Rubidium/Strontium Dating of Meteorites
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.
Rubidium-strontium (Rb-Sr) dating was the first technique in which the whole-rock isochron method was Earth: Accretion of the early Earth.
Different lithologies impure marble, eclogite and granitic orthogneiss sampled from a restricted area of the coesite-bearing Brossasco—Isasca Unit Dora Maira Massif have been investigated to examine the behaviour of 40 Ar— 39 Ar and Rb—Sr systems in phengites developed under ultrahigh-pressure UHP metamorphism.
Mineralogical and petrological data indicate that zoned phengites record distinct segments of the P — T path: prograde, peak to early retrograde in the marble, peak to early retrograde in the eclogite, and late retrograde in the orthogneiss. Besides major element zoning, ion microprobe analysis of phengite in the marble also reveals a pronounced zoning of trace elements including Rb and Sr. These data confirm previous reports on excess Ar and, more significantly, highlight that phengite acted as a closed system in the different lithologies and that chemical exchange, not volume diffusion, was the main factor controlling the rate of Ar transport.
Although this time interval matches Ar ages from the same sample, Rb—Sr data from phengite are not entirely consistent with the whole dataset. The oldest age obtained from a millimetre-sized grain fraction enriched in prograde—peak phengites may represent a minimum age estimate for the prograde phengite relics. Results highlight the potential of the in situ 40 Ar— 39 Ar laser technique in resolving discrete P — T stages experienced by eclogite-facies rocks provided that excess Ar is demonstrably a negligible factor , and confirm the potential of Rb—Sr internal mineral isochrons in providing precise crystallization ages for eclogite-facies mineral assemblages.
Dating eclogite-facies rocks and their subsequent retrogression at upper crustal levels represents an invaluable, essential tool for constraining the rate of exhumation of these rocks from mantle depths, thus allowing development of theoretical models. To temporally quantify geological processes, isotopic ages must be linked to a specific stage of the P — T —deformation evolution of a rock.
In the most popular approach, this link is established using the closure temperature concept T c ; Dodson, When interpreting isotopic ages in terms of temperature only, this concept has been used to derive the temperature—time path by analysing minerals with different T c. However, high-pressure HP and ultrahigh-pressure UHP metamorphic rocks are peculiar systems, which experienced extreme physical conditions characterized by limited aqueous fluids with restricted mobility, and consequently by limited mass transfer and exceedingly sluggish reaction kinetics.
In these circumstances, one cannot assume that radiogenic daughters diffusing out of a mineral are efficiently removed at the grain boundary and that the concentration of that isotope at the grain boundary is zero.
Single grain pyrite Rb-Sr dating of the Linglong gold deposit, eastern China
The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth. The equation is the one which describes radioactive decay:. Solving the equation for “age,” and incorporating the computation of the original quantity of parent isotope, we get:. Some assumptions have been made in the discussion of generic dating, for the sake of keeping the computation simple.
Such assumptions will not always be accurate in the real world. These include:.
The following radioactive decay processes have proven particularly useful in radioactive dating for geologic processes:. Note that uranium and uranium give rise to two of the natural radioactive series , but rubidium and potassium do not give rise to series. They each stop with a single daughter product which is stable. Ages determined by radioactive decay are always subject to assumptions about original concentrations of the isotopes.
The decay schemes which involve lead as a daughter element do offer a mechanism to test the assumptions. Common lead contains a mixture of four isotopes. Lead , which is not produced by radioactive decay provides a measure of what was “original” lead. It is observed that for most minerals, the proportions of the lead isotopes is very nearly constant, so the lead can be used to project the original quantities of lead and lead The two uranium-lead dates obtained from U and U have different half-lives, so if the date obtained from the two decays are in agreement, this adds confidence to the date.
They are not always the same, so some uncertainties arise in these processes.
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.
14C dating This method is only reliable for samples with a high Rb/Sr ratio (e.g., biotite) because in that case, a wrong value for (87Sr∕86Sr)∘ has only a.
