Isochron dating - CreationWiki, the encyclopedia of creation science
Isochron dating is a common radiometric dating technique applied to date According to theory, the sample starts out with daughter isotopes. A brief introduction to isochron dating methodology. For example, with Rb/Sr isochron dating, any age less than a few tens of millions of years. Some examples of isotope systems used to date geologic materials. . We can simplify our isochron equation somewhat by noting that if x is.
According to Brent Dalrymple In the initial state, the graph of daughter isotope to the third isotope versus parent isotope to the third isotope should result in a straight, horizontal line. The process of evaluating the daughter product as a ratio against another isotope of the same element is a valid method because, when a mineral or rock forms from a homogenous state, the elements that are assimilated into crystalline formation are very restricted.
The key to the formation of crystals in the rock is that the process is selective between elements, but is indifferent to isotopes of the same element.
Thus, the daughter product and any other isotopes of the same element will be incorporated into the minerals of a rock with the same ratio. This initial ratio allows the non-daughter product isotope to be representative of the initial amount of the daughter product Stassen To view an animation of how an isochron changes over time as decay occurs, see the following website: The amount of non-decay isotope in the sample does not change.
Thus, as decay occurs, the parent ratio decreases and the daughter ratio increases. On an isochron diagram, this change in ratios shifts each measurement from the sample up and to the left at a one-to-one rate. As time progresses, the line connecting the measurements within the sample moves counter-clockwise around a point intersecting the y-axis, a point that represents the initial ratios Dalrymple Once the ratios are plotted, the age of the rock being dated can be determined based on the slope of the line.
The steeper the slope of the line, the more decay has occurred in a sample and the older the sample is Dalrymple The features of the isochron method provide a way do reduce doubt and speculation about an age that is computed using these methods. Based on the assumptions of basic radioactive dating, the problem of an unknown initial amount of daughter isotope is eliminated by the definition of the isochron itself. The problem of contamination is "self-checking". If contamination has occurred within a sample, the ratios from the sample shouldn't fall on a line.
Instead, the points would be in a scatter on the graph. Points that do not fall on a straight line suggest contamination, and this invalidates the results.
However, by this same principle, points falling relatively close to a best fit line should provide an accurate date for the age of the rock being dated Stasser In most cases, the slope of the line generated by the isochron method gives an age for a rock sample of millions, or even billions of years.
In general, these ages are supported by the science community, who declare that the Earth is about 4. However, young-Earth creationists believe in an Earth that was created only 6, years ago. The old age provided by isochron dating methods obviously conflicts with the young age of only 6, years held by these creationists. The intent of the plot is to assess a correlation between: Meaning of the plot axes.
If the data points on the plot are colinear, and the line has a positive slope, it shows an extremely strong correlation between: The amount of P in each sample, and The extent to which it is enriched in D, relative to Di.
This is a necessary and expected consequence, if the additional D is a product of the decay of P in a closed system over time. It is not easily explained, in the general case, in any other way. Why isochron data are colinear The data points would be expected to start out on a line if certain initial conditions were met. Consider some molten rock in which isotopes and elements are distributed in a reasonably homogeneous manner.
Its composition would be represented as a single point on the isochron plot: Global composition of the melt. As the rock cools, minerals form. They "choose" atoms for inclusion by their chemical properties. This results in an identical Y-value for the data points representing each mineral matching the Y-value of the source material.
There are minor differences between isotopes of the same element, and in relatively rare circumstances it is possible to obtain some amount of differentiation between them. This is known as isotope fractionation. The effect is almost always a very small departure from homogeneous distribution of the isotopes -- perhaps enough to introduce an error of 0. In contrast, P is a different element with different chemical properties.
This results in a range of X-values for the data points representing individual minerals. Since the data points have the same Y-value and a range of X-values, they initially fall on a horizontal line: Differential migration of elements as minerals form. A horizontal line represents "zero age. In most cases, any age less than about P half-lives will include zero within its range of uncertainty.
The range of uncertainty varies, and may be as much as an order of magnitude different from the approximate value above. It depends on the accuracy of the measurements and the fit of the data to the line in each individual case.
That encompasses the entire young-Earth timescale thousands of times over.afrocolombianidad.info #18 - Absolute radiometric age dating of rocks and geologic materials
As more time passes and a significant amount of radioactive decay occurs, the quantity of P decreases by a noticeable amount in each sample, while the quantity of D increases by the same amount. This results in a movement of the data points to the left decreasing P and upwards increasing D. Since each atom of P decays to one atom of D, the data point for each sample will move along a path with a slope of As a result, the data points with the most P the right-most ones on the plot move the greatest distance per unit time.
The data points remain colinear as time passes, but the slope of the line increases: Movement of data points as decay occurs. The slope of the line is the ratio of enriched D to remaining P. Miscellaneous notes Age "uncertainty" When a "simple" dating method is performed, the result is a single number.
There is no good way to tell how close the computed result is likely to be to the actual age. An additional nice feature of isochron ages is that an "uncertainty" in the age is automatically computed from the fit of the data to a line.
A routine statistical operation on the set of data yields both a slope of the best-fit line an age and a variance in the slope an uncertainty in the age. The better the fit of the data to the line, the lower the uncertainty. For further information on fitting of lines to data also known as regression analysissee: Yorka short technical overview of a technique specially designed for assessing isochron fits.
Note that the methods used by isotope geologists as described by York are much more complicated than those described by Gonick. This will be discussed in more detail in the section on Gill's paper below. The "generic" method described by Gonick is easier to understand, but it does not handle such necessities as: Unfortunately, one must wade through some hefty math in order to understand the procedures used to fit isochron lines to data.
General comments on "dating assumptions" All radiometric dating methods require, in order to produce accurate ages, certain initial conditions and lack of contamination over time. The wonderful property of isochron methods is: This topic will be discussed in much more detail below. Where the simple methods will produce an incorrect age, isochron methods will generally indicate the unsuitability of the object for dating. Avoidance of generic dating's problems Now that the mechanics of plotting an isochron have been described, we will discuss the potential problems of the "simple" dating method with respect to isochron methods.
Initial daughter product The amount of initial D is not required or assumed to be zero. Living organisms continually exchange Carbon and Nitrogen with the atmosphere by breathing, feeding, and photosynthesis.
When an organism dies, the 14C decays back to 14N, with a half-life of 5, years. Measuring the amount of 14C in this dead material thus enables the determination of the time elapsed since the organism died.
Radiocarbon dates are obtained from such things as bones, teeth, charcoal, fossilized wood, and shells. Because of the short half-life of 14C, it is only used to date materials younger than about 70, years.
Other Uses of Isotopes Radioactivity is an important heat source in the Earth. Elements like K, U, Th, and Rb occur in quantities large enough to release a substantial amount of heat through radioactive decay. Thus radioactive isotopes have potential as fuel for such processes as mountain building, convection in the mantle to drive plate tectonics, and convection in the core to produce the Earth's magnetic Field. Initial isotopic ratios are useful as geochemical tracers.
Such tracers can be used to determine the origin of magmas and the chemical evolution of the Earth. Short-lived isotopes Isotopes made during nucleosynthesis that have nearly completely decayed away can give information on the time elapsed between nucleosynthesis and Earth Formation. Ratios of stable, low mass isotopes, like those of O, S, C, and H can be used as tracers, as well as geothermometers, since fractionation of light isotopes can take place as a result of chemical process.
We can thus use these ratios of light isotopes to shed light on processes and temperatures of past events. Radioactivity is a source of energy and thus can be exploited for human use - good and bad.