Methods for Dating the Age of Meteorites

Methods of Dating the Age of Meteorites

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 + (87Rboriginal87Rbnow)

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.’


Meteorites and Their Parent Planets, Harry Y. McSween, Cambridge University Press, 1987.

Thunderstones and Shooting Stars, Robert T. Dodd, Harvard University Press, 1986.

Created on 25 Sep 1998 by Jim Hurley

For additional information on radiometric dating, read the PSRD article by Alexander N. Krott: Dating the Earliest Solids in our Solar System , Sep 2002.

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.

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