ISOCHRON ROCK DATING IS FATALLY FLAWED
by William Overn
For Exciting New Work On Radiometric Dating Showing a Young Earth, Click HERE
ABSTRACT
Radiometric rock dating, the methodology of determining the date of
formation of a rock sample by the wellestablished rate of decay of the
isotopes contained, depends on accurately determination of the starting
points, the original concentrations of the isotopes. Many methods of
estimating these beginning concentrations have been proposed, but all
rest on tenuous assumptions which have limited their acceptance. This
paper attempts to show that the IsochronDiagram method contains a
logical flaw that invalidates it. This most accepted of all methods
has two variations, the mineral isochron and the wholerock isochron.
The logicallysound authenticating mechanism of the mineral isochron is
applied to the wholerock isochron, where it is invalid. The longterm
stability of the wholerock is applied to the mineral, where it is
inappropriate.
When the isochron data are the result of the rock being a blend of two
original species, the diagram is called a mixing line, having no time
significance. This paper shows that all wholerock isochrons are
necessarily mixing lines. It is noted that by analogy the mixingline
logic casts strong suspicion on the mineral isochron as well. Since
only wholerock isochrons play a significant role in the dating game
anyway, isotopic geochronology can be rather generally discredited.
Keywords:
Age
Geochronology
Isochron
Isotope
Oldearth
Radiometric
Youngearth
Introduction:
Thanks mainly to the fact that they appear to be so constant, the decay
rates of radioactive materials have become the primary mechanism for
attempting to discover the age of rocks.[5,16] In addition to a
constant rate of variation, however, any timing mechanism must also
have a calibrated beginning point. A number of methods have been tried
to calibrate the "radiometric clock". But they have all required
unprovable and apparently unwarranted assumptions. Faure, in his
textbook [9] refers to all of them as "assumed values" except for those
obtained by the "isochron", or similar linear method.
The linear methods are several, and have in common the reduction of the
data to a set which can yield a straightline plot. Many exceedingly
detailed descriptions of these methods are available.[1,2,5,16] A
summary description of the RbSr isochron is included below.
Arndts and Overn alerted the creationist community to the fact that in
spite of the mathematical rigor of the isochron, it also has
unwarranted assumptions, and the data carefully gathered and processed
to indicate immense ages can more appropriately be dismissed as
indicating the recent mixing of two or more magmas.[1,2,3] Dalrymple[6]
challenged our analysis with five points, all of which were promptly
and thoroughly refuted.[4]
In Dalrymple's latest book [7] he ignores the entire issue of the
wholerock isochron, only defending the mineral isochron. There is
sound logic supporting the mineral isochron, but another fatal flaw.
Individual mineral crystals are not closed systems. Even over the few
thousands of years available in the youngearth paradigm, they are
insufficiently stable to give acceptable data to the geochronologists.
The RbSr Isochron Method
Rubidium and strontium occur as trace elements in many common rock
types. Rubidium has two isotopes. ^{85}Rb (stable, abundance 72%) and ^{87}Rb
(radioactive). ^{87}Rb decays to ^{87}Sr with a halflife of (approximately)
48.8 billion years. Strontium is stable in all natural forms, and in
addition to the radiogenic ^{87}Sr (7%), has isotopes ^{88}Sr (82%), ^{86}Sr
(10%), and ^{84}Sr (<1%).
The general method of dating is to take several samples of the rock, to
determine the ratios of the RbSr isotopes in each, and by simultaneous
equations determine the probable beginning points for each, from which
the age may be determined.[16]
For the sake of compatibility with the available laboratory
instruments, the specific ratios chosen are ^{87}Rb^{86}Sr and ^{87}Sr^{86}Sr.
The algebra is equivalent to a simple straightline diagram as in
Figure 1. where points a, b, and c represent the samples.
Here is graphically represented the fact that the amount of daughter
isotope increases as the amount of parent increases in the sample. The
magnitude of that increase (i.e. the slope of the line) depends on the
time allowed for the decay process to transpire, or the age of the
rock. If we extrapolate down the line to the zero intercept, we have a
representation of a sample with no parent isotope to contribute to the
daughter concentration. This must represent the initial daughter
concentration.
The slope is the age and the intercept is the initial daughter ratio.
The scheme is mathematically sound. We must examine the assumptions.
For a problem to be solvable by simultaneous equations there must be as
many independent equations as there are unknowns. The unknowns are the
original ^{87}Sr^{86}Sr ratio for each sample and the age of each sample.
Each sample gives one equation, but introduces two additional unknowns.
Regardless of the number of samples, there are never enough equations
to cover all the unknowns.[16] These problems must be resolved by the
assumptions.
