The Age of the Moon
In this report, for example, we are told that using one radioactive dating technique , a lunar rock sample is 4, million years old, plus or minus. Scientists find the age of the Earth by using radiometric dating of rocks Modern radiometric dating methods came into prominence in the late radiometric dating to determine the ages of moon rocks, obtained by astronauts. the actual experiments -- like analyzing meteorites and moon rocks -- have One technique, potassium-argon dating, determines the age of a rock Over time, atoms of the radioactive form of potassium -- an isotope called.
In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter.
Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e. It is not affected by external factors such as temperaturepressurechemical environment, or presence of a magnetic or electric field.
For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.
Accuracy of radiometric dating[ edit ] Thermal ionization mass spectrometer used in radiometric dating. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.
It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron.
This can reduce the problem of contamination. In uranium—lead datingthe concordia diagram is used which also decreases the problem of nuclide loss.
Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established.
On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero.
The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy.
At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature.
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis.
Radiometric dating - Wikipedia
The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No. The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.
Modern dating methods[ edit ] Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth. In the century since then the techniques have been greatly improved and expanded.
The mass spectrometer was invented in the s and began to be used in radiometric dating in the s. Generally, we are told that scientists have ways to analyze the object they are dating so as to eliminate the uncertainties due to unknown processes that occurred in the past. One way this is done in many radioactive dating techniques is to use an isochron. However, a recent paper by Dr. Hayes has pointed out a problem with isochrons that has, until now, not been considered.
The elements rubidium and strontium are found in many rocks. One form of rubidium Rb is radioactive. As illustrated above, a neutron in a Rb atom can eject an electron often called a beta particlewhich has a negative charge.
Since a neutron has no charge, it must become positively charged after emitting an electron. In fact, it becomes a proton. This changes the chemical identity of the atom. It is no longer Rb; it is strontium Sr Sr is not radioactive, so the change is permanent.
Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
We know how long it takes Rb to turn into Sr, so in principle, if we analyze the amount of Rb and Sr in a rock, we should be able to tell how long the decay has been occurring. Of course, there are all sorts of uncertainties involved. How much Sr was in the rock when it first formed?
Was Rb or Sr added to the rock by some unknown process? Was one of them removed from the rock by some unknown process? The isochron is supposed to take care of such issues. Here is how one of those sources tried to spin the results. The 40KAr ages are for No.
Comparison of mineral and rock data demonstrates gas loss. The plagioclase for No. The concordance of He and Ar ages must be fortuitous. The maximum age is equal to the Rb-Sr age, and the general pattern is compatible with the Sr results.
Seven crystalline rock samples returned by Apollo 11 have been analyzed in detail by means of the 40ArAr dating technique. Potassium-argon ages, corrected for the effects of this loss, cluster relatively closely around the value of 3.