Fission track dating

When the isotope uranium-238 decays by fission in certain minerals, charged nuclei create a trail of damage, called a fission track. In transparent minerals, fission tracks can be enlarged by chemical etching until they are visible in an optical microscope. The age of the mineral can then be determined from the number of fission tracks and the uranium concentration in the mineral sample.

The fission track dating technique is applicable to any type of rock containing minerals or glasses that record fission tracks and have sufficient uranium concentration to produce a detectable number of fission events within an appropriate-sized sample. Samples containing uranium at a concentration of one part per million can easily be dated by this technique if they are more than 100,000 years old. Correspondingly higher concentrations of uranium are required for younger samples, and lower concentrations are required for older ones. Because of its broad applicability, the fission track dating method has been applied to all three major terrestrial rock classes—sedimentary, metamorphic, and igneous.

Nuclear fission is a process by which an unstable atomic nucleus splits into two smaller nuclei, or fission fragments. The fission process releases a large amount of energy, causing the fission fragments to fly apart. In solid matter, each fission fragment can travel about 10-20 micrometers before coming to rest. Because of their high energy, these fission fragments do not carry along all the electrons from the original atom. Therefore, they have a positive electric charge during their passage through the surrounding matter. The passage of a charged particle through certain types of material gives rise to localized damage along the path of that particle. The damage caused by fission fragments can be observed in some materials, making it possible to determine the number and location of fission decays that have taken place since that material was formed.

Spontaneous Fission

Spontaneous fission is a random radioactive decay process. For many elements with atomic numbers higher than that of lead, although alpha decay is usually the dominant mode of decay, spontaneous fission decay is also observed. The half-life (the time for half of any initial amount of an isotope to decay) for spontaneous fission has been measured for those elements having significant spontaneous fission decay. The age of a sample containing uranium-238 depends on the number of fission tracks, the half-life for fission decay, and the number of uranium-238 atoms present. The number of atoms can be found from mass spectroscopy of a small part of the sample or by methods involving neutron irradiation of the sample.

In natural mineral samples, uranium is the only element currently present for which spontaneous fission is significant. Because uranium-235 has a much longer spontaneous fission half-life and a much lower abundance than uranium-238, the major contribution comes from the fission of the latter. In minerals that still survive from the very early era of solar system formation, plutonium-244 can have contributed many more fissions than did uranium because plutonium-244 has a very short spontaneous fission half-life. Minerals old enough to display plutonium-244 fission tracks occur in meteorites and in some ancient lunar samples, but no terrestrial rocks preserving a record of such fission have yet been found.

Determining the Number of Fission Decays

The age of a natural mineral sample containing a significant amount of uranium can be calculated if the number of spontaneous fission decays since the formation of that mineral can be determined. The present abundance of uranium in the sample is first determined. The number of fission decays, along with the known spontaneous fission half-life of uranium-238 and the measured uranium abundance, is what is used to calculate the time that has elapsed since formation of the mineral.

The damage caused by the passage of each fission fragment is examined to determine the number of fission decays that have occurred in the mineral since its formation. In 1959, it was observed that the passage of fission fragments through natural silicate minerals gave rise to a population of short damage trails, which could be viewed in a transmission electron microscope. Because of the low uranium abundance in natural silicates and the high magnification required to observe these small damage trails, however, the use of the transmission electron microscope to count fission decays in such samples was not routinely practical. The major breakthrough permitting the routine fission track dating of natural minerals came in the 1960s, when a technique was developed to permit the damage trails to be observed at low magnification in an ordinary optical microscope. In these damage trails, chemical activity is greater than in the surrounding undamaged mineral. Certain chemical etches will attack the damage trail more rapidly than they will the surrounding crystal. Initially, the etch removes material along the damage trail. As the hole lengthens, however, the etch also attacks the walls, enlarging the diameter of the hole. These etched holes, or fission tracks, can then be easily counted using an optical microscope.

Ion Explosion Spike Model

The detailed mechanism by which a fission fragment interacts with the mineral structure to produce the damage trail has not been positively determined. The “ion explosion spike” model, however, is the generally accepted mechanism. In the ion explosion spike model, the positively charged fission fragment passes through a crystalline mineral, which consists of a periodic array of positively charged nuclei, each surrounded by orbiting electrons. The charged fission fragment removes electrons from some of the atoms along its path, leaving a line of positively charged ions in the crystal. If the electrical conductivity of the crystal is low, a significant time elapses before the ejected electrons can migrate back to the ionized nuclei, restoring local electric neutrality. During this time, the positively charged nuclei along the path of the fission fragment repel one another electrostatically, causing displacements in the crystal structure. Once electrical neutrality is restored, the displaced atoms remain. This damage trail is visible in a transmission electron microscope as a disruption of the periodic array structure or can be enlarged by chemical etching.