The same age
It is assumed that all samples analyzed together are the same age. The
word "isochron" (from the Greek "same time") symbolizes that. We do
not dispute this assumption.
The same initial strontium ratio
If all initial ^{87}Sr^{86}Sr ratios in the system are assumed to be the
same, the scheme can be made to work, as the unknowns are reduced to
two, the common age, and the common strontium ratio. Any two samples
may now introduce the required two equations, and any more beyond that
will simply improve the accuracy and the confidence level. This
assumption is outside the experience based on field data, however,
where the general case is that every sample has its own unique ratio.
However, it can be rationally assumed that each sample we find has its
own age and its particular rubidium concentration, which over time may
have imparted a unique portion of daughter isotope. The assumed
uniform strontium ratios should certainly be valid when applied to a
rock system solidifying from a uniform homogenized melt. We must
emphasize, however, that this enabling assumption must fail in the
absence of an initial homogenized melt.
A "closed" system
If isotopes have migrated in or out of the sample during the aging
period, the resulting data have no time significance. Isochrons are
thought to be self checking in this regard, since with several samples
an open system with random migration should scatter the points off of
the straight line. Indeed, it often happens that there is a scatter of
data, rendering the isochron worthless. But there are many occurrences
of isochrons having acceptably straightline form that are also
rejected. Often "metamorphism" is cited as the probable cause, the
system having opened, either partially or completely resetting the
clock. [11,19] In order to assure an acceptably closed system, samples
as large as 1 meter cubes have been suggested.[20] The assumption of a
closed system for many of the isochrons, if they have not been
questioned by the geochronologists, will not be challenged here. We
note that these are generally obtained on the samples of larger
dimensions, that is the wholerock isochrons.
Independent equations
If the equations are not independent, the problem cannot be solved.
This would be the case where all samples on the diagram plot on a
single point. Although the single point on the diagram is valid, there
is no way of finding a slope or intercept. If the melt were initially
homogeneous and remained closed, it could be expected still to be
homogeneous, and yield that singlepoint isochron. This should be the
general case of the wholerock isochron.
The need is to find samples with a variety of initial rubidium content
but still having initial strontium ratios that are known to be uniform.
The assumed initial homogeneous melt cannot be expected to give
wholerock samples with variable rubidium, but the assumed uniform
^{87}Sr^{86}Sr ratios demand such an initial homogeneous melt.
The mineral isochron solves the dilemma. The mineral crystals have
done the job in an elegant way. Crystals naturally form around a
specific chemical composition, each atom occupying its
naturallyassigned site. Foreign atoms just don't fit, either
electrochemically or physically, and are strongly rejected. Depending
on its concentration in the melt, a foreign element may have more or
less acceptance in a crystal, based on its chemical and physical
resemblance to one or another of the normal host elements. As the
crystals form, each different mineral type accepts a different trace
level of rubidium and of strontium. Because of their individual unique
chemistry they each extract a different amount of rubidium and of
strontium from the melt. The crystals of the individual minerals are
used as the rock samples in the mineral isochrons.
MIXING
Often an isochron yields an unacceptable slope, indicating an age much
too young or much too old to be compatible with the accepted model.
[19] Frequently the slope is negative.[18,14] A common explanation
for these cases is "mixing". It has always been recognized that the
same straightline plot as the isochron can be achieved if the original
melt were a mixture of two original homogenized pools.[12] Figure 1.
may also be used to illustrate this case. If points a and c are the
compositions of the two original pools that partially merged to form
the melt, any sample from the melt will occupy a place on a straight
line between them, such as point b. No sample will be found above a or
below c. Such a "mixing line" has no time significance, and the
textbook warns to be wary of accepting such mixing as a true isochron.
Faure's text also proposes a test for mixing. [13] If a plot of
^{87}Sr^{86}Sr vs 1/Sr (the concentration of strontium) shows a linear
relationship, then mixing is indicated. A brief study conducted in
1981 showed a high degree of correlation to this mixing test in the
isochrons being published.[3] A subsequent public dialog between
Dalrymple[6] and Arndts & Overn [4] concluded that although the mixing
test is strongly indicative of mixing, there are circumstances under
which mixing would not be detected by such a test, and others wherein
the test could give a false indication of mixing. The caution for the
geochronologist would be to suspect any isochron, since there is no way
to rule out mixing.
It is now clear, however, that there is at least one positive test for
mixing. It is the wholerock isochron itself. If the whole rock
yields samples that give a linear plot, whether the slope is positive
or negative, or whether the slope signifies an age that fits a
preconceived model or not, there is no other known mechanism outside of
mixing to which the data may be rationally ascribed.
Discussion
Mixing is an unfortunate misnomer that has become popular for
describing rocks formed from two or more original melts, or from a melt
becoming contaminated by isolated incorporation of local rock.
