Rb—Sr isotopic system

Fletcher IR, McNaughton NJ, Pidgeon RT, Rosman KJR (1997a) Sequential closure of K-Ca and Rb-Sr isotopic systems in Archaean micas. Chem Geol 138 289-301... 

The radiogenic Rb-Sr isotopic system has become a useful parameter in studies of ground-water systems, especially in assessing the origin(s) of the solutes. McNutt (2000) presents a detailed review of strontium isotopic behavior in water with emphasis on water-rock interaction see also Chapter 5.12. 

The geochemistry of marine sediments is a major source of information about the past environment. Of the many measurements that provide such information, those of the U-series nuclides are unusual in that they inform us about the rate and timescales of processes. Oceanic processes such as sedimentation, productivity, and circulation, typically occur on timescales too short to be assessed using parent-daughter isotope systems such as Rb-Sr or Sm-Nd. So the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides (principally or the U-series nuclides. This makes the U-series nuclides powerful allies in the quest to understand the past ocean-climate system and has led to their widespread application over the last decade. 

Figure 11.14 (A) Internal Rb-Sr isochron for a system composed of three crystalline phases of initial compositions Aq, Bq, and Q formed at time t = 0 and thereafter closed to isotopic exchanges up to time of measurement t, when they acquired compositions A, and C. (B) Effects of geochronological resetting resulting from metamorphism or interaction with fluids. X, X2, and X3 bulk isotopic compositions of the three rock assemblages. In cases of short-range isotopic reequilibration, the three assemblages define crystallization age and original ( Sr/ Sr)o of the system the three internal isochrons (concordant in this example) define resetting age.

Nd. From the high isotope ratio " Nd/ " Nd and the low isotope ratio it must be concluded that chain reactions have occurred. The age of the Oklo deposits was found by Rb/ Sr analysis to be a 1.7 10 y. At that time the concentration of in natural uranium was 3%. The presence of water in the sedimentary ore deposits led to high values of the resonance escape probability p (eq. (11.1)) and to criticality of the systems ( eff > ) ... 

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. 

Although the continental crust constitutes only 0.6% by mass of the silicate Earth, it contains a very large proportion of incompatible elements (20-70%, depending on element and model considered Rudnick and Fountain (1995)), which include the heat-producing elements and members of a number of radiogenic-isotope systems (Rb-Sr, U-Pb, Sm-Nd, Lu-Hf). Thus the continental crust factors prominently in any mass-balance calculation for the Earth as a whole and in estimates of the thermal structure of the Earth (Sclater et al., 1980). 

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