Rubidium/strontium dating

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). 

Halpern M (1966). Rubidium-strontium date from Mt. Byerly, West Antarctica. Earth Planet Sci Lett 1 455-457 Halpern M (1967) Rubidium-strontium isotopic age measurements of plutonic igneous rocks in eastern Ellsworth Land and northern Antarctic Peninsula, Antarctica. J Geophys Res 72 5133... 

Halpem M (1970) Rubidium-strontium date of possibly 3 billion years for a granitic rock fiom Antarctica. Science 169 977-978 Hambrey MJ, Barrett PJ, Robinson PH (1989) Stratigraphy. In Barrett PJ (ed) Antarctic Cenozoic History from the CIROS-1 Drill Hole, McMurdo Sound. DSIR BuU 245 23-48. Wellington, New Zealand... 

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. 

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) ... 

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... 

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. 

The rubidium isotope Rb is radioactive and makes up 27.8% of all natural rubidium. Because of that it is used in the rubidium/strontium technique for dating rocks. 

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. 

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... 

Determination of ages by the Sm/Nd method entails analyzing either individual minerals or cogenetic rock suites in which the ratios between the two are sufficiently different to define the slope of an isochron in coordinates of [ Nd/ Nd] and ] Sm/ Nd]. The method is especially suitable for mafic and ultramafic rocks, c the Rb/Sr method, which is best suited for acidic and intermediate igneous rocks enriched in rubidium and depleted in strontium. Since the rare earth elements are less mobile than the alkali metals and the alkaline earths, phenomena such as regional metamorphism have less effect on them. Hence, suitable rocks can be dated by the Sm/Nd method even if they have lost or gained rubidium and strontium and this makes the Sm/Nd method a useful complement to the R/Sr method. 

Rocks can also be dated by comparing the amounts of various other radionuclides, for example lead-206 and lead-204, potassium-40 and argon-40, rubidium-87 and strontium-87. Using isotope dating methods, the oldest rocks on the Earth are 3.7 x 10 years old which means that the Earth is at least this old, because the isotope method can only work from the time that the rocks became solid. For example, any helium atoms formed while the rocks were still liquid would have boiled off as gas. Rocks from the Moon and meteorites appear to be around 4.6 x 10 years old. 

---