Understand it to mean partial mixing, with resulting heterogeneity.
Complete mixing would result in homogeneity, and would give only a
single point to plot. No curve of any kind, nor even a scattering of
points would occur.
This homogeneity is the assumed starting point in the history of the
rock being dated. It then solidifies. But now, years later, we dig up
6 adjacent meter cubes of the rock, and discover that the normalized
ratio of the parent (and incidentally of the daughter) is different in
each cube, sufficient to plot as an "isochron". How can we
rationally accept the assumed initial homogeneity? We can not.
What is needed but missing in the whole rock isochron is a mechanism to
establish initial homogeneity, and then to extract heterogeneous
samples. The mineral crystals do the job in an elegant way. Each type
accepts a different level of contamination of the parent isotope,
chemically determined. One cannot rationally extend this process back
to the whole rock. It has been tried, but there is a fallacy . [5,20]
As we stated in 1986: [5]
The wholerock isochron is justified on the basis that migration of the
isotopes in a metamorphic event may be confined to distances of perhaps
1 cm. This is much larger than the average crystal size. Thus the
original constituents of each crystal will lie nearby. By taking
samples of 100cm dimensions, one could assure that the entire content
of the original crystals are well represented by the sample, with very
small error. However, this matrix is the original melt that was
theorized to be homogeneous. The ability to find differences in the
rubidium content among the samples violates the assumption of original
homogeneity. Original inhomogeneity is the only possible explanation:
in other words, mixing.
This method of justifying the wholerock isochron on the basis of the
mineral is logically unsound. Within the larger matrix the tiny
crystals may incorporate discrete trace elements and return them over
time. But they are powerless to alter the composition of the
wholerock matrix.
It is claimed that fractional crystallization of magmas and separation
of crystals from the remaining liquid result in suites of comagmatic
rocks of differing composition. [10]. This may be true, but there is
no experimental evidence that this can generally be applied to trace
elements that are foreign to the crystals. Add the fact that trace
elements are not securely held by crystals until temperatures are well
below the melting points, and this postulate falls far short of
explaining the variation in rubidium in wholerock isochrons. Mixing
is much preferred, particularly when it is noted that many data sets
have negative slope, where mixing is always the accepted explanation.
Often the negativeslope data pertain to large formations that
particularly fit the hypothesis of slow cooling from a melt. [15,18]
In the case of the mineral isochrons the scheme postulates an initial
homogeneous melt, represented by a single point on the diagram. As the
crystals form, their differential solubility will move their individual
points on the diagram horizontally , different distances. (Only
horizontally, since the vertical is a ratio of two isotopes of the same
element). The large volume of wholerock isochrons, however, shows the
general case to be an initial heterogeneous melt represented by the
kind of diagram published as an isochron, and which we conclude is
actually a mixing line. Any point in the melt can be represented as a
point on the straight line. When mineral crystals form, each crystal
will move its point off the straight line in one or the other
horizontal directions. The result is a scattering of the points. The
geochronologist discards it as one of the following:
A three or more part mixture,
Subsequent metamorphosis,
Not a closed system: In this case he recognizes that crystals
really cannot be expected to be a closed system. They tend to
continue to reject contaminants long after formation, the
mobilities of foreign elements in crystals being a whole school
of scientific study. The retention of trace elements in
crystals is so inadequate that it has been possible to
construct "Isochrons" from various parts of the same
crystal.[17] It is common that when the mineral isochron
fails, the geochronologist then produces a wholerock isochron
from the same formation.
The ability to obtain a wholerock diagram, straightline or not, can
be considered proof that the data represent a "mixing line" rather than
an "isochron". If mixing has not occurred, and the system has remained
closed, then the wholerock data must all lie on a single point. In
fact, even if the wholerock data show scatter, either mixing is
indicated  but of a complex nature, with more than two components 
or there have been subsequent alterations described as the system being
open, or both.
Has any legitimate isochron ever been formed? It is improbable. There
is ample evidence for mixing. Any "isochron" could be mixing. There is
no way to rule it out. All wholerock "isochrons" are mixing, and they
are approximately 90% of all published. Many of the remaining
(mineral) "isochrons" have a wholerock point located close enough to
the straight line to discredit them. Why should we expect any of the
others to be "true isochrons", since mixing has the strongest
probability?
If one possesses a strong faith in the antiquity of the rocks, one
could rationally expect that an occasional mineral isochron is
legitimate. But it would also require the wholerock diagram to be
concentrated in a single point. (Neither a straight line or scattered).
