Thermochronology

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Approximate temperature curve within depths of Earth. Shows that it gets hotter with increased depth. Upper mantle temperature profile.png
Approximate temperature curve within depths of Earth. Shows that it gets hotter with increased depth.

Thermochronology is the study of the thermal evolution of a region of a planet. Thermochronologists use radiometric dating along with the closure temperatures 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, and is closely associated with geochronology.

Contents

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. [1]

Common isotopic systems used for thermochronology include fission track dating in zircon, apatite, 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. [1]

Radiometric Dating

Radiometric dating is how geologist determine the age of a rock. In a closed system, the amount of radiogenic isotopes present in a sample is a direct function of time and the decay rate of the mineral. [2] 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. From the known parent isotopes and the decay constant, 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. [3] At high temperatures, the rocks will behave as if they are in an open system, which relates to the increased rate of diffusion of the daughter isotopes out of the mineral. At low temperatures, however, the rocks will behave as a closed system, meaning that all the products of decay are still found within the original host rock, and therefore more accurate to date. [3] 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. Closure temperature is specific for each mineral and can be very useful if multiple minerals are found in a sample. [4] This temperature is dependent on several assumptions, including: grain size and shape, a constant cooling rate, and chemical composition. [4]

Types of Dating associated with Thermochronology

Fission Track Dating

Fission tracks observed in a mineral under optical microscope. Cjkgfmj.jpg
Fission tracks observed in a mineral under optical microscope.

Fission track dating is the method used in thermochronology to find the approximate age of several uranium-rich minerals, such as apatite. When nuclear fission 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. [5] 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 microscopes. 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. [6]

At higher temperatures, fission tracks are known to anneal. [7] 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. [8] 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. [6] Approximate ages of minerals still reflect aspects of the thermal history of the sample, such as uplift and denudation. [6]

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. [9] The age can be found by knowing the half-life of potassium. [9]

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 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. [10]

(U-Th)/He Dating

(U-Th)/He dating is used to measure the age of a sample by measuring the amount of radiogenic helium (4He) present as a result of the alpha decay from uranium and thorium. 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. [11]

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, [12] exhumation of mountain belts, [13] hydrothermal ore deposits, [4] and even meteorites. [14] 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.

See also

Related Research Articles

Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of Earth itself, and can also be used to date a wide range of natural and man-made materials.

<span class="mw-page-title-main">Apatite</span> Mineral group, calcium phosphate

Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ion, respectively, in the crystal. The formula of the admixture of the three most common endmembers is written as Ca10(PO4)6(OH,F,Cl)2, and the crystal unit cell formulae of the individual minerals are written as Ca10(PO4)6(OH)2, Ca10(PO4)6F2 and Ca10(PO4)6Cl2.

<span class="mw-page-title-main">Geochronology</span> Science of determining the age of rocks, sediments and fossils

Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as paleomagnetism and stable isotope ratios. By combining multiple geochronological indicators the precision of the recovered age can be improved.

<span class="mw-page-title-main">Spontaneous fission</span> Form of radioactive decay that occurs randomly

Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 ; spontaneous breakdown into smaller nuclei and a few isolated nuclear particles becomes possible at greater atomic mass numbers.

<span class="mw-page-title-main">Natural nuclear fission reactor</span> Naturally occurring uranium self-sustaining nuclear chain reactions

A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kuroda. The remnants of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions have occurred in the past, can be verified by analysis of isotope ratios of uranium and of the fission products. An example of this phenomenon was discovered in 1972 in Oklo, Gabon by Francis Perrin under conditions very similar to Kuroda's predictions.

<span class="mw-page-title-main">Nuclear fission product</span> Atoms or particles produced by nuclear fission

Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy, and gamma rays. The two smaller nuclei are the fission products..

Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as feldspars, micas, clay minerals, tephra, and evaporites. In these materials, the decay product 40
Ar
 is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes). The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. These factors introduce error limits on the upper and lower bounds of dating, so that the final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure or open air. Time since recrystallization is calculated by measuring the ratio of the amount of 40
Ar
 accumulated to the amount of 40
K
 remaining. The long half-life of 40
K
 allows the method to be used to calculate the absolute age of samples older than a few thousand years.

Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.

