How reliable is geologic dating?
The K-Ar dating technique was one of the earliest isotope dating techniques, contrast, the Ar-Ar dating technique provided a wealth of precise ages and.  The inability of the radiocarbon and the K-Ar methods to cover this time period explains Human evolution demands precise dating of the relevant fossils. Argon–argon dating is a radiometric dating method invented to supersede potassium-argon (K/Ar) dating in accuracy. . UC Berkeley press release: " Precise dating of the destruction of Pompeii proves argon-argon method can reliably date.
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. 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–argon dating - Wikipedia
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. Funkhouser and Naughton were quite careful to point out that the apparent "ages" they measured were not geologically meaningful. Quite simply, xenoliths are one of the types of rocks that cannot be dated by K-Ar techniques.
Funkhouser and Naughton were able to determine that the excess gas resides primarily in fluid bubbles in the minerals of the xenoliths, where it cannot escape upon reaching the surface. Studies such as the one by Funkhouser and Naughton are done to determine which materials are suitable for dating and which are not, and to determine the cause of sometimes strange results. They are part of the continuing effort to learn. There have been two extensive K-Ar studies on historic lava flows Dalrymple, ; Krummenacher, that showed that excess argon is not a serious problem for dating lava flows.
Responses to specific creationist claims Wiens' online article, mentioned above, is an excellent resource for countering claims of creationists on the reliability of geologic dating.
In an appendix to this article, Wiens addresses and responds to a number of specific creationist criticisms. Here is a condensed summary of these items, quoted from Wiens' article [ Wiens ]: Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered.
This is not at all true, though it is implied by some young-earth literature. Radiometric dating is based on the half-lives of the radioactive isotopes. These half-lives have been measured over the last years. They are not calibrated by fossils. No one has measured the decay rates directly; we only know them from inference.
Decay rates have been directly measured over the last years. In some cases a batch of the pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed.
In many cases it is easier to detect radioactive decays by the energy burst that each decay gives off. For this a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter or gamma-ray detector. These instruments count the number of decays over a long time.
- Argon–argon dating
If the half-lives are billions of years, it is impossible to determine them from measuring over just a few years or decades. The example given in the section [in Wiens' article] titled, "The Radiometric Clocks" shows that an accurate determination of the half-life is easily achieved by direct counting of decays over a decade or shorter.
Additionally, lavas of historically known ages have been correctly dated even using methods with long half-lives. The decay rates are poorly known, so the dates are inaccurate.
Most of the decay rates used for dating rocks are known to within two percent. Such small uncertainties are no reason to dismiss radiometric dating. Whether a rock is million years or million years old does not make a great deal of difference.
To date a rock one must know the original amount of the parent element. But there is no way to measure how much parent element was originally there. It is very easy to calculate the original parent abundance, but that information is not needed to date the rock.
All of the dating schemes work from knowing the present abundances of the parent and daughter isotopes. There is little or no way to tell how much of the decay product, that is, the daughter isotope, was originally in the rock, leading to anomalously old ages.
A good part of [Wiens' article] is devoted to explaining how one can tell how much of a given element or isotope was originally present.
Usually it involves using more than one sample from a given rock. It is done by comparing the ratios of parent and daughter isotopes relative to a stable isotope for samples with different relative amounts of the parent isotope.
From this one can determine how much of the daughter isotope would be present if there had been no parent isotope. This is the same as the initial amount it would not change if there were no parent isotope to decay. Figures 4 and 5 [in Wiens' article], and the accompanying explanation, tell how this is done most of the time. There are only a few different dating methods.
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There are actually many more methods out there. Well over forty different radiometric dating methods are in use, and a number of non-radiogenic methods not even mentioned here. A young-Earth research group reported that they sent a rock erupted in from Mount Saint Helens volcano to a dating lab and got back a potassium-argon age of several million years. This shows we should not trust radiometric dating.
There are indeed ways to "trick" radiometric dating if a single dating method is improperly used on a sample. Anyone can move the hands on a clock and get the wrong time. Likewise, people actively looking for incorrect radiometric dates can in fact get them.
Geologists have known for over forty years that the potassium-argon method cannot be used on rocks only twenty to thirty years old. Publicizing this incorrect age as a completely new finding was inappropriate.