Often a whole rock point is put on a mineral diagram. That does not
meet the criterion. Several wholerock samples must be obtained, using
the same techniques required for the wholerock method. Their
individual data points must be identical, i.e. superimposed on the
diagram. At that point mixing would not have been ruled out, but all
available tests requiring mixing would have been eliminated.
In the dialog with Dalrymple [4] it was noted that he is unwilling to
defend the wholerock isochron. In his latest book [7] on the age of
the earth he has included a section that describes the elegant process
with which crystals (minerals) give the necessary heterogeneity to make
the system work. He also shows why the mineral isochron cannot be
relied upon for dating, but does not state that conclusion. He
carefully avoids describing the wholerock method, which leads the
casual reader to conclude that it is validated by the same processes as
is the mineral method. Nothing could be farther from the case.
Dalrymple has seen our initial critique of the wholerock method, [5]
and is obviously reluctant to forthrightly claim any scientific
merit for it. He has clearly sidestepped the issue.
Dalrymple [7] does not depend directly on isochron dating of rocks to
date the earth, but rather on the leadisotope ratios. He must be
commended for his carefully pointing out the many assumptions involved.
However, he finally ignores them and claims that the age has been
determined within a very narrow margin.
His ultimate method is to take the radiometric ages of lead ores (Circa
2.63.5 Ga) and correct to the beginning. Again I point out that the
"isochrons" used to date the ores, as well as those of the meteorites,
that add so much to Dalrymple's confidence in the method, are most
probably mixing. Note tables 7.4 and 7.5, [Ref 7] which give many
meteorite ages. Almost all are wholerock.
Additionally note that with all his enthusiasm for the isochron,
Dalrymple characterizes the method as a "first approximation" [8]
As has been pointed out many times before, all radiometric methods
including the linearplot techniques have been effectively "calibrated"
to the fossil dates by selecting among the discordant data those that
fit the accepted stratigraphic model. [16] Since the proponents of
the isochrons don't take them at face value, others should by equally
wary.
See also: "Still No Proof For Ancient Age A Response" by W. M. Overn and Russell T. Arndts
A technical analysis of "Isochrons" as defended by Dalrymple against creationist criticism, showing that despite mathematical sophistication, they are unreliable and are calibrated to "known ages" using the geologic column.
For Exciting New Work On Radiometric Dating Showing a Young Earth, Click HERE
BIBLIOGRAPHY
[1] Arndts, R. & Overn, W. 1981 "Pseudo Concordance in UPb Dating"
BibleScience Newsletter 19(2):1.
[2] Arndts, R. & Overn, W. 1981 "Isochrons" BibleScience Newsletter
19(4):56.
[3] Arndts, R., Kramer, M. & Overn, W. 1981 "Proof of the Validity of
the Mixing Model" BibleScience Newsletter 19(8):1.
[4] Arndts, R. & Overn, W. Proceedings of 1985 Creation Conference
North Coast BibleScience Association, Cleveland, Ohio.
[5] Arndts, R. & Overn, W. 1986 "Radiometric Dating  An unconvincing
Art" Proceedings of the First International Conference on Creationism
Vol 2, Creation Science Fellowship, Pittsburgh, Pennsylvania, pp
167173.
[6] Dalrymple, G. B. 1984 "How Old is the Earth? A Reply to {at}Scientific
Creationism' " Proceedings of the 63rd Annual Meeting of the Pacific
Division AAAS 1(3):8486
[7] Dalrymple, G. B. 1992 The Age of the Earth
[8] Ibid p. 402.
[9] Faure, L. 1977 Principles of Isotope Geology John Wiley & Sons,
Inc. New York, New York. p.78
[10] Ibid p. 79.
[11] Ibid p. 8387.
[12] Ibid p. 97105.
[13] Ibid p. 101.
[14] Jager, E. & Hunziker, J. C., eds, 1979 Lectures in Isotope Geology
SpringerVerlaug, Berlin, Heidelberg and New York, p. 36
[15] Ibid p. 142144
[16] Overn, W. 1986 "The Truth About Radiometric Dating" Proceedings
of the First International Conference on Creationism Vol 1, Creation
Science Fellowship, Pittsburgh, Pennsylvania, pp 101104.
[17] Scharer, V. & Allegre, C. 1982 "Uranium  Lead System in Fragments
of a Single Zircon Grain" Nature 295 (Feb.): 585
[18] Tilton, G. R. & Barreio, B. A. 1979 "Origin of Lead in Andean
CalcAlkaline Lavas, Southern Peru" Science 210, 12451247
[19] Woodmorappe, John 1979 "Radiometric Geochronology Reappraised"
Creation Research Quarterly 16, 102129
[20] York, D. & Farquhar, R. M. 1972 The Earth's Age and Geochronology
Pergamon Press, New York, pp. 80 ff.