The damage to the crystal structure can, however, be removed (or “annealed”) by heating the mineral. The temperature required to anneal fission tracks depends on both the type of mineral and the duration of exposure to heat. For time scales appropriate to most geological measurements (thousands of years or longer), the track annealing temperatures of common minerals range from less than 100 degrees Celsius to more than 600 degrees. Thus, the age actually measured by fission track dating is the time interval from the present back to the time when the mineral was last heated above its annealing temperature.

Chemical etches appropriate to reveal fission tracks have been found for more than one hundred minerals. Some etches are quite simple. For example, a boiling sodium hydroxide solution is appropriate for the common mineral feldspar. However, some other minerals, such as olivine, require etches that are mixtures of several chemicals. Despite the fact that the ion explosion model seems applicable only to crystalline solids, fission tracks can also be revealed in most glasses when a hydrofluoric acid etch is used.

Dating of Volcanic Material

The products of a volcanic eruption are frequently quite rich in uranium and volcanic glasses can be dated by the fission track method. While the major terrestrial volcanoes themselves can be dated with greater precision using other techniques, volcanic debris dated by fission track techniques has proved useful in establishing the time scale for sedimentary accumulation on the ocean bottom. Wind-blown debris from major volcanic eruptions frequently accumulates as discrete layers of ash in the deposited sediments. Tiny volcanic glass fragments from the ash layers of ocean bottom cores collected by the Deep Sea Drilling Project have been dated using fission tracks, providing a chronological framework for the sediment deposition.

Fission track dating has proved to be especially valuable in the investigation of ocean-bottom, or seafloor, spreading. A model for the evolution of the floors of the ocean basins proposed that the mid-ocean ridges were sites where fresh, hot lava intrusions were deposited. After cooling, the lava would be displaced horizontally, and a new deposition would occur at the ridge. Thus, age determinations for ocean-bottom rocks at various distances from a mid-ocean ridge would constitute a direct test of the seafloor spreading hypothesis. Potassium-argon dating had been applied to such rocks, but the ages obtained were unreliable because of the way argon-40 reacts with lava. The fission track dating technique was applied to samples taken at the Mid-Atlantic Ridge and at various distances up to 140 kilometers from the ridge. The results showed material at the ridge to have an age of 10,000 years before the present and material from the most distant point to have an age of 16 million years. As the distance of the sample from the mid-ocean ridge increased, the fission track age also increased. Thus, the ocean-bottom spreading hypothesis was supported.

Dating of Meteorites and Lunar Material

The fission track technique has also allowed scientists to date meteoritic impact events. Such events produce impact glass, formed from the local rock and soil that was melted by the impact event. The age of the impact glass thus dates the impact event.

Additionally, fission tracks have proved useful in determining the ages of meteoritic and lunar samples. Generally, minerals extracted from meteorites give ages of about 4.5 billion years, consistent with the age of the solar system inferred by other radioactive dating techniques. Mineral grains extracted from some meteorites, and in rarer cases grains from lunar samples, however, exhibit far more fission tracks than would be produced in 4.5 billion years by the uranium in the samples. Once all other sources of tracks were excluded, the investigators attributed these tracks to the fission of now-extinct plutonium-244, which was present in the very early solar system. Given that the half-life of plutonium-244 is only 80 million years, minerals containing a substantial number of fission tracks from plutonium must have formed and cooled to below the annealing temperature within a few hundred million years of the last addition of fresh radioactive material to the solar system. Thus, grains exhibiting plutonium-244 fission tracks formed very early in the evolution of the solar system.