Argon–argondating is a radiometric dating method invented to supersede potassium–argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. 40Ar/39Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (39K) into the radioactive 39Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the 40K/40Ar* ratio, and thus to calculate the age of the unknown sample. 40Ar* refers to the radiogenic 40Ar, i.e. the 40Ar produced from radioactive decay of 40K. 40Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the 36Ar and assuming that 40Ar is found in a constant ratio to 36Ar in atmospheric gases.

Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range.

Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range, in contrast with relative dating, which places events in order without any measure of the age between events.

<span class="mw-page-title-main">Fission track dating</span> Radiometric dating technique

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. Fission-track dating is a relatively simple method of radiometric dating that has made a significant impact on understanding the thermal history of continental crust, the timing of volcanic events, and the source and age of different archeological artifacts. The method involves using the number of fission events produced from the spontaneous decay of uranium-238 in common accessory minerals to date the time of rock cooling below closure temperature. Fission tracks are sensitive to heat, and therefore the technique is useful at unraveling the thermal evolution of rocks and minerals. Most current research using fission tracks is aimed at: a) understanding the evolution of mountain belts; b) determining the source or provenance of sediments; c) studying the thermal evolution of basins; d) determining the age of poorly dated strata; and e) dating and provenance determination of archeological artifacts.

Potassium-40 (40K) is a radioactive isotope of potassium which has a long half-life of 1.25 billion years. It makes up about 0.012% of the total amount of potassium found in nature.

Helium dating may refer to the traditional uranium–thorium dating or to a variety of He diffusion methods that utilize the mobility of He atoms to determine the thermal history of a rock. Helium diffusion experiments are often used to help interpret information retrieved from U–Th/He thermochronometric experiments. Kinematic parameters derived from He diffusion is done through estimating He diffusion over a range of temperatures. The use of density functional theory helps in estimating energy barriers for He to overcome as it diffuses across various crystallographic directions. Discrepancies, however, between observed and predicted He diffusion rates is still a problem and likely stem from unresolved problems in crystal defects and radiation damage in natural grains as opposed to theoretical grains. Depending on the mineral analyzed there are different assumptions to be made on He mobility. For example, He diffusion in minerals such as zircon, rutile, and monazite have been shown to be strongly anisotropic.

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 system has cooled so that there is no longer any significant diffusion of the parent or daughter isotopes out of the system and into the external environment. The concept's initial mathematical formulation was presented in a seminal paper by Martin H. Dodson, "Closure temperature in cooling geochronological and petrological systems" in the journal Contributions to Mineralogy and Petrology, 1973, with refinements to a usable experimental formulation by other scientists in later years. This temperature varies broadly among different minerals and also differs depending on the parent and daughter atoms being considered. It is specific to a particular material and isotopic system.

<span class="mw-page-title-main">Radiogenic nuclide</span>

A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive or stable.

A river anticline is a geologic structure that is formed by the focused uplift of rock caused by high erosion rates from large rivers relative to the surrounding areas. An anticline is a fold that is concave down, whose limbs are dipping away from its axis, and whose oldest units are in the middle of the fold. These features form in a number of structural settings. In the case of river anticlines, they form due to high erosion rates, usually in orogenic settings. In a mountain building setting, like that of the Himalaya or the Andes, erosion rates are high and the river anticline's fold axis will trend parallel to a major river. When river anticlines form, they have a zone of uplift between 50-80 kilometers wide along the rivers that form them.

Provenance in geology, is the reconstruction of the origin of sediments. The Earth is a dynamic planet, and all rocks are subject to transition between the three main rock types: sedimentary, metamorphic, and igneous rocks. Rocks exposed to the surface are sooner or later broken down into sediments. Sediments are expected to be able to provide evidence of the erosional history of their parent source rocks. The purpose of provenance study is to restore the tectonic, paleo-geographic and paleo-climatic history.

Potassium–calcium dating, abbreviated K–Ca dating, is a radiometric dating method used in geochronology. It is based upon measuring the ratio of a parent isotope of potassium to a daughter isotope of calcium. This form of radioactive decay is accomplished through beta decay.

<span class="mw-page-title-main">Optically stimulated luminescence thermochronometry</span>

Optically stimulated luminescence (OSL) thermochronometry is a dating method used to determine the time since quartz and/or feldspar began to store charge as it cools through the effective closure temperature. The closure temperature for quartz and Na-rich K-feldspar is 30-35 °C and 25 °C respectively. When quartz and feldspar are beneath the earth, they are hot. They cool when any geological process e.g. focused erosion causes their exhumation to the earth surface. As they cool, they trap electron charges originating from within the crystal lattice. These charges are accommodated within crystallographic defects or vacancies in their crystal lattices as the mineral cools below the closure temperature.

