Argon Geochronology Methods
Jan 1, Isotopic dating of geological samples using the K/Ar method and its variant, the 40Ar/39Ar dating technique, provides ages that in favourable. The 40Ar/39Ar step-heating method can be applied successfully to any sample which is suitable for K-Ar dating. The step-heating approach is particularly. Because 39ArK can only be produced by a fast neutron all samples dated by the 40Ar/39Ar technique must be.
Argon–argon dating - Wikipedia
Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon. Argon can mobilized into or out of a rock or mineral through alteration and thermal processes. Like Potassium, Argon cannot be significantly fractionated in nature. However, 40Ar is the decay product of 40K and therefore will increase in quantity over time. The quantity of 40Ar produced in a rock or mineral over time can be determined by substracting the amount known to be contained in the atmosphere.
This ratio is The decay scheme is electron capture and positron decay.
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The material in question is a closed system. 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.Tephra Conference 2014 - New Generation Mass-Spectrometers Offer Improved 40Ar/39Ar Dating of Tephra
The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses. The actual accumulation of 40Ar 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. Volume diffusion depends on the cooling rate, the activation energy for diffusion, and the geometry and size of the diffusion domain.
The closure temperatures of the minerals dated in this project will be discussed in chapter 0. In order to determine the irradiation conditions, a standard mineral of known K-Ar age is irradiated with the samples to be dated.
This way an irradiation parameter J can be defined: J can be calculated for different positions within the irradiation capsule by measuring standards from different positions and interpolating between them. Once J is known, ages for the unknown samples can be calculated: In the laser probe analysis method, the sample is placed in an ultrahigh vacuum system and the argon is extracted from the sample by heating with a diffuse laser beam.
More details on this dating method can be found in McDougall and Harrison,Kelly, and Wijbrans et al. In this study hornblende, biotite and muscovite were used.
It can occur in a variety of igneous and metamorphic rocks.
Primary hornblende typically occurs in intermediate plutonic rocks and in the alkali basalts and calc-alkaline series.