Analysis of the radiometric dating methods shows that some of these ages are Clague, D.A., and Dalrymplye, G.B., , Cretaceous K-Ar ages of volcanic rocks Principles, techniques and applications to geochronology: San Francisco . Potassium-argon dating; principles, techniques, and applications to geochronology. Responsibility: [by] G. Brent Dalrymple and Marvin A. Lanphere. Imprint: San. Potassium-Argon Dating. Principles, Techniques and Applications to Geochronology. G. Brent Dalrymple and Marvin A. Lanphere. Freeman, San Francisco.
Argon is then condensated on ac- tive charcoal cooled with liquid nitrogen. Other noble gas such as Helium, Neon and Krypton may also be present, but He is not condensated at the temperature of liquid nitrogen in such low-pressure conditions. Neon and Krypton, if present, have a very low concen- tration and they do not, anyway, influence the meas- urement within the mass-spectrometer due to their low mass.
Purified argon is then introduced in the mass spec- trometer. The most widespread technique of measure- ment uses the isotopic dilution of Argon by adding pure 38Ar as a tracer. The quantity of 38Ar added to the sam- ple is known from standards, the K content and the age Fig. Decay scheme of Potassium The measurement of 36Ar and ation, while radiogenic argon already accumulated will 38Ar in laboratory thus can be used to estimate the lev- stay in the dominent part of secondary phase of neo- el of 40Ar corresponding to atmospheric contamina- formation: While the two uppermost cases of remobilization of potassium and argon always can be identified from care- Practically, this correction is based on the measurement full petrographical and mineralogical analyses, the case of 36Ar which is five times more abundant than 38Ar, of Argon inheritance is harder to characterize.
Such an and hence is easier to determine accurately. Note that, inheritance corresponds to either: In such cristallizing condi- tions, radiogenic 40Ar previously produced by the decay 2. Hence, the higher the uncertainty on the correc- from the surrounding host rocks may also be mechani- tion of atmospheric contamination will be.
In contrast, cally incorporated in the arising magma toward the sur- older samples will be characterized by a higher content face and bring inherited Argon.
Argon Geochronology Methods
Such xenoliths, howev- of radiogenic argon, and the uncertainty associated er, will be recognizable from carefull petrographic with the correction of atmospheric contamination will examination and removed from carefull sample prepa- rapidly vanish and become negligible. Old rocks, the surface, i. Nevertheless, mineral transformations can in some cases affect the reliability 2.
With this technique, a part of the potassium from the sample is transformed into argon by neutron activation 2. Argon Loss in a nuclear reactor. In such cas- ogenic 40Ar, from the same aliquote.
We will see later that by SigugeirssonNaughtonMerrihue Argon potentially remaining in the system if Argon and Mitchell These autors precised the effects of from the heated rock has not been completely diffused neutronic irradiation and optimized the conditions of can also pose severe problems to get a meaningfull age the analytical procedure.
Harisson and McDougall of formation. Potassium Lixiviation The transformation of 39K into 39Ar involves a neu- If the transformation occurs at the surface, under am- tron-proton reaction: Kelley et alii ; diation are adapted to form, according to the age and 2.
Artifi- cial 39Ar produced by sample irradiation is radioactive 2. Analyses on separated Grains with a half-life period of years. Variations in its con- While the K-Ar conventionnal technique requires a centration at the timescale of the measurement are large and homogeneous mineral preparation to make thus negligible.
The age and Such a possibility is particularly appropriate to date het- the K-concentration of these standards have been erogeneous deposits including minerals of various previously determined by conventionnal techniques, provenance, e. Before irradiation, the reference stan- statistical analysis can be applied to distinguish xenolitic dards are distributed at different heights in the tube to inherited grains from juvenile magmatic grains.
It is be irradiated, along with the geological samples to even possible to use a laser to volatilize a micro quanti- date. With this procedure, to old samples. A law of interpolation is then applied, 2. Degasing with a Step-Heating Procedure which allows to constrain precisely the effects of irra- Since the age of the mineral is deduced directly from dation for each of the samples to date.
Once the sample is fused, all Argon has been re- measured for the standard. The successive steps will form a plateau Fig. How- necessary to know the weight of sample from which ever, as shown in Figure 3b, it is possible to observe two Argon is extracted; distinct plateaux, at low and high temperature, respec- - the respective abundances between the father 40K tively.
Such a kind of diagram indicates a re-opening of deduced from 39K, i. The apparent ages obtained at high very low, requiring high-sensitivity collector systems temperatures will constrain the age of mineral forma- such as electron multipliers, which are now currently tion.
Perpustakaan Puslit Geoteknologi
If a plateau exists at low temperatures, the age of used in modern mass-spectrometry. Limitations with various characteristics of Argon retentivity with respect to temperature. Irradiation of the samples in a fast neutron nuclear re- This approach is powerfull if applied to geological actor involves several nuclear reactions which produce samples sufficiently old to have been subjected to such a number of radioactive elements other than 39Ar. For thermic or tectonic crises.
