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.