Argon-argon dating
Encyclopedia
Argon-argon dating is a radiometric dating
method invented to supersede potassium-argon (K/Ar) dating
in accuracy. The older method required two samples for dating while the newer method requires only one. This newer method converts a stable form of potassium (39K
) into 39Ar
while irradiated with neutrons in a nuclear reactor
.
The decay
ratio of 40K
to 40Ar*
( * means daughter Ar atoms) is used to compute a radiometric date for a geological
event, particularly the eruption and cooling of igneous rock
and minerals.
The sample is heated in increments (step heating) which releases argon from different reservoirs within the crystal grain. Each step produces argon with a certain 40Ar:39Ar ratio, and only when 80% or more of these steps are within acceptable error is the crystal's age known. Dating via 40Ar/39Ar geochronology is generally accurate to within 1-2% for properly collected and irradiated and treated samples.
where λ is the radioactive decay constant of 40K (approximately 5.5 x 10−10 year−1, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the 40Ar*/39Ar ratio.
.
as the age typically reflects the time when a mineral cooled through its closure temperature. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on fault
systems is also possible with the 40Ar/39Ar method. Different minerals have different closure temperatures; biotite
is ~300°C, muscovite
is about 400°C and hornblende
has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock.
Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. 40Ar/39Ar geochronology assumes that a rock retains all of its 40Ar after cooling past the closing temperature and that this was properly sampled during analysis.
This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified.
(see also Kerr) reports that a correction of 0.65% is needed. Thus the K-T Extinction (when the dinosaurs died out) - previously dated at 65.0 or 65.5 million years ago - is more accurately dated to 66.0 Ma. Similarly, the Permian-Triassic extinction is now dated at 252.5 Ma, which is effectively coincident with the age determined by other means for Siberian Traps
basalt flows.
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
method invented to supersede potassium-argon (K/Ar) dating
Potassium-argon dating
Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...
in accuracy. The older method required two samples for dating while the newer method requires only one. This newer method converts a stable form of potassium (39K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
) into 39Ar
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
while irradiated with neutrons in a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
.
The decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
ratio of 40K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
to 40Ar*
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
( * means daughter Ar atoms) is used to compute a radiometric date for a geological
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
event, particularly the eruption and cooling of igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
and minerals.
Method
The sample is generally crushed and single crystals of a mineral hand-selected for analysis. These are then irradiated to produce 39Ar from 39K. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Dating relies on the conversion of K to Ar, and accurate measurement of this conversion.The sample is heated in increments (step heating) which releases argon from different reservoirs within the crystal grain. Each step produces argon with a certain 40Ar:39Ar ratio, and only when 80% or more of these steps are within acceptable error is the crystal's age known. Dating via 40Ar/39Ar geochronology is generally accurate to within 1-2% for properly collected and irradiated and treated samples.
Age equation
The age of a sample is given by the age equation:where λ is the radioactive decay constant of 40K (approximately 5.5 x 10−10 year−1, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the 40Ar*/39Ar ratio.
Relative dating only
The 40Ar/39Ar method only measures relative dates. In order for an age to be calculated by the 40Ar/39Ar technique, the J parameter must be determined by irradiating the unknown sample along with a sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by 40Ar/39Ar, it must be first determined by another isotopic dating method. The method most commonly used to date the primary standard is the conventional K/Ar techniquePotassium-argon dating
Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...
.
Applications
The primary use for 40Ar/39Ar geochronology is dating metamorphic and igneous minerals. 40Ar/39Ar is unlikely to provide the age of intrusions of graniteGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
as the age typically reflects the time when a mineral cooled through its closure temperature. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on fault
Geologic fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of tectonic forces...
systems is also possible with the 40Ar/39Ar method. Different minerals have different closure temperatures; biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
is ~300°C, muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
is about 400°C and hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock.
Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. 40Ar/39Ar geochronology assumes that a rock retains all of its 40Ar after cooling past the closing temperature and that this was properly sampled during analysis.
This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified.
Recalibration
One problem with argon-argon dating has been a slight discrepancy with other methods of dating. Recent work by Kuiper(see also Kerr) reports that a correction of 0.65% is needed. Thus the K-T Extinction (when the dinosaurs died out) - previously dated at 65.0 or 65.5 million years ago - is more accurately dated to 66.0 Ma. Similarly, the Permian-Triassic extinction is now dated at 252.5 Ma, which is effectively coincident with the age determined by other means for Siberian Traps
Siberian Traps
The Siberian Traps form a large region of volcanic rock, known as a large igneous province, in the Russian region of Siberia. The massive eruptive event which formed the traps, one of the largest known volcanic events of the last 500 million years of Earth's geological history, continued for...
basalt flows.
External links
- UC Berkeley press release: "Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years"
- The New Mexico Geochronology Research Laboratory http://www.ees.nmt.edu/Geol/labs/Argon_Lab/NMGRL_homepage.html
- Berkeley Geochronology CenterBerkeley Geochronology CenterThe Berkeley Geochronology Center is a non-profit geochronology research institute in Berkeley, California. It was originally a research group in the laboratory of University of California Berkeley geophysicist and geochronologist Garniss Curtis, now professor emeritus...
http://bgc.org/facilities/argon_lab.html - Argon Isotope Facility http://www.gla.ac.uk/departments/suerc/nercfacilities/argonisotopefacility/ of the Scottish Universities Environmental Research Council