Fractionation radiogenic isotopes

Heterogeneities in stable isotopes are difficult to detect, because stable isotope ratios are affected by the various partial melting-crystal fractionation processes that are governed by temperature-dependent fractionation factors between residual crystals and partial melt and between cumulate crystals and residual liquid. Unlike radiogenic isotopes, stable isotopes are also fractionated by low temperature surface processes. Therefore, they offer a potentially important means by which recycled crustal material can be distinguished from intra-mantle fractionation processes. 

If an element has two or more natural non-radiogenic isotopes, their isotopic ratio can be used to correct the mass fractionation in other isotope ratios of the same element. For example, " Nd/ Nd (=0.7219) in natural samples can be used to correct mass fractionation for " Nd/ Nd, where Nd is a radiogenic isotope from parent Sm. As another example, Sr/ Sr (=0.1194) in natural samples is used to correct mass fractionation in mass spectrometer for Sr/ Sr ratio, where Sr is a radiogenic isotope from parent Rb. 

Hf-isotope ratios also fall close or within the field of crustal rocks (Fig. 2.4 Table 2.2). This has been interpreted as evidence that metasomatic modification of lamproitic mantle sources in Tuscany was provided by addition of crustal material (e.g. metapelites). Interestingly, the mantle-normalised incompatible element patterns of Tuscany lamproites are similar to those of gneisses and schists in almost eveiy detail. This has been interpreted as evidence for addition of bulk upper crustal material to the mantle, with little element fractionation during metasomatism and the subsequent partial melting (Peccerillo 2002). This makes the Tuscany Province a zone where the upper mantle magmas look very much like the upper crust in terms of trace element and radiogenic isotope compositions. 

Mantle-derived materials have a range of elemental-abundance patterns due to various fractionation processes, although the mantle pattern has been inferred from measured isotopic variations and radiogenic isotope-production ratios (see PorceUi and Ballentine, 2002). From the " He/ Ne production ratio and the average coexisting shifts in " He/ He and Ne/ Ne in the mantle relative to the primordial compositions, a ratio of He/ Ne = 11 is obtained. This is greater than the more recent estimate of 1.9 for the solar nebula (see discussion in Porcelli and Pepin (2000)). Similarly, using the mantle " °Ar/2 Ar... 

At equilibrium, the activity ratio between any two members of a decay series is 1.00. However, at and near the Earth s surface, disequilibrium of the various nuclides of the uranium series is found to occur. The disequilibrium is especially pronounced in groundwaters (Cherdyntsev, 1971 Osmond and Cowart, 1976). The fractionation of the nuclides can occur as a result of chemical differences between elements, the fractionation of isotopes of a given element may occur because of preferential leaching of one (because of its radiogenic origin), or by the direct action of recoil during radioactive decay (Osmond and Cowart, 1976). 

Radiogenic isotopes have proved a most powerful tool for understanding mantle processes. By studying mantle peridotites and mantle-derived melts from a variety of different geological time periods it is possible to define evolutionary curves for the different isotopic systems within the mantle. These curves, when plotted on isotope ratio versus time diagrams, can be used to characterize the chemical evolution of the mantle over time. Deviations from the chondritic trend are used to identify chemical fractionation events in the mantle during Earth history. Of particular... 

---