Sr rb dating
Rb—Sr geochronology. Parent — daughter ratio : The ratio of rubidium Rb to strontium Sr. The daughter nuclide Sr is represented by Sr, which is stable and not subject to radiogenic ingrowth and constitutes approximately 9. Initial isotope equilibrium : The assumption that at the time of formation of a rock, all phases therein share the same Sr isotope composition; a prerequisite for an isochron. Isochron : A best-fit line of three or more phases in a Nicolaysen diagram with its slope corresponding to an age of phases that are in initial isotope equilibrium.
SEE VIDEO BY TOPIC: Isochrone equation Rb Sr methodContent:
In this article I shall introduce the Rb-Sr dating method, and explain how it works; in the process the reader should learn to appreciate the general reasoning behind the isochron method. There are three isotopes used in Rb-Sr dating.
It produces the stable daughter isotope 87 Sr strontium by beta minus decay. The 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 same. As rubidium easily substitutes chemically for potassium, it can be found doing so in small quantities in potassium-containing minerals such as biotite , potassium feldspar , and 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 87 Sr present initially. When we produced the formula for K-Ar dating , it 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 87 Sr in particular. 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 method , although the two are not exactly alike.
The reasoning, then, goes like this. 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: as we have noted, rubidium substitutes for potassium, and strontium for calcium.
Now consider the distribution of the two strontium isotopes 87 Sr and 86 Sr. The initial state of the rock may therefore be schematically represented by the graph to the right, which shows the initial states of four minerals imaginatively named A, B, C, and D.
Now consider what will happen to this system over time, as the 87 Rb decays to 87 Sr. If you have difficulty seeing this, try considering the extremal case of a mineral which contains no rubidium at all. The effect of the decay process on the isotope ratios can again be plotted on a graph, as shown to the right. So now we can find a date for the rock.
This one additional piece of information about the initial state of the rock allows us to calculate its age. As with the other methods we've discussed so far, the Rb-Sr method will only work if nothing but the passage of time has affected the distribution of the key isotopes within the rock. And of course this is not necessarily the case.
Hydrothermal or metasomatic events may have added or subtracted rubidium and strontium to or from the rocks since their formation; or a metamorphic event may have redistributed the rubidium or strontium among its constituent minerals , which would also interfere with the method. However, barring an extraordinary coincidence, the result of such events will be that when we draw the isochron diagram, the minerals will no longer lie on a straight line.
A small deviation from a straight line tells us that there is some uncertainty about the date, and this degree of uncertainty can be calculated; and if we get something which is nothing like a straight line, then the method simply doesn't supply us with a date. So just as step heating in Ar-Ar dating protects us from error, so too does the isochron method in Rb-Sr dating: it may not always lead us to the right date, but it is a good safeguard against our accepting one that is wrong.
There is, however, one potential source of error which will not show up on the isochron diagram, since it is expected to produce a straight line. Suppose that the original source of the rock was two different magmas call them X and Y imperfectly mixed together so that some parts of the rock will be all X, some all Y, some part X and part Y in varying proportions. Then these different parts of the rock, when analyzed for their isotopic composition, will plot in a straight line on the isochron diagram; and the slope of this line, and the point at which it intercepts the vertical axis, will have nothing to do with the age of the rock, and everything to do with the compositions of X and Y.
About half the time this will produce a straight line with negative slope: that is, it will slope down from left to right instead of up. Such a line must necessarily be produced by mixing, since a real isochron will always have positive slope: the rarity of such an occurrence tells us that mixing of this type must itself be rare. It can happen that if we produce a mixing plot for a perfectly good isochron, it will by some statistical fluke produce a straight line on the mixing plot; we would then be throwing out a perfectly good date.
However, this is worth it: it would, as I say, require a fluke for this to happen, so if we reject dates based on the mixing plot, then we will be throwing out a hundred bad dates for every good one. From Wikibooks, open books for an open world.
The radioactive decay of rubidium 87 Rb to strontium 87 Sr was the first widely used dating system that utilized the isochron method. Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium compared with the stable isotopes. A ratio for average continental crust of about 0. This difference may appear small, but, considering that modern instruments can make the determination to a few parts in 70,, it is quite significant. Dissolved strontium in the oceans today has a value of 0.
Sr isochron plots distinguish three different suites of granitic rocks in the vicinity of Okanagan Lake. The secular increase in initial ratios reflects crustal thickening and evolution. If you have an individual subscription to this journal, or if you have purchased this article through Pay-Per-view , you can gain access by logging in with your username and password here:. Advanced Search. All Journals Journal.
Alkali Metal Dating, Rb-Sr Dating Model: Radioactive Dating, Part 4
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 Strassmann , who 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. As a result, Rb is enriched in crustal rocks.
Continue to access RSC content when you are not at your institution. Follow our step-by-step guide. In situ dating of K-rich minerals, e.
Canadian Journal of Earth Sciences
In this article I shall introduce the Rb-Sr dating method, and explain how it works; in the process the reader should learn to appreciate the general reasoning behind the isochron method. There are three isotopes used in Rb-Sr dating. It produces the stable daughter isotope 87 Sr strontium by beta minus decay. The 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 same.
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.
На экран выплыла надпись: КЛЮЧ К ШИФРУ-УБИЙЦЕ ПОДТВЕРЖДЕН - Укрепить защитные стены! - приказал Джабба. Но Соши, опередив его, уже отдала команду. - Утечка прекратилась! - крикнул техник. - Вторжение прекращено. Наверху, на экране ВР, возникла первая из пяти защитных стен.
Черные атакующие линии начали исчезать. - Происходит восстановление! - кричал Джабба.
Он тяжко вздохнул: какое все это имеет значение. Он профессор лингвистики, а не физики. - Атакующие линии готовятся к подтверждению доступа. - Господи! - Джабба в отчаянии промычал нечто нечленораздельное.
Хейл теряет самообладание, и у него всего два выхода: выбраться из шифровалки или сесть за решетку. Внутренний голос подсказывал ей, что лучше всего было бы дождаться звонка Дэвида и использовать его ключ, но она понимала, что он может его и не найти. Сьюзан задумалась о том, почему он задерживается так долго, но ей пришлось забыть о тревоге за него и двигаться вслед за шефом.
Стратмор бесшумно спускался по ступенькам.
Он искал нужные слова. - У вас есть кое-что, что я должен получить. Эти слова оказались не самыми подходящими.
Ангел-хранитель. - И, полагаю, если с Танкадо что-нибудь случится, эта загадочная личность продаст ключ.
Это был тот самый парень, за которым он гнался от автобусной остановки. Беккер мрачно оглядел море красно-бело-синих причесок. - Что у них с волосами? - превозмогая боль, спросил он, показывая рукой на остальных пассажиров. - Они все… - Красно-бело-синие? - подсказал парень. Беккер кивнул, стараясь не смотреть на серебряную дужку в верхней губе парня.
НАЙТИ: ЗАМОК ЭКРАНА Монитор показал десяток невинных находок - и ни одного намека на копию ее персонального кода в компьютере Хейла.
Сьюзан шумно вздохнула. Какими же программами он пользовался. Открыв меню последних программ, она обнаружила, что это был сервер электронной почты. Сьюзан обшарила весь жесткий диск и в конце концов нашла папку электронной почты, тщательно запрятанную среди других директорий. Открыв ее, она увидела несколько дополнительных папок; создавалось впечатление, что у Хейла было множество почтовых адресов.
Один из них, к ее удивлению, был адресом анонимного провайдера.
Просто позор. - Могу я для вас что-нибудь сделать. Клушар задумался, польщенный оказанным вниманием.