Rubidium-87 dating

Bunsen, Monatsber. Akad. Berliny i860 (1861), 221 (new alkali metal dated Heidelberg, 3 May i860) 1861, 273 (casium, dated Heidelberg, 23 February 1861) Kirchhoff and Bunsen, Ann. Phys.y 1861, cxiii, 337-81 (rubidium, dated June 1861) Bunsen, Ann. Phys.y 1863, cxix, i-ii (casium, dated 26 April) Ann.y 1863, cxxv, 367-8 Ges. Abhl.y iii, 252-93. 

Rubidium-87 emits beta-particles and decomposes to strontium. The age of some rocks and minerals can be measured by the determination of the ratio of the mbidium isotope to the strontium isotope (see Radioisotopes). The technique has also been studied in dating human artifacts. Rubidium has also been used in photoelectric cells. Rubidium compounds act as catalysts in some organic reactions, although the use is mainly restricted to that of a cocatalyst. 

A popular method used to date rocks is the potassium-argon method. Potassium is abundant in rocks such as feldspars, hornblendes, and micas. The K-Ar method has been used to date the Earth and its geologic formations. It has also been applied to determine magnetic reversals that have taken place throughout the Earth s history. Another method used in geologic dating is the rubidium-strontium, Rb-Sr, method. Some of the oldest rocks on Earth have been dated with this method, providing evidence that the Earth is approximately 5 billion years old. The method has also been used to date moon rocks and meteorites. 

There are two basic ways to apply the 87Rb-87Sr technique to natural samples. The original method is to simply measure the isotopic composition of strontium and the abundance of rubidium in a rock and then calculate a date. If the rock contains no common strontium, a date can be calculated from ... 

A wide variety of rock types can be dated by the 87Rb-87Sr system, provided that the samples satisfy the assumptions that the system was initially isotopically homogeneous (had a uniform 87Sr/86Sr ratio) and did not gain or lose rubidium or strontium after if formed. Both... 

In the rubidium-strontium age dating method, radioactive 87Rb isotope with a natural isotope abundance of 27.85 % and a half-life of 4.8 x 1010 a is fundamental to the 3 decay to the isobar 87 Sr. The equation for the Rb-Sr method can be derived from Equation (8.9) ... 

Radioactive dating -use of radioisotopes [RADIOISOTOPES] (Vol 20) -use of rubidium compounds [RUBIDIUM AND RUBIDIUM COMPOUNDS] (Vol 21)... 

The ion microprobe has also been applied in a preliminary fashion to the rubidium-strontium dating technique. The correlation of the ion mieroprobe results with the independently determined isochron indicates that it may be possible to obtain useful results for samples on a micrometer scale from this dating technique. 

Compston, W., and R. T. Pidgeon Rubidium-strontium dating of shales by the total-rock method. J. Geophys. Res. 67, 3493—3502 (1962). 

CIDEP and CIDNP in Metal and Metallorganic Systems. To date, very few CIDEP studies of metal systems have been reported (139). These included the earlier study of rubidium and cesium systems... 

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

River Station Date Discharge Cond. pH SPM Vanadium Chromium Manganese Cobalt Nickel CoQper Zinc Arsenic Rubidium Strontium Cesium Barium Lead Uranium... 

Cffisium, rubidium, ammonium, and certain organic trichlormolyb-dates have been prepared.1... 

Isochron age calculations are commonly made for the Rb-Sr (rubidium-strontium], Sm-Nd (samarium-neodymium], and U-Pb (uranium-lead] radioactive systems. They are most commonly applied to whole-rock systems, that is, a suite of samples thought to have formed at the same time, such as an igneous plu-ton or a suite of lavas. Isochron age calculations may also be made for a suite of minerals in a rock, in which case they date the time at which the minerals lost isotopic contact with each other, that is, became closed systems. This approach can be useful in dating metamorphism. 

With its predictable and unchanging rates, radioactive decay has provided scientists with a technique for determining the age of fossils, geological formations, and human artifacts. Using a knowledge of the half-life of a given radioisotope, one can estimate the age of an object in which the iso- tope is found. Four different isotopes are commonly used for dating objects carbon-14, uranium-238, rubidium-87, and potassium-40. Now look at one of these techniques in more detail. 

To date objects that are more than 60 000 years old, techniques involving other isotopes must be used. Rocks and minerals that are up to billions of years old can be dated on the basis of the decay of radioisotopes with long half-lives, such as potassium-40 tm = 1.25 billion years), uranium-238 (ti/2 = 4.5 billion years), and rubidium-87 = 48 billion years). Such techniques are illustrated in Figure 21.9. 

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