The 40Ar/39Ar laboratory - Fred Jourdan
The 40Ar/39Ar dating method is used to measure the age and timing of a large variety Nb: A price discount might be applied if more than 10 samples are to be . Western Australian Argon Isotope Facility Ar-Ar laser dating laboratory, . This price includes use of the equipment, and also covers the cost of neutron. Overview of Argon Methods · Mineral/Rock Samples with Ar-dating potential · Links Here we explain the most common formats used to represent argon data.
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.
Average Cost of a Date in Each State
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.
Argon Geochronology Methods
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. The results from a laser probe can be plotted in several graphical ways, including a map of a grain showing lateral argon distribution. Total fusion is performed using a laser and results are commonly plotted on probability distribution diagrams or ideograms.
For the J to be determined, a standard of known age must be irradiated with the samples of unknown age. The error box associated with the ages may be plotted as 1 or 2 sigma, depending on the preference of the user. The margin including steps D through G is called a plateau. It has been commonly defined by the strict criteria of Fleck et al Steps D through G define a plateau meeting these criteria. Steps C and H do not because, while overlapping the immediately adjacient heating steps, they do not overlap step F.
Bureau Laboratories: Argon Lab: Data
The NMGRL no longer relies on the plateau definition of Fleck et albut rather tests the statistical precision of a number of contiguous heating steps using the criteria of Mahon The plateau age is calculated by weighting each step by the inverse of the variance. The plateau error is calculated using the method of Taylor The age immediately above the x-axis is the total gas or integrated age.
The integrated age is calculated by weighting the individual steps by the fraction of 39Ar released.
Commonly plotted as log values, these ratios help to indicate what mineral phases may be degassing at certain temperatures. However, effects such as alteration often contaminate the sample with large quantities of atmospheric 40Ar overwhelming the radiogenic 40Ar.
Inverse Isochron Properties of an inverse isochron: Argon data can be plotted on an isochron to help assess the isotopic composition of Ar trapped at the time of argon closure, thereby testing the assumption that trapped argon isotopes have the composition of modern atmosphere which is implicit in age spectra. Ratios are corrected for extraction line and mass spectrometer blank contributions and nucleogenic isotopes. Correction for atmospheric argon is not applied.
Commonly accepted MSWD values are less than 2.