(K/Ar) Potassium Argon Dating Techniques I
K-Ar dating has played a key role in unraveling the temporal patterns of hominid In the process of reworking, these ashes can pick up pre-existing detrital . D. 40Ar/39Ar Laser-Probe Dating of North American Tektite Fragments from. Learn how potassium-argon isotopic dating works and how it is especially useful for determining Scientist Using Furnace for Dating Process. How potassium-argon dating works Photo Wikipedia by Tas Walker One of the most widely used dating methods is the potassium-argon.
It would be assumed that there was something wrong with the idea or the data, and a new scenario would be sought. Some papers give evidence of presenting filtered data. What is meant by filtered data, is that they only present the data that agrees with evolutionary thinking.
The other data is eliminated. Potassium-argon dating and the Cenozoic mammalian chronology of North America. Am J Sci ; This paper is now considered to be a classic paper. Yet they use biotite in an uncritical manor in other areas where the dates they obtained matched their expectations.
On Pagewe can also note: Thus, of some 65 samples collected by M. Skinner, only 10 could be used. Sometimes the whole rock basalt date is reported, but sometimes only a mineral fraction is reported from the basalt, like biotite or sanidine. Why is it that one type of date is used one time and not at another time, is not discussed in the paper. As Paul Giem notes: Thus one could pick the dates that fit one's expectations and create a very impressive list of dates with close agreement without there being more than a general correlation of most dates with one's expectations.
They think of the long age scenario of evolution as being fact. They do not believe that there is any alternative way to look at history. So when the data does not come out right, it is only natural that they assume that there is something wrong with the dates that do not fit the long age viewpoint. However, when they turn around and say that the data supports the evolutionary viewpoint and not the Creationary viewpoint.
This is not right! The data does not support long ages. So, many people try to say something like: But this is not true either, the weight of evidence does not prove anything.
We do not have an issue of weight of evidence. Rather, what we have is weight of interpretation! This controversy is not over data. The data can go either way.
Very intelligent people believe in the long history of the earth and they have good data to support them. There is no question about it. However, I look at that same data and I come to very different conclusions. This process is legitimate! There is such a thing as multiple interpretation to the data base.
There is no proof for either position. The Assumptions used in K-Ar Dating On this web page I want to discuss a possible scenario that would allow K-Ar dates to indicate a short age chronology. Such a discussion might never be allowed in normal scientific circles because of the assumptions they choose to believe as being true. There is such a strong consensus of opinion on K-Ar dating and other similar topics that deal with the history of the Earth that alternative viewpoints are probably viewed as being counterproductive.
Before we start, lets look at the specific K-Ar dating assumptions. The rate of decay half-lifeand the branching ratio, of K have not changed. The material in question lost all its argon at an identifiable time, the reset time. No argon has been lost since the time the rock was reset, or set to zero. No potassium has been gained or lost since the reset time, except by decay. The ratio of K to total K is constant. The total K, Ar, and Ar in the material in question can all be measured accurately.
The seventh assumption is one that scientists are doing their best to fulfill. We should also be able to safely make this assumption.
The sixth assumption is also fairly secure. When the concentrations of the various K isotopes are measured, the results are always the same. The fifth assumption is fairly safe. There are some cases where K has been gained or lost; However, the mineral itself has been noticeably altered. The fourth assumption is probably satisfied for most samples. However, this is an assumption that could be challenged. If the rock was heated in the presence of Argon from the earth's mantle, or perhaps in some primordial Argon which might have had a higher concentration of Ar 36; we might have problems making this assumption.
According to most texts on Potassium-Argon dating, the third assumption is fairly commonly violated. Metamorphism, weathering, and reheating are some of the processes that are mentioned to cause a loss of Argon in the crystal of a rock. Most sedimentary rocks are thought to lose Argon because the crystal structure leaks Argon. A loss of Argon would cause the rock to date younger than it should according to evolutionary thought. This is probably the assumption that scientists make when they choose to present filtered data in a scientific paper.
They see the young dates as those samples that have lost Argon. It is an assumption that they probably view as having no alternatives, yet if this same issue was ever pursued, it might uncover other possibilities suggesting a short age time scenario. Another possibility is that the second assumption is being violated rather than the third. Some samples will not be fully reset, initially. Thus these rocks give a date which is older than what normally would happen if the rocks were fully reset.
These older dating rocks give the kind of dates as expected by the scientific community. On the other hand, those rocks that date younger, would not need to have had Argon leak from the crystal after the time when the reseting process occurs. Instead, the rock was probably more completely reset when it was molten. This means that there was less Argon in the rocks to begin with, because the younger dating rocks were more fully set to zero in the reseting process.
The second assumption sounds logical at first. Many text books say it is self-evident. The Age of the Earth. Stanford University Press, p.
