I had written: "Your comment presupposes that the presuppositions
of these radiometric dating methods are valid. But if the
isotopic data is better understood in terms of geochemical or
non-radiogenic geophysical causes, the data is not meaningful in
a chronological sense".
Bill Hamilton wrote on 28th March:
"What do you mean by this? Do you have references? I am not a
geologist, but I understand radioactive decay and I have read a
few references on radiometric dating methods and the rules
geologists use to qualify samples to reduce the possibility of
errors. As nearly as I can tell, their science is sound."
In my original post to Steve, I illustrated my point with two
examples: the theoretical distinction between "normal" and
"abnormal" lead isotopes (which is hard to defend), and
alternative explanations of isochrons. I will pick up on the
latter to develop the argument, basing it on Rb/Sr dating. In
what follows, I am not throwing stones at geochronologists -
their methodology is sound GIVEN THEIR PREMISES. What we are
looking at is an alternative framework for interpretation - one
that is open to the idea that the isotopic ratios are not
necessarily caused by the passage of time. (I should add that
I last looked seriously at this subject about 10 years ago - but
I'm not aware of significant developments over the last decade).
1. Isochrons are not a sign of closed systems.
The thought sometimes expressed is that BECAUSE a straight line
is produced, the data must be "reliable" and the conclusions
"accurate". This is of significance because both Sr and Rb
isotope values are quite variable in igneous rocks. Can we be
sure that the system is closed and isotope migration has not
occurred?
It is common to identify metamorphic or deformation events which
disturb or reset the isotopic ratios. However, it is generally
assumed that the system is closed outside these events - so that
the dates obtained are dates of the disturbances.
Matsua (1974) has shown that "a linear isochron can be obtained
even for an open system". "Such open systems likely exist in
Nature. Hence, we must be very careful to conclude the age on
the basis of an isochron".
Sometimes, an identifiable disturbance is found which correlates
with the resultant date - and the interpretation placed on the
data is not only consistent but considered meaningful. But is
this optimism justified? Field and Raheim (1979) published their
data regarding "A geologically meaningless Rb-Sr total rock
isochron". Although the rock suites used showed "evidence of
only minor secondary alteration effects", they had to conclude
that "An isochron so produced is geologically meaningless, that
is, the apparent age is unrelated to any geological event".
Bell and Blenkinsop (1978) address the problem of open systems:
"The susceptibility of fine-grained, volcanic rocks to open-
system behaviour during regional metamorphic activity has long
been suspected in Rb/Sr geochronology". Their paper not only
confirms suspicions, but relates Rb/Sr ratios to bulk chemical
composition. Their paper discusses the problem of getting any
meaningful information from the analyses.
Are meaningless isochrons the norm or the exception? Are
premises relevant here?
2. Isochrons can be produced by the mixing of source materials.
Suppose that a two-phase model of rock magma be considered. This
is not a way-out suggestion as bimodality is not unusual in
igneous terrains. Whilst models of magma evolution are less
complex, most will agree that there are many unknowns between
generation and emplacement. In our two phases, we have
differences in the Rb and Sr contents.
The resultant rocks produced from these magmas will yield
isochrons which are unrelated to age. The isochron is a
geochemical signature, not a mark of the passage of time. The
theory for this seems to be well-recognised in the literature,
but I have not got a reference to the mathematical derivation to
report.
Dupre et al (1982) discuss isotopic variations on the island of
Terceira. they report observations "in agreement with a model
of mixing between two different mantle components, ..." They
obtained isochrons using lead isotopes, but stated: "These
straight lines must be considered as mixing lines with no
chronometric significance".
Shaffer and Faure (1976) looked at strontium ratios in sediments
taken from the Ross Sea. Their conclusions are:
"The isotopic compositions of strontium and concentrations of
rubidium and strontium as well as those of several detrital
minerals in the <100-mesh noncarbonate fractions vary
systematically throughout the Ross Sea. These variations can be
attributed to mixing of two detrital components which are
weathering products of old sialic rocks and young basaltic
volcanics in Antarctica. ... Linear correlations of 87Sr/86Sr and
87Rb/86Sr ratios are due to mixing of two components and bear no
relation to the age of the rocks in Antarctica or to the time of
deposition of the sediment."
Again I ask: if magma mixing is a mechanism for producing
isochrons, why do we not hear more about it? Are there ways of
distinguishing between age-related isochrons and geochemical
isochrons (due to mixing)? (There is one method - see below).
Are people even asking these questions? How far are premises
important here?
3. The phenomenon of pseudoisochrons.
According to Brooks et al (1976), "One serious consequence of the
mantle isochron model is that crystallisation ages determined on
basic igneous rocks by the Rb-Sr whole-rock technique can be
greater than the true age by many hundreds of millions of years.
This problem of inherited age is more serious for younger rocks,
and there are well-documented instances of conflicts between
stratigraphic age and Rb-Sr age in the literature." This
phenomenon of "inherited age" is the theme of their paper.
They distinguish a pseudoisochron from a simple isochron plot in
the following way. The simple plot is of current-day values, but
the pseudoisochron used values corrected back to the time of
crystallisation of the magma. However, in practice, "For the
most part the pseudoisochrons come from young volcanic terranes
in which no age correction of the measured present-day Sr
isotopic composition is necessary."
