes in rocks due to fluids, temperature, and pressure. Zheng admits:
‘In some cases, gain or loss of Rb and Sr from the rocks is so regular that
a linear array can be produced on the conventional isochron diagram and a biased
isochron results from the altered rocks to give spurious age and initial 87Sr/86Sr
ratio estimates.’7
At the end of his paper, Zheng wrote:
‘In conclusion, some of the basic assumptions of the conventional Rb-Sr isochron
method have to be modified and an observed isochron does not certainly define a
valid age information for a geological system, even if a goodness of fit of the
experimental data points is obtained in plotting 87Sr/86Sr
vs. 87Rb/86Sr. This problem cannot be overlooked, especially
in evaluating the numerical time scale. Similar questions can also arise in applying
Sm-Nd and U-Pb isochron methods.’8
And as if to make the point even more succinctly and clearly, Zheng also wrote in
the abstract (or summary) of his paper:
‘As it is impossible to distinguish a valid isochron from an apparent isochron
in the light of Rb-Sr isotopic data alone, caution must be taken in explaining the
Rb-Sr isochron age of any geological system.’9
One could hardly expect a more emphatic and complete ‘demolition job’
on the isochron dating method than that! Notice also that Zheng extends his criticism
to the traditional uranium-lead (UPb) and currently-in-vogue samarium-neodymium
(Sm-Nd) isochron methods.
Conclusions
Given now these criticisms from an evolutionist geochemist/geochronologist in the
open scientific literature, one wonders how quickly geochronologists world-wide
will rigorously re-examine the isochron method and the results it has produced over
the past few decades. Of course, abandoning the method could hardly be countenanced,
as it would mean abandoning what has become one of the foundational cornerstones
to the whole evolutionary view of the geological development of the earth with its
millions of years time-scale.
Nevertheless, this ‘attack’ on radioactive dating by an evolutionist
in the open scientific literature is a timely reminder that there are problems with
these methods. Christians need not compromise with the evolutionists’ time-scale
because it is being propped up by these faulty dating methods. Rather, we should
place our confidence in the 6,000-7,000 year chronology given us by our Creator
in His eyewitness account of what happened. Furthermore, the Creator has given us
abundant scientific evidence that His Word is true ‘from the beginning’.
References
- Humphreys, D.R., 1991. Evidences for
a young world. Creation 13(3): 28–31.
Return to text
- Zheng, Y.-F., 1989. Influences of the nature of the initial Rb-Sr
system on isochron validity. Chemical Geology (Isotope Geoscience Section),
vol. 80, pp. 1-16. Return to text
- Zheng, Ref. 1, pp. 1-2. The references quoted are:
- Baadsgaard, H., Lambert, R.st.J. and Krupicka, J., 1976. Mineral isotopic age relationships
in the polymetamorphic Amitsoq gneisses, Godthaab District, West Greenland. Geochimicael
et Cosmochimica Acta, vol. 40, pp. 513–527.
- Bell, K. and Powell, J.L., 1969. Strontium isotopic studies of alkalic rocks: the
potassium-rich lavas of the Birunga and Toro-Ankole regions, East and Central Equatorial
Africa. Journal of Petrology, vol. 10, pp. 536–572.
- Betton, PJ., 1979. Isotopic evidence for crustal contamination in the Karroo rhyolites
of Swaziland. Earth and Planetary Science Letters, vol. 45, pp. 263–274.
- Brooks, C, James, D.E. and Hart, S.R., 1976a. Ancient lithosphere: its role in young
continental volcanism. Science, vol. 193, pp. 1086–1094.
- Brooks, C., Hart, S.R., Hofmann, A. and James, D.E., 1976b. Rb-Sr mantle isochrons
from oceanic regions. Earth and Planetary Science Letters, vol. 32, pp.
51–61.
- Chrisloph, G., 1986. Isochron or mixing line? Proceedings of the 4th Workshop
Meeting on Isotopes in Nature, Leipzig, pp. 197–207.
- Compston, W. and Chappell, B.W., 1979. Srisotope evolution of granitoid source rocks.
In: M.W. McElhinny (editor), The Earth: Its Origin, Structure and
Evolution, Academic Press, London, pp. 377–426.
- Faure, G., 1977. Principles of Isotope Geology, Wiley, New York.
- Field, D. and RaHeim, A., 1980. Secondary geological meaningless Rb-Sr isochrons,
low 87Sr/86Sr initial ratios and crustal residence times of high-grade gneisses.
Lithos, vol. 13, pp. 295–304.
- Haack, U., Hoefs,J. and Gohn E., 1982. Constraints on the origin of Damaran granites
by Rb/Sr and ∂18O data. Contributions to Mineralogy and Petrology,
vol. 79, pp. 279–289.
- Kohler, H. and Muller-Sohnius, D., 1980. Rb-Sr systematics on paragneiss series
from the Bavarian Moldanubium, Germany. Contributions to Mineralogy and Petrology,
vol. 71, pp. 387–392.
- Munksgaard, N.C., 1984. High ∂18O and possible pre-eruptional
Rb-Sr isochrons in cordierite-bearing Neogene volcanics from SE Spain. Contributions
to Mineralogy and Petrology, vol. 84, pp. 281–291.
- Qin, Z–W., 1988. Mix-isochron and its significance in isotopic chronology.
Science Sinica, vol. B28, pp. 97–108.
- Roddick, J.C. and Compston, W., 1977. Strontium isotopic equilibration: a solution
to a paradox. Earth and Planetary Science Letters, vol. 34, pp. 238–246.
- Zheng, Y.-F., 1986. Crust-mantle Rb-Sr mixing isochron and its geological significance.
Terra Cognita, vol. 6, p. 151 (abstract).
Return to text
- Zheng, Ref. 1, p. 2. Return to text
- Arndts, R. and Ovem, W., 1981. Radiometric dating, isochrons, and
the mixing model. Bible-Science Newsletter, February, March, April and
August, 1981 issues. Return to text
- Arndts, R. and Overn W., 1981. Radiometric Dating, Isochrons,
and the Mixing Model, Bible-Science Association, Minneapolis, USA, reprint
series, p. 25. Return to text
- Zheng, Ref. 1, p. 13. Return to text
- Zheng, Ref. 1, p. 14. Return to text
- Zheng, Ref. 1, p. 1. Return to text
|