This is because Ar 40 is an inert gas that does not combine chemically with any other element and so escapes easily from rocks when they are heated. Thus, while a rock is molten the Ar 40 formed by the decay of K 40 escapes from the liquid. The first assumption is often challenged by some creationists.
They think that the radioactivity could have speeded up during the flood producing dates with long ages.
But there is no known mechanism to explain or predict the increased rate of radioactivity. However there may be a new development in the field of nuclear reactions that could change this situation.
People around the world are working on active "Cold Fusion" reactions. There is another group that has been conducting experiments for the express purpose of speeding up the transmutation process thus changing the half-life characteristics of radioactive materials.
Some of these reactions occur under admittedly extremely mild conditions, However, it is another question to suppose that these newly discovered processes can occur or did occur in natural conditions, in the history of our world. Dating mechanisms such as Carbon, work within the creationary paradigm without the need of having a change in half-lives.
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.
The percentage of detrital illite in each fraction is then determined using any suitable quantitative analysis technique. Through use of K-Ar dating the mean age of total illite in each fraction is also ascertained. For each fraction, the average age of total illite is correlated with the percentage of detrital illite to develop a linear relationship between the age of total illite and the percentage of detrital illite.
Preferably, the linear relationship is determined using least-squares regression analysis. For a preselected percentage of detrital illite, the mean age of illite corresponding to that percentage can then be determined using the linear relationship. The linear relationship can be used to determine the illite age for any detrital percentage from zero percent detrital illite to one hundred percent detrital illite.
The age corresponding to zero percent detrital illite is called the "end member" age of diagenetic illite and the age corresponding to one hundred percent detrital illite is called the "end member" age of detrital illite. The practice of this invention effectively determines the end member age of diagenetic illite and the end member age of detrital illite in a sample that contains both detrital and diagenetic components without physically separating the components.
The practice of this invention will be described using a shale sample that contains both diagenetic illite and detrital illite. The amount of sample conventionally used in K-Ar dating methods may be used in the practice of this invention. A 50 gram initial sample is normally sufficient. If the sample is coated with drilling mud or other extraneous materials, it is cleaned using a brush, pick, or other suitable cleaning means.
Before analyzing the rock sample in accordance with the process of this invention, the sample needs to be disaggregated.
It is important to take the rock apart into its individual grains without creating any new grains and particularly without creating any new small grains. For this reason the sample is preferably not ground. Disaggregation can be performed using conventional disaggregation techniques. Some samples will fall apart in water. Other samples will fall apart by repeated freezing and thawing. Most rocks disaggregate when subjected to freezing and thawing hundreds of times. Other suitable disaggregation methods may include high-frequency sonic vibration while submerged in a liquid such as water.
Potassium-argon (K-Ar) dating (video) | Khan Academy
The sample may optionally be treated with a buffered acid to remove carbonates to further enhance disaggregation. Acetic acid buffered with sodium acetate may be used for this purpose to bring the pH of the solution to about 5. To further aid disaggregation, following the buffer acid treatment the sample may optionally be treated with hydrogen peroxide to partially dissolve organic matter which may be holding rock particles together.
These chemical treatment steps may be interspersed with additional ultrasonic disaggregation. Next, the aqueous suspension containing the clay particles is separated into a plurality of fractions so that each fraction will have a different mean age of illite.
A preferred way of making this separation is to separate the particles by size.
Coarser-sized particles tend to contain more detrital illite than finer-sized particles. Therefore, separating the sample into a plurality of fractions with each fraction having different sized particles that fall within a narrow size range will nearly always produce fractions having different mean ages of illite.
One way of separating the sample into a plurality of fractions in accordance with this invention is to subject the aqueous suspension to centrifugation and re-suspension. A minimum of two fractions are required to practice this invention. However, preferably the sample is separated into three or more fractions. Increasing the number of fractions dated in the practice of this invention reduces statistical error. For purposes of clarity and simplicity of presentation, three fractions were separated in this description.
To prepare the first coarse fraction, the sample was centrifuged at sufficient speeds and for a time period calculated based on settling rates according to Stokes' law to settle particles larger than about 0. The second medium fraction was separated in a similar manner to separate out particles ranging in size between 0.
The third fine fraction contained particles smaller than 0. These particle sizes are provided in this description for illustration purposes. Other size distributions for the fractions may be used in the practice of this invention and as mentioned above additional fractions may be made. A wide size range of particles in each fraction is not desirable because different size particles of the same mineral were commonly formed within the rock sample at different times and they may have different potassium and argon contents.
For this reason, a wide range of particle sizes can lead to an inhomogeneous fraction and therefore less difference in age between fractions. The size selected for each fraction can be suitably selected by those skilled in the art, taking into account the natural grain size of illite in the sample and the number of fractions to be analyzed. It has been observed, however, that particles larger than about 2 microns are more likely to contain unwanted minerals such as feldspars.
Other techniques can be used in the practice of this invention to separate the sample into a plurality of fractions having different mean ages of illite.