The Rb-Sr system

Ganguly J. and Ruitz J. (1986). Time-temperature relation of mineral isochrons A thermodynamic model, and illustrative examples for the Rb-Sr system. Earth Planet Set Letters, 81 338-348. 

An isochron diagrana is a bivariate plot of measured parent-daughter isotope ratios for a suite of cogenedc samples. Where the sample suite defines a Linear array, this is said to be an isochron and the slope of the line is proportional to the age of the sample suite. Consider as an example the Rb-Sr system. The total number of... 

These values indicate the extent to which an element is fractionated into the crust relative to the depleted mande. For example, Rb is the element most concentrated in the crust reladve to the depleted mande whereas Sr, Sm and Lu are the least concentrated. Alternatively, a parent-daughter element-pair may behave coherendy and not be fractionated and yet behave in a very different manner from the parent-daughter pair of another isotopic system. A good example is the contrast between the Sm-Nd system, in which both elements share very similar chemical and physical characteristics, and the Rb-Sr system, in which the elements are... 

The Sm/Nd ratio of the condnental crust is highly fractionated (<1.0) relative to CHUR and shows retarded Nd/ Nd evoludon with time (Figure 6.16). This is the opposite of the Rb-Sr system where the strontium-isotopic evolution of the crust shows accelerated evoludon with dme reladve to the mantle. 

The slope of the line connecting the open boxes depends on the time elapsed since the six minerals last equilibrated with each other or, in other words, since closure of the Rb-Sr system for these minerals. In this example, 40 million years (Ma) elapsed since closure. 

Figure 3 Crust-mantle differentiation patterns for the decay systems Rb-Sr, Sm-Nd, Lu-Hf, and Re-Os. The diagram illustrates the depletion-enrichment relationships of the parent-daughter pairs, which lead to the isotopic differences between continental crust and the residual mantle. For example, the Sm/Nd ratio is increased, whereas the Rb/Sr ratio is decreased in the residual mantle. This leads to the isotopic correlation in mantle-derived rocks plotted in Figure 4(a). The construction is similar to that used in Figure 2, but D values have been adjusted slightly for greater clarity.

Because of the numerous secondary effects that modify the whole-rock elemental and isotopic compositions, our discussion below will focus solely on mineral-separate analyses for the Rb-Sr, Sm-Nd, Lu-Hf, and U-Pb isotope systems. In comparison, the Re-Os system is much more robust to the effects of... 

Shirey S. B. (1991) The Rb-Sr, Sm-Nd and Re-Os isotopic systems a summary and comparison of their applications to the cosmochronology and geochronology of igneous rocks. In Applications of Radiogenic Isotope Systems to Problems in Geology, Short Course Handbook (eds. L. Heaman and J. Ludden). Mineral. Assoc. Can., Nepean, vol. 19, pp. 109-166. 

Jenkin G. R. T., EUam R. M., Rogers G., and Stuart E. M. (2001) An investigation of closure temperature of the biotite Rb-Sr system the importance of cation exchange. Geochim. 

Extensive studies of peridotite xenoliths using the Rb-Sr, Sm-Nd and U-Pb isotope systems have... 

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. 

Model results. The primary conclusion of the model is that Loihi and HIMU characteristics (for Sr, Pb, and He) can be generated from subducted components sequestered in two deep reservoirs. A nonlinear inversion was used to maximize the fit to the observations of continent concentrations and reservoir isotope ratios within the bounds of assigned uncertainties in the fluxes and fractionation coefficients. This was done using the U-Pb and Rb-Sr systems as well. In the best fit results, the D layer is 250-km thick (5% of the total mantle) and the RDM is 500-km thick. 

Because of the long half-lives of these nuclides they must have been formed at the time of (or possibly even before) the formation of the solar system and of the earth. When the earth s crust solidified, these radionuclides became trtq[>ped in rocks. As they decayed, decay products accumulated in the closed rock environment. By measuring the amount of parent and daughter nuclides, it is possible with the half-life to calculate how long this environment (e.g. a rock formation) has existed. This is the bases for nuclear dating (also called "radioactive clocks ), and almost all of the nuclides in Table 5.2 can be used for this purpose. In 5.8 we discuss dating methods for the K—Ar and Rb—Sr systems. 

Some uncertainty is associated with the use of the KI Ar ratio because of the possibility of the loss of gaseous argon from minerals. An alternative method is based on Rb/Sr system ... 

The Rb-Sr systematics indicate that the flows as well as the dikes of Vestfjella originated in an open magmatic system in which the chemical composition of the magma was altered by interaction with the Precambrian basement rocks that were characterized by having higher Sr/ Sr ratios but lower strontium concentration than the mantle-derived basalt magma. 

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