Fission Track Dating

 

 

     Fission tracks are disruptions of the crystalline matrix in mineral crystals and glasses. A single disruption is caused by the release of 20 million electron volts, which if released over 1 seconds time, equals about 3.2 x 10 -12 watts. This energy release is produced by the spontanaeous splitting, or fission, of a radioactive atom. The disruptive force travels outward from the original atom in two opposite directions along a plane of the crystal. This creates a double pointed spike-shaped region of weakened crystalline bonding. It can be seen under an electron microscope as a darker or lighter spike a few nanometers wide  and 10-20 micrometers long.

     Of the elements capable of fission, only U235, U238, and Th232 are commonly seen as trace constituents of many minerals and glasses, and of these, only U238 undergoes spontanaeous fission frequently enough to account for almost all natural fission tracks. This is true even though the U238 isotope is a very small, though constant, fraction of the total uranium content, almost all of which is U235. Normal radioactive decay of uranium and thorium to lead, through emission of alpha and smaller particles, does damage to the crystalline matrix but not anything visible.

     Many mineral crystals, including apatite, zircon, sphene, titanite, biotite, mica, barite, epidote, and garnet, natural glasses including obsidian, pitchstone, and tephras, and man-made ceramics, contain the proper amount of uranium to allow dating. The proper amount varies according to the age of the material because too much, particularly in very old materials, produces so many tracks that they hide each other, and too little in younger materials won't produce enough tracks for statistical analysis.

     Depending upon the crystalline composition, strength, and other factors, their fission tracks shorten and disappear with increasing temperatures. Thus, the ages derived from the number of tracks reflect the time passed since last exposed to high temperatures. Statistical analysis of the track lengths can show periods of exposure to temperatures high enough to shorten, but not eliminate the tracks. Temperature exposures at certain levels will shorten or eliminate tracks in some minerals and not others, so rocks composed of both can have their temperature history determined.

     Sampling for fission track analysis depends on the question involved. Hearth materials and ceramics show the time of last firing. Rates of uplift and exhumation can be determined by sampling along vertical transects. River and basin sediment samples can be sourced to various uplift regions.

     Most samples require crushing followed by separation of required crystals. Density tables and heavy liquids are used to separate density fractions containing them and electromagnets are used to remove magnetic components. Isolated crystal grains are then mounted in a durable matrix like epoxy or teflon, and brought to a high polish.

     Etching with a strong acid or base is required to enlarge the tracks into normal microscopic visibility range. The solvent type, etch duration, and temperature are adjusted according to mineral type and expected age. Surface contaminants and alpha damage can affect the etching results.

     Many grains are rejected because of size, poor etching, mis-alignment to the crystalline axis, cracks, scratches, and inclusions. Good grains are marked for counting, and a specific area is counted, thus determining the spontanaeous track density in square centimeters.

     Numerous means are used to determine or estimate the uranium content, from which the constant relative U238 content is made. With the spontanaeous track density, the U238 content of that particular area, and the known frequency of U238 fission, the length of time passed since reaching closure temperature can be reliably estimated.

     Fission track analysis provides useful information ranging from recent radioactivity exposure to meteorites possibly formed during the nova which created our solar system. It correlates well with all other dating methods, and is particularly useful with 40Ar/39Ar, K/Ar, U/Pb, UTh/He, paleomagnetism, stratigraphy, and biostratigraphic studies. Applications range from deep borehole, to basement rock, to ocean bottom sediments, to basin sediments, to tephra in ice layers, to mountain cliffs.

     Archaeologists apply dates derived from pottery, glass, and hearth stones to sites, cultures, and other associated items. Anthropologists date tephra layers above and below the earliest hominid fossils and trace our family tree through many changes and around the world. Paleontologists date fossils and life traces associated with FT datable minerals throughout the spectrum of life on earth.

     Geological processes are most often studied with fission track analysis. These include cataclysmic and slow motion events, landscape devolution/development studies, and geochemical changes during formation and deformation.

     Volcanic eruptions, earthquakes, fault slips, floods, tidal waves, and meteoric impacts have been studied using fission track analysis. Volcanic tephra are a particularly good source of fission track dates because of their geologically instantaneous happenstance, their homogeneity, and their typical proper uranium constituency.

     Slow geologic events contribute to most of what we see on earth, and much more no longer visible. Continental plates, sea floors, volcanic arcs and other mountain ranges move continuously though slowly. When plates collide, some is subducted and some create accretionary complexes or mountain ranges. The timing of these actions can be derived from temperature changes as the rock is pushed from hot depths and cools, thereby beginning to accumulate fission tracks. These and interior mountain ranges expose their rates of uplift by passing through the cooling depths at varying speeds. Sea floor spreading rates are determined by older cooling ages found further from the rift. Volcanic arc motion leaves older fission tracks behind.

     As mountains rise, wind, water, and gravity attacks newly exposed surfaces, reducing them to sediments. Exhumation rates and sedimentation rates are reflected in the amount of sediment produced over time. The proportions provenanced to particular orogenies can be determined by timing peaks in the sediments fission tracks. The type of sediment produced, such as wind vs. water borne, can give indications as to climate when the mountains were exhumed.

     Thermal histories of geologic processes derived from fission track and other dating methods, have provided insight into metamorphism; rock, mineral and crystal formation and deposition; hydrocarbon exploration; and radioactive waste disposal siting.

 

 

 

 

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