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Historical Geology/Rb-Sr dating
With heat, daughter isotopes diffuse out of their host minerals but are incorporated into other minerals in the rock. When the rock again cools, the minerals close and again accumulate daughter products to record the time since the second event. Remarkably, the isotopes remain within the rock sample analyzed, and so a suite of whole rocks can still provide a valid primary age.
1 orebody for pyrite Rb-Sr dating. They are hosted in quartz albitite or muscovite-quartz schist. Among these samples, no obvious post-ore.
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. Thus, if well-dated, unaltered fossil shells containing strontium from ancient seawater are analyzed, changes in this ratio with time can be observed and applied in reverse to estimate the time when fossils of unknown age were deposited. The rubidium—strontium pair is ideally suited for the isochron dating of igneous rocks.
As a liquid rock cools, first one mineral and then another achieves saturation and precipitates, each extracting specific elements in the process. Strontium is extracted in many minerals that are formed early, whereas rubidium is gradually concentrated in the final liquid phase. In practice, rock samples weighing several kilograms each are collected from a suite of rocks that are believed to have been part of a single homogeneous liquid prior to solidification.
The samples are crushed and homogenized to produce a fine representative rock powder from which a fraction of a gram is withdrawn and dissolved in the presence of appropriate isotopic traces, or spikes.
Alkali Metal Dating, Rb-Sr Dating Model: Radioactive Dating, Part 4
Radiometric dating is a means of determining the “age” of a mineral specimen by determining the relative amounts present of certain radioactive elements. By “age” we mean the elapsed time from when the mineral specimen was formed. Radioactive elements “decay” that is, change into other elements by “half lives.
The Rb-Sr method is commonly used to date. Rb-rich rock samples were analysed by this method. What accounts for huge range in Rb/Sr ratios of rocks? 1.
The Rb-Sr beta-decay dating system is one of the most attractive tools in geochronology, as Rb is sufficiently abundant in common K-bearing minerals like biotite, muscovite and K-feldspar. This allows dating of a wide variety of rocks e. However, this advantage was to date negatively counteracted by the lack of a suitable in-situ technique, as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope.
A reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer ICP-MS allows exactly this, the online chemical separation of two different elements. Coupled to a laser ablation LA system, in-situ Rb-Sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates Sr from Rb.
We present here a simple procedure in which Rb-Sr ages can be obtained from a suite of individual phases in regular thin sections. Results are presented for a variety of magmatic rocks with well-established thermal records: a sample each from the Klokken syenodiorite Greenland; Ma , the Ulvo alkaligabbro Sweden; Ma and a pegmatite from the Bohus granite Sweden; Ma. Obtained in situ Rb-Sr isochron ages are accurate Sidansvarig: Webbredaktion Sidan uppdaterades: English Lyssna.
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Rubidium-strontium dating , method of estimating the age of rocks, minerals, and meteorites from measurements of the amount of the stable isotope strontium formed by the decay of the unstable isotope rubidium that was present in the rock at the time of its formation. Rubidium comprises The method is applicable to very old rocks because the transformation is extremely slow: the half-life, or time required for half the initial quantity of rubidium to disappear, is approximately 50 billion years.
Most minerals that contain rubidium also have some strontium incorporated when the mineral was formed, so a correction must be made for this initial amount of strontium to obtain the radiogenic increment i. Rubidium-strontium dating. Article Media.
Rubidium-strontium isochrons can be used to calculate the last time of radioactive decay of 87Rb. There is no way of anticipating what the 87Sr is at can be learned from the data, but it does not give an unambiguous date since melting.
Rubidium strontium dating example This shows that the main method by the nuclei in geochronological dating service o2 rubidium strontium Radiometric dating method of time the age dating 5. Here you will decay. Rubidium 87 nucleus will decay of dating? All of relative dating method is to. Rb-Rich minerals such as trace elements in the rock composition and rubidium—strontium method the quantities they.
Ice cores are the isochron for extremely old rocks absolute dating the ratio of carbon isotopes.
Rb sr dating example
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. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer.
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