Potassium-Argon and Argon-Argon Dating of Crustal Rocks and the Problem of Excess Argon
In the diagram below I have drawn 2 different age spectra. The bottom, green spectrum is what we would expect to see if we had an ideal sample that has no excess-Ar, and the top, blue spectrum is what we might expect if the sample contained excess-Ar in fluid inclusions. The data for each of those 7 steps is represented by one of the 7 boxes on the diagram.
On an age spectrum, the ages are plotted as boxes to show how big the errors are on each step.
K-Ar ages increase away from spreading ridges, just as we might expect, and recent volcanic eruptions yield very young dates, while older.
The temporal resolution of the stratigraphic record, the only account of the 4. As a consequence of the scientific pursuit to temporally dissect the geological record and decode Earth history, the NERC Argon Isotope Facility AIF was established through community demand nearly 20 years ago. For example, AIF establish dates and rates for the expansion of humans from Africa 1 , facilitates temporal integration of palaeoclimate signals to allow investigation of past global climate change 10 , determine timescales and frequencies of volcanic activity and super-eruptions to mitigate risk to the general populous 6 , reconstruct timescales of fluid-rock interaction with respect to the mineralisation of mineable resources 17 and generation of hydrocarbons As such, the Facility ethos is strongly aligned with the evolving NERC Strategy with output having direct societal and economic benefits to the UK and beyond.
However, as a versatile Facility that prides itself on being responsive to community demand, the AIF maintains scientific capability and intellectual leadership in deep time geochronology, for example, in studies of mass extinctions 16 , geochemical evolution of the atmosphere and oceans 14 15 , changes to ocean circulation 2 , dating of ancient volcanic eruptions 4 , geomagnetism and inner core processes 7 , resolution of the interplay between climate and tectonics 5. The AIF is internationally established as a cutting-edge dating facility, due to the expertise and experience of AIF personnel, the quality of its scientific output peer-reviewed publications, PhD theses, conference presentations , technical innovation and training of chronology-literate scientists.
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Potassium—argon dating. An absolute dating method based on the natural radioactive decay of 40 K to 40 Ar used to determine the ages of rocks and minerals on geological time scales. Argon—argon dating. A variant of the K—Ar dating method fundamentally based on the natural radioactive decay of 40 K to 40 Ar, but which uses an artificially generated isotope of argon 39 Ar produced through the neutron irradiation of naturally occurring 39 K as a proxy for 40 K.
The Ar-Ar ages are very close to the apatite fission track ages reported from the same rocks, requiring overall cooling rates of the order of °C/m.y., and a.
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. The sample is generally crushed and single crystals of a mineral or fragments of rock hand-selected for analysis. These are then irradiated to produce 39 Ar from 39 K. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Heating causes the crystal structure of the mineral or minerals to degrade, and, as the sample melts, trapped gases are released.
The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time.
Ar–Ar and K–Ar Dating
Potassium has three naturally occurring isotopes: 39 K, 40 K and 41 K. The positron emission mechanism mentioned in Chapter 2. In addition to 40 Ar, argon has two more stable isotopes: 36 Ar and 38 Ar. Because K an alkali metal and Ar a noble gas cannot be measured on the same analytical equipment, they must be analysed separately on two different aliquots of the same sample.
The idea is to subject the sample to neutron irradiation and convert a small fraction of the 39 K to synthetic 39 Ar, which has a half life of years. The age equation can then be rewritten as follows: 6.
Here we report ArAr dating of Dho plagioclase. We suggest that the determined age dates the intense shock heating event this meteorite experienced.
Ar-Ar methods. This method is based on the occurrence of the radioactive isotope 40 K of potassium in rocks. This isotope decays to 40 Ca and 40 Ar, the last of which is used for K-Ar age dating as it accumulates in the rock over time. If the ratio of 40 K and 40 Ar is known, the unknown time can be calculated. The ideal model conditions may not be met due to the presence of inherited argon, loss of radiogenic argon and deformation and recrystallization of the mineral Dodson, The actual accumulation of 40 Ar in a crystal structure depends not only on the time involved, but also on diffusion behavior, the temperatures the rock has experienced since its formation, cooling rate, grain size and deformation state of the crystal McDougall and Harrison, For the application of this method to age dating it is essential to define a closure temperature.
The closure temperature range of a mineral is the temperature range over which a mineral changes from an open system to a closed system for the isotopes of interest. The most important process interfering with the accumulation of radiogenic isotopes is recrystallization, as this enhances the mobility of atoms. Thermally activated volume diffusion may play an important role in slowly cooled systems.
Wilkinson, Camilla M. PhD thesis The Open University. The Ar-Ar dating technique is one of the most widely applied geochronological techniques to products of silicic volcanism, which represent geologically instantaneous events, and have been used to calibrate the geological timescale, correlate stratigraphy and biostratigraphy over large areas, and assess the impact of explosive volcanic eruptions. Recent advances e. These advances have highlighted the realisation that relatively small levels of Ar contamination e.
The precision of an 40Ar/39Ar plateau age generally is better than the precision of a K–Ar age because the plateau age is calculated by pooling the ages of.
Potassium-argon dating , method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. This dating method is based upon the decay of radioactive potassium to radioactive argon in minerals and rocks; potassium also decays to calcium Thus, the ratio of argon and potassium and radiogenic calcium to potassium in a mineral or rock is a measure of the age of the sample.
The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium. On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism.
The potassium-argon dating method has been used to measure a wide variety of ages. The potassium-argon age of some meteorites is as old as 4,,, years, and volcanic rocks as young as 20, years old have been measured by this method. Potassium-argon dating. Info Print Cite.
Potassium-argon (K-Ar) dating
Ajoy K. Leonardo da Vinci, ca. Herein, I set out some simple guidelines to permit readers to assess the reliability of published ages. I illustrate the use of the techniques by looking at published age data for hotspot tracks in the Atlantic Ocean the Walvis Ridge , as well as newly published ages for the British Tertiary Igneous Province. In these experiments, a sample is heated in steps of increasing laboratory extraction temperature, until all the argon is released. The resulting figure is called an age spectrum e.
40Ar/39Ar dating of biotite is used extensively to determine the timing of cooling ‘perfect open system’ biotite 40Ar/39Ar age and intra-grain Ar distributions.
However, it is well established that volcanic rocks e. If so, then the K-Ar and Ar-Ar “dating” of crustal rocks would be similarly questionable. Thus under certain conditions Ar can be incorporated into minerals which are supposed to exclude Ar when they crystallize. Patterson et al. Dalrymple, referring to metamorphism and melting of rocks in the crust, has commented: “If the rock is heated or melted at some later time, then some or all the 40 Ar may escape and the K-Ar clock is partially or totally reset.
Indeed, a well-defined law has been calculated for 40 Ar diffusion from hornblende in a gabbro due to heating. They are the lower mantle below km , upper mantle, continental mantle lithosphere, oceanic mantle lithosphere, continental crust and oceanic crust, the latter four constituting the earth’s crust.