The advantage is to eliminate the or KF, irradiated in the same conditions as the samples dominant part of atmospheric contamination at low to date. Similarly, the artificial production of 36Ar, 37Ar, temperatures, and hence to increase the amount of ra- 38Ar, and 39Ar, mainly from Calcium and Chlorine, need diogenic argon released at higher temperatures. Argon to be accounted for.
Different reactions and argon isotopes produced during fast neutron irradiation of a sample. The amounts of to Holocene volcanic Eruptions 36Ar and 39Ar produced by the neutron activation of in Southern Italy Calcium are deduced from the measurement of 37Ar, Despite the rather long radioactive period of 40K, and which also results from a neutron alpha nuclear reac- the presence of 40Ar in the atmosphere making neces- tion but is exclusively produced from Calcium.
K-Ar dating up to kyrs with the Cassignol technique rected itself of the 36Ar produced from calcium by neu- tron activation. Their age is known with a relative un- agreement with available radiocarbon ages.
Principle of atmospheric correction and signal calibration. Thus, K-Ar conventional K-Ar or in the argon-argon techniques. The whole argon measure- the last thousands of years. A peculiar analytical proce- ment procedure thus corresponds to a double compar- dure is thus necessary to detect radiogenic 40Ar accu- ison with atmospheric argon: The second one by using a known quantity of at- For an identical quantity of 40Ar, the difference between mospheric argon, which allows the volumetric deter- the 36Ar signals measured from the sample and from mination of the number of atoms of radiogenic 40Ar ex- pure atmosphere respectively determines the propor- tracted during fusion of the sample, independently tion or radiogenic argon in the sample.
Potassium-argon dating; principles, techniques, and applications to geochronology [book review]
The total and permanent measurements Bard et alii Since coral growth in- purification of argon during the whole measuring volves CO2 consumption and Ca and Th concretion process, the simultaneous collection of argon ion from marine water, a given coral sample can be dated beams, the isotopic measurement focused on the natu- with both radiocarbon and U-Th methods.
The com- ral argon isotopes, without any artificial isotope added, parison was completed between 30, years and pres- and the steadiness of the ion source makes it possible. All the points of our study lies on the with a relative discrepancy of only a few permill over curve of Bard et alii within the range of analytical one week running.
The Cassignol-Gillot technique allows the detection of amounts of radiogenic argon as low as of 0. Despite sev- ported in Figure 6. This likely corresponds to a ences, see Albore-Livadie et alii Albore-Livadie et significant excess of 14C in atmosphere around the last alii proposed new AMS 14C dating of the Avelino glacial maximum. Re-calibration of the radiocarbon eruption, the products of which recovered human sites ages with dendro-chronology is now available for the from the Brass age several tens of kilometers ne of the period covering the last 11, years Bard et alii Vogel et alii The systematic comparison was realized by which lies on the prehistoric village from the Brass age comparing radiocarbon and U-Th mass spectrometric destructed by the eruption; the second in the Cava No- 63 P.
These two sites are separated one from minerals from historical lavas remains limited by the each other by several kilometers. It confirms the reliability Romano M. In the case of a volcanic mineral, this means rapid cooling. Likewise, potassium has not been gained or lost. The decay constants of 40K are accurately known. Argon loss and excess argon are two common problems that may cause erroneous ages to be determined. Excess argon may be derived from the mantle, as bubbles trapped in a melt, in the case of a magma.
Both techniques rely on the measurement of a daughter isotope 40Ar and a parent isotope. Because the relative abundances of the potassium isotopes are known, the 39ArK produced from 39K by a fast neutron reaction can be used as a proxy for potassium. Instead, the ratios of the different argon isotopes are measured, yielding more precise and accurate results. The amount of 39ArK produced in any given irradiation will be dependant on the amount of 39K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39K.
However, because each of these parameters is difficult to determine independantly, a mineral standard, or monitor, of known age is irradiated with the samples of unknown age.
The monitor flux can then be extrapolated to the samples, thereby determining their flux. This flux is known as the 'J' and can be determined by the following equation: In addition to 39Ar production from 39K, several other 'interference' reactions occur during irradiation of the samples.
Other isotopes of argon are produced from potassium, calcium, argon and chlorine. As the table above illustrates, several "undesirable" reactions occur on isotopes present within every geologic sample. These reactor produced isotopes of argon must be corrected for in order to determine an accurate age. The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses.
For example, to determine the amount of reactor produced 40Ar from 40K, potassium-rich glass is irradiated with the samples. The desirable production of 38Ar from 37Cl allows us to determine how much chlorine is present in our samples.
Multiple argon extractions can be performed on a sample in several ways. Step-heating is the most common way and involves either a furnace or a laser to uniformily heat the sample to evolve argon. The individual ages from each heating step are then graphically plotted on an age spectrum or an isochron. Mechanical crushing is also a technique capable of releasing argon from a single sample in multiple steps.
Laser probes also allow multiple ages to be determined on a single sample aliquot, but do so using accurate and precise spatial control. For example, laser spot sizes of microns or less allow a user to extract multiple argon samples from across a small mica or feldspar grain.