Historical Geology/Rb-Sr dating - Wikibooks, open books for an open world
rubidium—strontium dating A radiometric dating method based on the radioactive decay of 87Rb to 87Sr. Rubidium has two isotopes (85Rb %, 87Rb. Dating - Rubidium–strontium method: The radioactive decay of rubidium ( 87Rb) to This equation is that of a straight line of the form y = b + xm, where y . Most work to date has centred around rhenium- or osmium-enriched minerals. Although we now recognize lots of problems with that calculation, the . We can measure the present ratios of (87Sr/86Sr)t and (87Rb/86Sr)t.
As rubidium easily substitutes chemically for potassium, it can be found doing so in small quantities in potassium-containing minerals such as biotitepotassium feldsparand hornblende. The quantity will be small because there is much more potassium than rubidium in the Universe. But there is no reason at all to suppose that there was no 87Sr present initially. When we produced the formula for K-Ar datingit was reasonable enough to think that there was little to no argon present in the original state of the rock, because argon is an inert gas, does not take part in chemical processes, and so in particular does not take part in mineral formation.
Strontium, on the other hand, does take part in chemical reactions, and can substitute chemically for such elements as calcium, which is commonly found in igneous rocks. So we have every reason to think that rocks when they form do incorporate strontium, and 87Sr in particular.
The isochron diagram[ edit ] However, there is still a way to extract a date from the rock. In the reasoning that follows, the reader may recognize a sort of family resemblance to the reasoning behind step heating in the Ar-Ar methodalthough the two are not exactly alike. The reasoning, then, goes like this.
Calculating Rb-Sr Isochrons
When an igneous rock is first formed, its minerals will contain varying concentrations of rubidium and strontium, with some minerals being high in rubidium and low in strontium, others being high in strontium and low in rubidium. We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: Now consider the distribution of the two strontium isotopes 87Sr and 86Sr.Isochrone equation Rb Sr method
The rock itself gives the integratedmore gradual increase. Approaches to this ideal case are commonly observed, but peculiar results are found in situations where the heating is minimal.
Epidote, a low-temperature alteration mineral with a very high concentration of radiogenic strontium, has been found in rocks wherein biotite has lost strontium by diffusion. The rock itself has a much lower ratio, so that it did not take part in this exchange. Although rubidium—strontium dating is not as precise as the uranium—lead method, it was the first to be exploited and has provided much of the prevailing knowledge of Earth history.
The procedures of sample preparationchemical separation, and mass spectrometry are relatively easy to carry out, and datable minerals occur in most rocks. Precise ages can be obtained on high-level rocks i. The mobility of rubidium in deep-level crustal fluids and melts that can infiltrate other rocks during metamorphism as well as in fluids involved in weathering can complicate the results. Samarium—neodymium method The radioactive decay of samarium of mass Sm to neodymium of mass Nd has been shown to be capable of providing useful isochron ages for certain geologic materials.
Both parent and daughter belong to the rare-earth element group, which is itself the subject of numerous geologic investigations.
All members of this group have similar chemical properties and charge, but differ significantly in size. Because of this, they are selectively removed as different minerals are precipitated from a melt. In the opposite sense, their relative abundance in a melt can indicate the presence of certain residual minerals during partial melting.
Unlike rubidium, which is enriched over strontium in the crust, samarium is relatively enriched with respect to neodymium in the mantle. Consequently, a volcanic rock composed of melted crust would have elevated radiogenic strontium values and depressed radiogenic neodymium values with respect to the mantle. As a parent—daughter pair, samarium and neodymium are unique in that both have very similar chemical properties, and so loss by diffusion may be reduced.
Their low concentrations in surface waters indicates that changes during low-temperature alteration and weathering are less likely. Their presence in certain minerals in water-deposited gold veins, however, does suggest mobility under certain conditions. In addition, their behaviour under high-temperature metamorphic conditions is as yet poorly documented. The exploitation of the samarium—neodymium pair for dating only became possible when several technical difficulties were overcome.
Procedures to separate these very similar elements and methods of measuring neodymium isotope ratios with uncertainties of only a few parts inhad to be developed. In theory, the samarium—neodymium method is identical to the rubidium—strontium approach. Both use the isochron method to display and evaluate data. In the case of samarium—neodymium dating, however, the chemical similarity of parent and daughter adds another complication because fractionation during crystallization is extremely limited.
This makes the isochrons short and adds further to the necessity for high precision. With modern analytical methods, however, uncertainties in measured ages have been reduced to 20 million years for the oldest rocks and meteorites.
Mineral isochrons provide the best results.
Calculating Rb-Sr Isochrons
The equation relating present-day neodymium isotopic abundance as the sum of the initial ratios and radiogenic additions is that of a straight line, as discussed earlier for rubidium—strontium. Other successful examples have been reported where rocks with open rubidium—strontium systems have been shown to have closed samarium—neodymium systems. In other examples, the ages of rocks with insufficient rubidium for dating have been successfully determined.
There is considerable promise for dating garneta common metamorphic mineral, because it is known to concentrate the parent isotope.
In general, the use of the samarium—neodymium method as a dating tool is limited by the fact that other methods mainly the uranium—lead approach are more precise and require fewer analyses. In the case of meteorites and lunar rocks where samples are limited and minerals for other dating methods are not available, the samarium—neodymium method can provide the best ages possible.
Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock generally during potassic alteration or calcic albitisation alteration.
Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed. Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement. A Rb-Sr date which is at variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock.
Geochronology[ edit ] The Rb-Sr dating method has been used extensively in dating terrestrial and lunar rocks, and meteorites. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. Although this is a potential source of error for terrestrial rocks, it is irrelevant for lunar rocks and meteorites, as there are no chemical weathering reactions in those environments.