Can rb-sr isochron dating understand
Petrology Tulane University Prof. Stephen A. Nelson Radiometric Dating Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Then, in , radioactivity was discovered.
The rubidium-strontium dating method is a radiometric dating technique used by scientists to determine the age of rocks and minerals from the quantities they contain of specific isotopes of rubidium 87 Rb and strontium 87 Sr, 86 Sr. Development of this process was aided by German chemists Otto Hahn and Fritz Strassmannwho later went on to discover nuclear fission in December The utility of the rubidium - strontium isotope system results from the fact that 87 Rb one of two naturally occurring isotopes of rubidium decays to 87 Sr with a half-life of In addition, Rb is a highly incompatible element that, during partial melting of the mantle, prefers to join the magmatic melt rather than remain in mantle minerals.
Decay scheme of K-Ar, U-Pb and Sm-Nd, petrogenetic implications-part B
As a result, Rb is enriched in crustal rocks. The radiogenic daughter, 87 Sr, is produced in this decay process and was produced in rounds of stellar nucleosynthesis predating the creation of the Solar System.
Historical Geology/Rb-Sr dating
During fractional crystallizationSr tends to become concentrated in plagioclaseleaving Rb in the liquid phase. Highest ratios 10 or higher occur in pegmatites. For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldsparK-feldsparhornblendebiotiteand muscovite.
Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt. The ideal scenario according to Bowen's reaction series would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende i.
This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different.
The isotope 86 Sr is non-radiogenic in origin and does not change, but 87 Sr is produced by the radioactive decay of 87 Rb.
The Rb-Sr method. usually the isochron method is employed to determine the age and initial. Sr/ Sr ratio of a suite of rock samples. 87Rb/86Sr y = 0 5 10 15 20 25 30 35 8 7 S r / 8 6 S r Rb-Sr isochron diagram for a series of cogenetic rock samples formed at the same time ( Sr/ 86 Sr) 0. The isotopes . There are three isotopes used in Rb-Sr dating. 87 Rb (rubidium) is an unstable isotope with a half-life of about 49 billion years. It produces the stable daughter isotope 87 Sr (strontium) by beta minus rhodeshotel.net third isotope we need to consider is 86 Sr, which is stable and is not radiogenic, meaning that in any closed system the quantity of 86 Sr will remain the . Dating - Dating - Rubidium-strontium method: The radioactive decay of rubidium (87Rb) to strontium (87Sr) was the first widely used dating system that utilized the isochron method. Rubidium is a relatively abundant trace element in Earth's crust and can be found in many common rock-forming minerals in which it substitutes for the major element potassium.
There is no way of anticipating what the 87 Sr is at the time of melt, but if there is 87 Rb present then it will increase with time as the rubidium isotope decays. That is what makes this a useful clock.
Rubidium-strontium isochrons will be formed at any time after crystallization of a rock provided the initial conditions are met. Different minerals which make up the rock will in general include different amounts of rubidium 87 Rb in their structures, and those which have more rubidium at the time of crystallization will have more radioactive decays and gain more of the daughter product 87 Sr.
The precise nature of the radioactive decay process predicts that all the minerals should lie along a straight line, an isochron.
Dating Rocks with the Rb-Sr "Isochron" Method. Page 5 of As you know, there are numerous radioactive isotopes that can be used for numeric dating. All of the dating methods rely on the fundamental principles of radioactive decay, but the specific materials that can be dated and the exact procedures for calculating a date are very different.
The longer the time interval, the more the decay and the steeper the slope of the isochron line. Rubidium-strontium Rb-Sr dating was the first technique in which the whole-rock isochron method was extensively employed. Certain rocks that cooled quickly at the surface were found to give precisely defined linear isochrons, but many others did not.
Some studies have shown that rubidium is very mobile both in fluids that migrate through the rock as it cools and in fluids that are present as the rock undergoes chemical weathering. Similar studies have shown that the samarium - neodymium Sm-Nd parent-daughter pair is more resistant to secondary migration but that, in this instance, sufficient initial spread in the abundance of the parent isotope is difficult to achieve.
When an igneous rock crystallizes, a wide variety of major and trace minerals may form, each concentrating certain elements and radioactive trace elements within the rock.
By careful selection, certain minerals that contain little or no daughter element but abundant parent element can be analyzed.
Rb-sr isochron dating
In this case, a graph can be set up in which the slope of the line may be computed from an assumed value for the initial ratio, and it is usually possible to show that uncertainties related to this assumption are negligible. This is possible in potassium-argon K-Ar datingfor example, because most minerals do not take argon into their structures initially.
In rubidium-strontium datingmicas exclude strontium when they form but accept much rubidium. In uranium-lead U-Pb dating of zirconthe zircon is found to exclude initial lead almost completely.
Minerals too are predictable chemical compounds that can be shown to form at specific temperatures and remain closed up to certain temperatures if a rock has been reheated or altered.
A rock, on the other hand, may contain minerals formed at more than one time under a variety of conditions.
Under such circumstances the isolation and analysis of certain minerals can indicate at what time these conditions prevailed. If a simple mineral is widespread in the geologic recor it is more valuable for dating as more units can be measured for age and compared by the same method. However, if a single parent-daughter pair that is amenable to precise analysis can be measured in a variety of minerals, the ages of a wide variety of rock types can be determined by a single method without the need for intercalibration.
Dating - Dating - The isochron method: Many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughter-type isotopes already exists. These isotopes did not come from radioactive decay in the system but rather formed during the original creation of the elements. In this case, it is a big . The isochron equation is applied just like that for the Rb-Sr system, by determining the Nd/ Nd and Sm/ Nd ratios on several minerals with a mass spectrometer and then from the slope determine the age of the rock. Rubidium-Strontium Isochrons Click on the illustration to step through the discussion. Rubidium-strontium isochrons can be used to calculate the last time of complete melting of a rock. The slope of the isochron line gives a measurement of the time since the last complete melting of the rock. It also gives the initial concentrations of strontium at the time of melting by projecting the.
In some cases the discovery of a rare trace mineral results in a major breakthrough as it allows precise ages to be determined in formerly undatable units.