The paper documents a whole series of "anomalous dates" from
tertiary volcanics. Of particular interest are two reported by
Leeman (1974). Hawaiites from the Western Grand Canyon have an
apparent age of 1300 million years, and alkali basalts from the
same area are dated as 1100 million years. I'll comment further
on these below. The authors suggest three ways of getting
pseudoisochrons:
1. Mixing of heterogeneous magmas.
2. Selective melting of heterogenous mantle material of the same
age or different ages.
3. Disequilibrium melting of homogeneous mantle in which
individual mineral phases are in isotopic disequilibrium.
"Distinguishing between these possibilities is difficult as none
of the proposed mechanisms leads to a uniquely identifiable
compositional property in the rocks, other than that of Pb and
Sr isotopic variation ..."
"One means of evaluating pseudoisochrons is to determine whether
they involve identifiable mixing-line chemistry. Rubidium-
Strontium variation diagrams and plots of 87Sr/86Sr against 1/Sr
can be used for this purpose. Examination of such plots for the
data [used in this study] reveals that a good pseudoisochron is
commonly accompanied by a good positive correlation between
87Sr/86Sr and 1/Sr, but no correlation at all between Rb and Sr.
The absence of an Rb-Sr correlation is inconsistent with simple
mixing. Thus, while we cannot discount mixing of heterogeneous
mantle material, the data in general seem to preclude simple two-
component magma mixing as an important factor in most
pseudoisochrons."
I do have a problem with the last point above. I would say that
a correlation of 87Sr/86Sr and 1/Sr is indicative of mixing. Are
the 87Sr isotopes due to radioactive decay of 87Rb, or are they
dependent on the concentration of Sr in the rock? The positive
correlations reported indicate that the strontium isotope
concentrations are directly dependent on the strontium present
in the rock. At the moment, I do not follow Brooks argument -
if anyone can see the logic behind it, I will be glad to hear
from you.
The Grand Canyon lavas mentioned above are the ones studied by
Steve Austin - some discussion took place on this last year.
Austin has obtained values of 1.3 billion years for the Tertiary
basalts of Grand Canyon: something for which he is using to
question the appropriateness of using Rb/Sr dating. The
important point here is that his findings are is close agreement
with Leeman (1974). To nit-pick at possible selective use of
data seems to be an unwise response to Austin. The argument of
Brooks et al (1976) is that Austin has found a pseusoisochron -
that conveys information about the earth's mantle and mechanisms
of petrogenesis. At the moment, unless I can get my question
(previous paragraph) answered, I think the magma mixing
explanation is to be considered the most likely - which implies
that the isochrons have geochemical, but not geochronological,
information.
Even if Brooks et al are right, the number of variables involved
becomes very large. We have a situation where the isotope
variations cannot be interpreted without a context:
"Effective use of the mantle isochron concept requires knowledge
of actual crystallisation ages (so that the measured isochron can
be divided into its pre- and post-crystallisation components) and
determination of isochron parameters on rocks that have been
subjected to minimal postmelting processes (fractional
crystallisation, wall rock contamination, and so forth)..."
The net effect is to "fit" the data into an accepted paradigm.
Conclusion
I have discussed (1) the problem of open systems, (2) the
alternative explanation of magma mixing, and (3) pseudoisochrons.
The picture emerging is one of paradigm-dependency. The
presuppositions are essential for the method to deliver results.
The objectivity of radiometric dating (at least affecting Rb/Sr
dating) is an illusion. My personal conclusion (stated in a
previous post) is that all dating methods are subject to
presupposition problems of this kind - and that we really have
no valid ways to establish a chronology of earth history.
References
Bell, K. and Blenkinsop, J. 1978. Reset Rb/Sr whole-rock
systems and chemical control. Nature, 273(15 June), 532-534.
Brooks, C., James, D.E. and Hart, S.R. 1976. Ancient
lithosphere: its role in young continental volcanism. Science.
193(17 September), 1086- 1094.
Dupre, B., Lambret, B. and Allegre, A.J. 1982. Isotopic
variations within a single oceanic island: the Terceira case.
Nature, 299(14 October), 620-622.
Field, D. and Raheim. 1979. A geologically meaningless Rb-Sr
total rock isochron. Nature, 282(29 November), 497-499.
Leeman, W.P. 1974. Late Cenozoic alkali-rich basalt from the
Western Grand Canyon Area, Utah and Arizona: isotopic composition
of strontium. Geological Society of America Bulletin.
85(November), 1691-1696.
Matsuda, J. 1974. A virtual Rb-Sr isochron for an open system.
Geochemical Journal. 8, 153-155.
Shaffer, N.R. and Faure, G. 1976. Regional variations of
87Sr/86Sr ratios and mineral compositions of sediment from the
Ross Sea, Antarctica. Geological Society of America Bulletin.
87(October), 1491-1500.
------------------------
Best wishes,
*** From David J. Tyler, CDT Department, Hollings Faculty,
Manchester Metropolitan University, UK.
Telephone: 0161-247-2636 ***