References

  1. 1 2 Zentilli, M.; Reynolds, P.H. (1992). Low temperature thermochronology. Mineralogical Association of Canada. OCLC   26628421.
  2. ↑ Misra, Kula C. (2012). Introduction to Geochemistry : Principles and Applications. John Wiley & Sons, Incorporated. pp. 225–232. ISBN   978-1-4051-2142-2.
  3. 1 2 Braun, Jean, 1961- Beek, Peter van der, 1967- Batt, Geoffrey (2012). Quantitative thermochronology : numerical methods for the interpretation of thermochronological data. Cambridge University Press. ISBN   978-1-107-40715-2. OCLC   819316615.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. 1 2 3 Mclnnes, Brent I. A.; Evans, Noreen J.; Fu, Frank Q.; Garwin, Steve (2005-12-31), "18. Application of Thermochronology to Hydrothermal Ore Deposits", Low-Temperature Thermochronology, De Gruyter, pp. 467–498, doi:10.1515/9781501509575-020, ISBN   978-1-5015-0957-5
  5. ↑ Wagner, G.; Haute, P. van den (2012-12-06). Fission-Track Dating. Springer Science & Business Media. ISBN   9789401124782.
  6. 1 2 3 Gleadow, Andrew J. W.; Belton, David X.; Kohn, Barry P.; Brown, Roderick W. (2002-01-01). "Fission Track Dating of Phosphate Minerals and the Thermochronology of Apatite". Reviews in Mineralogy and Geochemistry. 48 (1): 579–630. Bibcode:2002RvMG...48..579G. doi:10.2138/rmg.2002.48.16. ISSN   1529-6466.
  7. ↑ Michels, Joseph W. (1972). "Dating Methods". Annual Review of Anthropology. 1 (1): 113–126. doi:10.1146/annurev.an.01.100172.000553. ISSN   0084-6570.
  8. ↑ McInnes, B. I. A. (2005-01-01). "Application of Thermochronology to Hydrothermal Ore Deposits". Reviews in Mineralogy and Geochemistry. 58 (1): 467–498. Bibcode:2005RvMG...58..467M. doi:10.2138/rmg.2005.58.18. ISSN   1529-6466.
  9. 1 2 McDougall, Ian. (1988). Geochronology and thermochronology by the p40 sAr/ p39 sAr method. Oxford University Press. OCLC   270672499.
  10. ↑ KUIPER, K (2004). "40Ar/39Ar ages of tephras intercalated in astronomically tuned Neogene sedimentary sequences in the eastern Mediterranean*1". Earth and Planetary Science Letters. 222 (2): 583–597. doi:10.1016/s0012-821x(04)00177-3. ISSN   0012-821X.
  11. ↑ Farley, K. A. (2000-02-10). "Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite". Journal of Geophysical Research: Solid Earth. 105 (B2): 2903–2914. Bibcode:2000JGR...105.2903F. doi: 10.1029/1999jb900348 . ISSN   0148-0227.
  12. ↑ Stockli, Daniel F. (2005-12-31), "16. Application of Low-Temperature Thermochronometry to Extensional Tectonic Settings", in Reiners, Peter W; Ehlers, Todd A (eds.), Low-Temperature Thermochronology, De Gruyter, pp. 411–448, doi:10.1515/9781501509575-018, ISBN   978-1-5015-0957-5
  13. ↑ Spotila, James A. (2005-12-31), "17. Applications of Low-Temperature Thermochronometry to Quantification of Recent Exhumation in Mountain Belts", in Reiners, Peter W; Ehlers, Todd A (eds.), Low-Temperature Thermochronology, De Gruyter, pp. 449–466, doi:10.1515/9781501509575-019, ISBN   978-1-5015-0957-5
  14. ↑ Min, Kyoungwon (2005-12-31), "21. Low-Temperature Thermochronometry of Meteorites", in Reiners, Peter W; Ehlers, Todd A (eds.), Low-Temperature Thermochronology, De Gruyter, pp. 567–588, doi:10.1515/9781501509575-023, ISBN   978-1-5015-0957-5
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