# Geochronology/Fission track dating

Dating techniques of interest to archaeologists include thermoluminescence, optically stimulated luminescence, electron spin resonance, and fission track dating, as well as techniques that depend on annual bands or layers, such as dendrochronology, tephrochronology, and varve chronology.

## Fission fragments

Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses.

## Thermal neutrons

To determine the uranium content, several methods have been used: one is by neutron irradiation, where the sample is irradiated with thermal neutrons in a nuclear reactor, with an external detector, such as mica, affixed to the grain surface, where the neutron irradiation induces fission of uranium-235 in the sample, and the resulting induced tracks are used to determine the uranium content of the sample because the $^{235}$U:$^{238}$U ratio is well known and assumed constant in nature; however, it is not always constant.

## Low energy neutrons

Absorptive reactions with prompt reactions - Low energy neutrons are typically detected indirectly through absorption reactions. Typical absorber materials used have high cross sections for absorption of neutrons and include Helium-3, Lithium-6, Boron-10, and Uranium-235 Each of these reacts by emission of high energy ionized particles, the ionization track of which can be detected by a number of means. Commonly used reactions include $^3$He(n,p) $^3$H, $^6$Li(n,α) $^3$H, $^{10}$B(n,α) $^7$Li and the fission of uranium.

## Poloniums

α-Po crystallizes in a simple cubic lattice.
Native polonium may occur in minerals like pitchblende due to the decay of uranium. But, when the uranium is chemically bound, the polonium is likely to be also.
β-Po has a rhombohedral (trigonal) crystal structure.
"Solid diorite and gabbro rock, which had previously crystallized from magma, has been subjected to repeated cataclasis and recrystallization. This has happened without melting; and the cataclasis provided openings for the introduction of uranium-bearing fluids and for the modification of these rocks to granite by silication and cation deletion."
"In uranium ore-fields the extra uranium provides an abundant source of inert radon gas; and it is this gas that diffuses in ambient fluids so that incipient biotite and fluorite crystallization is exposed to it. Radon ($^{222}$Rn) decays and Po isotopes nucleate in the rapidly growing biotite (and fluorite) crystals whence they are positioned to produce the Po halos."
On the lower right is a photograph showing radioactive decay halos along a crack in biotite.
On the left is an example of groundwater incursion that has moved through a nearby fault. The groundwater has picked up dissolved uranium compounds and moved downward through adjacent porous sandstones. Uraninite then precipitated around a tongue of groundwater, resulting in the roll front seen in the image on the left.

## Latent nuclear particle tracks

"Fresh" (latent or unetched) californium-252 fission tracks in a chromite (FeCr$_2$O$_4$) grain from the Allende meteorite, showing up in a weak-beam darkfield TEM image which lights up the strain-fields around the 40Å-diameter track-damage cores. This work confirmed chromite's ability to record nuclear particle tracks in spite of its relatively low resistivity.

## Atacamaities

Atacamaites are black-colored impact splash glasses recovered from the Atacama Desert in Chile, South America. They've been interpreted as being deposited relatively close to the impact site, which is currently undiscovered. Many impact splash glasses are given the generic name "tektite", but that term appears to be restricted to objects deposited far from an impact site. Both proximal and distal impact glasses form when a meteoroid or asteroid impacts Earth. The impact event pulverizes, ejects, heats, and melts target rocks. As the material falls back to Earth, rapid cooling of the melt results in glass.
Atacamaite morphology ranges from bulbous to elongated to raindrop-shaped to dumbbell-shaped to irregular. Compositionally, they are close to dacite, a type of volcanic rock. Atacamaite chemistry shows that they also consist of some meteoritic material. The impacting object was probably a group IIAB iron (the other types of meteorites are stony and stony-iron). Fission-track dating indicates the impact occurred during the Late Miocene, about 7.83 million years ago.
Atacamaite Strewn Field is east of the town of Paposo & south-southeast of the town of Antofagasta, Atacama Desert, northern Chile.

## Zircons

Fission-track ages on detrital zircon can be as young as 1 Ma to as old as 2000 Ma.
"If the rock is heated high enough, >120°C for apatite, all tracks will disappear. Zircon and sphene loose their tracks at higher temperatures =ca.200° and =ca.300°C, respectively."

## Apatites

"Obtaining accurate and precise apatite fission-track (AFT) ages depends on the availability of high-quality apatite grains from a sample, ideally with high spontaneous fission-track densities (c. >1.10$^5$ tracks.cm$^{−2}$)."
Apatites of "bedrock samples from young orogenic belts or low-grade metamorphic samples with low U contents yield low spontaneous fission-track densities."
Fission tracks in apatite are commonly used to determine the thermal histories of orogenic belts and of sediments in sedimentary basins.

## Porphyry copper metallogenesis

"Although the tectonic uplift of the 3000 m to 5000 m high range has generally been assumed to be mostly Miocene in age, field relationships suggest that the Domeyko Fault System and tectonic uplift were active as early as the Eocene, coinciding with porphyry copper emplacement between 41 Ma and 30 Ma. Apatite fission track (FT) thermochronology provides both age data and a time-temperature history for rocks since they cooled below a temperature of ca. 125 degrees C (equivalent to a depth of 4 km to 5 km under normal geothermal gradients) on their way to the surface during exhumation, or after a heating event."
"Apatite FT data from the Paleozoic crystalline basement of the Domeyko Cordillera indicate that at least 4 km to 5 km of rocks were eroded during exhumation of this tectonic block between ca. 50 Ma to 30 Ma (Middle Eocene to Early Oligocene), a time that immediately precedes and overlaps with the emplacement of giant porphyry copper deposits. The FT data constrain the age and duration of a period of crustal thickening and extensive erosion known as the Incaic compression, an event recognized in the Andes of Chile and Peru."