Thermochronology is the study of the thermal evolution of a region of a planet. Thermochronologists use

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...

along with the

closure temperature In radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature is the temperature at which a syste ...

s that represent the temperature of the mineral being studied at the time given by the date recorded to understand the thermal history of a specific rock, mineral, or geologic unit. It is a subfield within

geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, and is closely associated with

geochronology Geochronology is the science of Chronological dating, determining the age of rock (geology), rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, ...

. A typical thermochronological study will involve the dates of a number of rock samples from different areas in a region, often from a vertical transect along a steep canyon, cliff face, or slope. These samples are then dated. With some knowledge of the subsurface thermal structure, these dates are translated into depths and times at which that particular sample was at the mineral's closure temperature. If the rock is today at the surface, this process gives the exhumation rate of the rock. Common isotopic systems used for thermochronology include fission track dating in

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

, titanite, natural glasses, and other uranium-rich mineral grains. Others include potassium-argon and argon-argon dating in apatite, and (U-Th)/He dating zircon and apatite.

Radiometric Dating

Radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...

is how geologist determine the age of a rock. In a

closed system A closed system is a natural physical system that does not allow transfer of matter in or out of the system, althoughin the contexts of physics, chemistry, engineering, etc.the transfer of energy (e.g. as work or heat) is allowed. Physics In cl ...

, the amount of radiogenic isotopes present in a sample is a direct function of time and the decay rate of the mineral. Therefore, to find the age of a sample, geologists find the ratio of daughter isotopes to remaining parent isotopes present in the mineral through different methods, such as

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

. From the known parent isotopes and the

decay constant A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and (lambda) is a positive rate ...

, we can then determine the age. Different ions can be analyzed for this and are called different dating. For thermochronology, the ages associated with these isotopic ratios is directly linked with the sample's thermal history. At high temperatures, the rocks will behave as if they are in an open system, which relates to the increased rate of

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

of the daughter isotopes out of the mineral. At low temperatures, however, the rocks will behave as a

, meaning that all the products of decay are still found within the original host rock, and therefore more accurate to date. The same mineral can switch between these two systems of behavior, but not instantaneously. In order to switch over, the rock must first reach its

. Closure temperature is specific for each mineral and can be very useful if multiple minerals are found in a sample. This temperature is dependent on several assumptions, including:

grain size Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...

and shape, a constant cooling rate, and chemical composition.

Types of Dating associated with Thermochronology

Fission Track Dating

Fission track dating is the method used in thermochronology to find the approximate age of several uranium-rich minerals, such as

. When

nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactiv ...

of uranium-238 ( ²³⁸U) happens in inorganic materials, damage tracks are created. These are due to a fast charged particle, released from the decay of Uranium, creating a thin trail of damage along its trajectory through the solid. To better study the fission tracks created, the natural damage tracks are further enlarged by chemical etching so they can be viewed under ordinary

optical microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of micros ...

s. The age of the mineral is then determined by first knowing the spontaneous rate of fission decay, and then measuring the number of tracks accumulated over the mineral's lifetime as well as estimating the amount of Uranium still present. At higher temperatures, fission tracks are known to anneal. Therefore, exact dating of samples is very hard. Absolute age can only be determined if the sample has cooled rapidly and remain undisturbed at or close to the surface. The environmental conditions, such as pressure and temperature, and their effects on the fission track on the atomic level still remains unclear. However, the stability of the fission tracks can generally be narrowed down to temperature and time. Approximate ages of minerals still reflect aspects of the thermal history of the sample, such as uplift and

denudation Denudation is the geological process in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interchang ...

Potassium-Argon/Argon-Argon Dating

Potassium-Argon/Argon-Argon dating is applied in thermochronology in order to find the age of the minerals, such as apatite. Potassium-argon (K-Ar) dating is concerned with determining the amount of the product of radioactive decay of isotopic potassium (⁴⁰K) into its decay product of isotopic argon (⁴⁰Ar). Because the ⁴⁰Ar is able to escape in liquids, such as molten rock, but accumulates when the rock solidifies, or recrystallizes, geologists are able to measure the time since recrystallization by looking at the ratio of the amount of ⁴⁰Ar that has accumulated to the ⁴⁰K remaining. The age can be found by knowing the

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of potassium. Argon-argon dating uses the ratio of ⁴⁰Ar to ³⁹Ar as a proxy for ⁴⁰K to find the date of a sample. This method has been adopted because it only requires one measurement of an isotope. To do this, the nucleus of the argon isotope needs to be irradiated from a

nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...

in order to convert the stable isotope ³⁹K to radioactive ⁴⁰Ar. In order to measure the age of the rock, you have to repeat this process in a sample of known age in order to compare the ratios.

(U-Th)/He Dating

(U-Th)/He dating is used to measure the age of a sample by measuring the amount of radiogenic

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

(⁴He) present as a result of the

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

from

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

and

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

. This helium product is kept in the mineral until the closure temperature is reached, and therefore can be determinant of the thermal evolution of the mineral. As in fission track dating, the exact age of the sample is difficult to determine. If the temperature goes above the closure temperature the product of decay, helium, diffuses to the atmosphere and the dating then resets.

Applications

By determining the relative date and temperature of a sample being studied, geologists are able to understand the structural information of the deposits. Thermochronology is used in a wide variety of subjects today, such as tectonic studies, exhumation of mountain belts, hydrothermal ore deposits, and even meteorites. Understanding the thermal history of an area, such as its exhumation rate, crystallization duration, and more, can be applicable in a wide variety of fields and help understand the history of earth and its thermal evolution.

References

{{Reflist Radiometric dating