The decay of rubidium-87 (⁸⁷Rb) into strontium-87 (⁸⁷Sr) is perhaps the
most useful scheme for geologic age determination. The same problem
shows up here, but at least there is a way out of the wilderness. It is
not exactly simple, but a consideration of it is fundamental to
understanding the process of nuclear dating. The figure shows patterns
from mass spectrometer charts; each peak represents an isotope of
strontium, and the height of every peak is proportional to the relative
abundance of that isotope. In the figure, A shows the mass-spectrum of a
rock or mineral containing COMMON strontium (which is a mixture of
several isotopes). The peak of ⁸⁷Sr is small compared to the others. B
shows the mass-spectrum of strontium from an old rubidium-rich mineral
CRYSTAL, drawn to the same scale, as far as the nonradiogenic isotopes,
⁸⁴Sr, ⁸⁶Sr, and ⁸⁸Sr, are concerned. The ⁸⁷Sr peak in this spectrum is
obviously larger than in the common strontium in A. This is because this
isotope is radiogenic and has been accumulating from the decay of
rubidium since this crystal was formed.
The question we must answer is: How much of this ⁸⁷Sr was formed from
⁸⁷Rb decay and how much originally was present in the crystal as an
impurity? If the amount of this ORIGINAL strontium is not too large, the
problem can be solved by simple arithmetic.
First, we must find a good sample of common strontium—that is, ordinary
strontium, the kind shown at left in the figure. We cannot require that
this strontium be entirely uncontaminated by radiogenic strontium,
because all strontium is more or less contaminated. What we need is
strontium contaminated to _just the same extent_ as the strontium that
was taken as an impurity into the closed system when it first formed. In
geological specimens such a material is usually available.
[Illustration: _Drawings of mass-spectrometer charts showing the
isotopic spectra of two kinds of strontium: (A) common strontium and (B)
strontium from an old mineral rich in rubidium._ (_See page 32 for photo
of a mass spectrometer._)]
[Illustration: _Rubidium-87 and strontium-87 fall on the same spot in
the mass spectrum. Therefore, rubidium must be separated chemically from
strontium before the strontium can be analyzed in a mass spectrometer.
It is done with ion-exchange columns. Four of them are shown in this
photograph. The author of this booklet is adding a sample, dissolved in
a few drops of hydrochloric acid, to the second column._]
Let us take as our closed system a mica crystal in a mass of granite.
Mica contains a fair amount of rubidium, and it retains its radiogenic
strontium very well. Furthermore, mica crystals are often associated or
even intergrown with the slender, rod-shaped crystals of a mineral
called apatite—a phosphate of calcium. It is justifiable, on the basis
of geological knowledge, to say that the mica and the apatite grew at
roughly the same time and thus presumably from the same liquid medium
that became granite when it later solidified. Now strontium is
geochemically similar to calcium, and some strontium will have gone into
the apatite crystal in place of calcium. Apatite contains no
alkalis—hence apatite will have virtually no rubidium (which is an
alkali) in it to contaminate the ⁸⁷Sr. Consequently, when we find
apatite in an old granite, we know the apatite will still contain the
kind of common strontium that was taken into the mica crystal when it
grew originally.
We can separate the apatite from the granite by standard mineralogical
techniques, extract the strontium from the apatite chemically, and
analyze it on a mass spectrometer to obtain the isotopic spectrum—the
relative amount of each isotope that is present. We can then perform the
same isotopic analysis on the strontium extracted from the mica, and
subtract the original (apatite) strontium from the total (mica)
strontium, to obtain the radiogenic component or daughter product. (See
page 32 for details of this method.)
Obviously, there is a certain error associated with every isotopic
analysis, so such a calculation is meaningful only when the radiogenic
component is large compared with the error in the measurement of
isotopic abundance. When one large quantity must be subtracted from
another large quantity to obtain a small difference, there is an obvious
limit to how much one can trust the result. The absolute accuracy in
measuring strontium isotope abundance is a few tenths of 1%, using the
best mass spectrometers now available. In practice, one can trust a
calculated age for a specimen only when the ⁸⁷Sr is as little as about
5% radiogenic. The results do not mean much when only 1 or 2% is
radiogenic.
[Illustration: _A sample of granite being made ready for crushing and
mineral separation._]