Plutonium-244 fission tracks have been used in the development of a technique to determine the rate at which the parent bodies of the meteorites cooled. These cooling rates then permit the sizes of the parent bodies to be inferred. Different minerals have different track annealing temperatures. The plutonium in meteorites is generally concentrated in phosphate minerals such as merrillite, which have very low track annealing temperatures (about 100 degrees Celsius for merrillite). These plutonium-rich minerals, however, occasionally occur adjacent to plutonium-poor silicate grains. As the fission fragments have ranges of 10-20 micrometers, some fission decays near the merrillite-silicate contact surface produce fission tracks in the silicate minerals. Typical silicates have track annealing temperatures of about 300 degrees Celsius. Thus, the plutonium-244 fission fragments from the merrillite begin to produce tracks in the adjacent silicate before the tracks were recorded in the merrillite itself.

Because the plutonium-244 decay rate is known, a comparison of the number of fission tracks in the silicates with the number in the merrillite gives the time it took for the meteorite to cool from the track annealing temperature of the silicate to that of the merrillite. When the ordinary chondrite meteorite St. Severin was examined by this technique, a cooling rate of about 1 degree per million years was found. Cooling-rate data obtained on a number of ordinary chondrites suggest that these meteorites come from below the surfaces of asteroidal-sized parent bodies, no more than about 300 kilometers in diameter.

Use in

Because fission track dating measures the time interval since the last heating of the mineral above the track annealing temperature, it has proven to be particularly valuable in archaeology. Certain archaeological objects, such as pieces of pottery, are heated when they are manufactured. The age of manufacture of such an object can be determined if it or mineral grains within it record fission tracks.

The earliest application of fission track dating to an archaeological sample was to an obsidian knife blade found by L. S. B. Leakey in Kenya. The texture of the blade indicated that it had been heated after its manufacture. A small fragment of the knife blade, about one-tenth of a gram, was found to be about 3,700 years old. Fission track dating was subsequently applied to samples from the Olduvai Gorge beds, from which Leakey's team recovered the specimen of Zinjanthropus, a very early humanoid. Potassium-argon dating of volcanic material from this bed suggested that the actual age of Zinjanthropus was almost twice as great as had been inferred from the fossils associated with the bed. Fission track dating of volcanic pumice from the bed gave an age of 2 million years, consistent with the potassium-argon age. This confirmation of the age of Zinjanthropus resolved the controversy.

Many of the clays used in the manufacture of pottery contain crystals of zircon, a mineral rich in uranium. The high temperature the pottery reaches in the kiln erases all the tracks previously recorded in the zircons, and their high uranium concentration permits even short intervals since the heating to be established with reasonable precision. In one such study done in Japan, nine zircon-containing samples of pottery were dated, giving ages ranging from 700 to 2,300 years. This fission track technique has also been applied to many human-made glass samples, doped at high uranium concentrations (sometimes up to 1 percent) to color them. These uranium-rich glass samples can be dated by the fission track technique after only a few years of track accumulation.

Comparison with Other Techniques

The fission track dating technique has been applied to a wide variety of terrestrial and extraterrestrial materials. The main advantage of this technique is the large span of ages over which it can be employed, permitting the ages of objects from only tens of years old to more than 4.5 billion years old to be established. Although the ages obtained by this technique are generally not as precise as those available through radiocarbon and potassium-argon dating, fission track ages are frequently useful to confirm ages obtained by these other techniques when their applicability to the particular sample is questionable. Where such comparisons can be made, fission track dating has been shown to give correct ages ranging from less than a year to more than a billion years.

Because a large number of individual fission tracks must be counted if age is to be determined with a high degree of precision, fission track dating results are generally considered less reliable than those of techniques that use mass spectrometers to determine isotopic ratios. Fission track dating is particularly valuable for the range of ages from 40,000 years before the present, where radiocarbon dating ceases to be accurate, to about a billion years, where potassium-argon dating is relatively easy. Fission track dating is also applicable to very small samples. Individual mineral grains as small as one milligram in mass have been dated by this technique.

The fission track dating method has been adopted by many laboratories throughout the world because of its simplicity, broad applicability, and low cost. Analyses can be performed in laboratories equipped with only simple chemical etching facilities and optical microscopes. Fission tracks are recorded in a wide variety of crystals and glasses, and the technique is applicable over a wide range of sample ages.

Principal Terms

crystal: a solid having a regular periodic arrangement of atoms

fission fragment: one of the lighter nuclei resulting from the fission of a heavier element

fission track: the damage along the path of a fission fragment traveling through an insulating solid material

glass: a solid that has no regular periodic arrangement of atoms

isotopes: two atoms of the same element having different numbers of neutrons and thus different atomic weights

spontaneous fission: the splitting of an unstable atomic nucleus into two smaller nuclei

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