Meteorites are among the oldest
objects we know about - formed about 4.5 billion years ago. But how
do scientists know this? This article describes the principles and
methods used to make that determination.
There are well-known methods of finding the ages of some natural
objects. Trees undergo spurts in growth in the spring and summer
months while becoming somewhat dormant in the fall and winter
months. When a tree is cut down, these periods are exhibited in a
cross section of the trunk in the form of rings. Simply counting the
number of rings will give one a fairly good idea of the age of the
tree. Periods of heavy rain and lots of sunshine will make larger gaps
of growth in the rings, while periods of drought might make it
difficult to count individual rings.
When determining the ages of very old objects, the only suitable
clocks we have found involve the measurement of decay products of
radioactive isotopes.
Isotopes are atoms of the same element with different
amounts of neutrons. Some isotopes are stable, whereas others are
radioactive and decay into other components called daughter
isotopes. For example, hydrogen has two stable isotopes 1H
(ordinary hydrogen), 2H (deuterium), and one radioactive
isotope 3H (tritium). The superscript denotes the atomic weight
of the isotope (the number of protons and neutrons).
Radioactive isotopes decay according to a power law, and the
typical unit given for this is called the half-life of the
isotope. When a given quantity of an isotope is created (in a
supernovae, for example), after the half-life has expired, 50% of the
parent isotope will have decomposed into daughter isotopes. After the
second half-life has elapsed, yet another 50% of the remaining parent
isotope will decay into daughter isotopes, and so on. For all
practical purposes, the original isotope is considered extinct after 6
half-life intervals.
Some of the isotopes and their daughters are shown in the
following table (from Dodd):
The isotopes above the line in that figure are now extinct, since there
are no means of replenishing the parent isotope in the Solar System.
Note that there are vast ranges of time exhibited in the decay
rates, allowing a suitable measure if one knows or guesses the
approximate age.
The clock most suitable for meteorites is the decay of Rubidium
(87Rb) into Strontium (87Sr), which has a
half-life of about 49 billion years. The manner in which the age is
determined is based on calculating ratios of these isotopes, as the
following calculation will show:
We know if there is some 87Rb present in the meteorite,
that there will also be the decay product 87Sr. However,
there will also be some unknown amount of 87Sr that was in
the meteorite when it formed. We can state mathematically, that the
amount of 87Sr present now, must have come from the amount
that was there originally, plus any decay product from
87Rb:
87Srnow =
87Sroriginal + (87Rboriginal - 87Rbnow)
The term in parenthesis, the amount of 87Rb that decayed into
87Sr can be related by the radioactive decay law:
87Rboriginal =
87Rbnow * (elt)
where, e is the base of the natural logarithm, l is the rate of radioactive decay,
and t is the elapsed time.
By substituting that in the original equation we get:
87Srnow =
87Sroriginal +
87Rbnow * (elt - 1)
Along with 87Sr, 86Sr also occurs
in meteorites, but it is not a decay product and its amount does not change
over time, so we can divide this constant in the above equation without
changing the equality:
87Srnow / 86Sr =
87Sroriginal / 86Sr +
87Rbnow / 86Sr * (elt - 1)
Note that this is the equation of a line in the form
y = mx + b
where, m, the slope, is (elt - 1)
and b, the y intercept, is the original strontium isotope
ratio.
Two of these quantities can be measured:
87Srnow / 86Sr and
87Rbnow / 86Sr. By taking samples
from various parts of a meteorite and plotting these results, the
data will fall on a straight line whose slope characterizes the age of the
meteorite. These lines are called isochrons, an example
for the meteorite Tieschitz (fall, 1878, Czechoslovakia, unequilibrated H3)
is shown in the following figure (from McSween):
How are these Measured?
Scientists use a mass spectrometer to obtain these ratios.
A small portion of a meteorite is vaporized in the device forming
ions. These ions are accelerated in an electric field through
collimating slits and subject to a magnetic field which causes the
ions to follow a curved path. The ions are deflected according to
their mass. By adjustment of the strength of the magnetic field and
suitable placement of an ion collector, the different isotopes can be
measured with precision.
Complications
There are some things that affect these measurements. Thermal processes
that may occur during meteorite impact in the lifetime of the specimen
can reset some of the atomic clocks, mixing components and releasing important
gases such as 129Xe and 40Ar.
In practice, several isotope systems and several samples are used
to determine the ages. Meteorites that are mostly unaltered (petrological
type 3) serve as the best samples.
Epilog: Jan. 24, 1999
I received private communications from scientists about this paper,
which was based mainly on work done in the 1980's. Nowadays,
146Sm - 142Nd with a mean life of 1.49 x 108
years is also used, along with other methods to date meteorites.
In one note, from Dr. Bogard at NASA, it was mentioned to me that:
"You refer to extinct nuclides 14C, 26Al, and 129I. Only the latter
two "extinct" nuclides are used in dating. The use of 14C in
meteorite dating is solely based on its production by cosmic rays (and
for terrestrial samples, with its production in the atmosphere). 26Al
and some other nuclides not mentioned are also used in this way.
Thus, although "extinct", these nuclides are present in meteorites,
but produced by a more recent process.
"The idea that Rb-Sr is the most used chronometer for meteorites is
largely based on work done 10-30 years ago. Increasingly, the other
techniques are used, such that probably no one technique dominates for
meteorite dating. Rb-Sr is a good example for explaining the process,
however.
"Near the end you imply that low petrologic type chondrites are the
most easily dated. Actually, meteorites that formed by melting, e.g.,
the various types of achondrites, usually give more precise ages.
Type-3 chondrites can contain phases with slightly different ages, and
some phases have been slightly altered by parent body processes."
References
Meteorites and Their Parent Planets, Harry Y. McSween, Cambridge University Press, 1987.
Thunderstones and Shooting Stars, Robert T. Dodd, Harvard University Press, 1986.
Another PSRD article, Using Aluminum-26 as a Clock for Early Solar System Events , Sep 2002, by Ernst Zinner, examines how Al-26 can be used as a fine-scale chronometer for early solar system events.
A detailed description of radiometric dating using the isochron method can be found on the website of Chris Stassen.
