Depleted mantle strontium isotopes

In the early days of mantle geochemistry, the composition of the bulk silicate earth, also called primitive mantle (i.e., mantle prior to the formation of any crust see Chapter 2.01) was not known for strontium isotopes because of the obvious depletion of rubidium of the Earth relative to chondritic meteorites (Gast, 1960). 

Zhu and MacDougall (1998) showed that calcium isotope composition of the shells was both species and temperamre dependent and that river water is depleted by 2%o relative to seawater. The temperature dependency of calcium isotope fractionation enables this tracer to be a potential paleotem-peramre proxy (Nagler et al., 2000). The seawater Cal" Ca secular variations are indicated by the results of De La Rocha and De Paolo (2000) for the last 160 Myr, and by the data of Zhu (1999) for the entire 3.4 Gyr of earth history. The latter indicate that, in analogy to strontium isotopes (Veizer and Compston, 1976), the Archean samples have C Ca ratios similar to the earth mantle, with the crustal-like values first appearing at the Archean/Proterozoic transition. However, in contrast to the strontium isotope trend, the calcium isotope ratios appear to dip towards mantle values also at —1.6 Gyr ago. 

Nd/ Nd low 8nneodymium isotopic compositions, the remarkable aspect of the frequency distribution for CLM as a whole is that the pronounced mode is within 58 units of bulk Earth and the mean 8j,jd value is 1.8. On this basis, the dominant neodymium isotopic characteristic of CLM is not enriched but is close to, or slightly more depleted than bulk Earth. Of course, if Depleted Mantle is used as a reference point then the CLM mode is enriched. The strontium isotope frequency distribution has a long tail out to very radiogenic ( enriched ) compositions but the mean Sr/ Sr is 0.7047 very close to estimates of bulk Earth. It is important to bear in mind that this statistical view of CLM geochemistry could... 

Figure 6.H The evolution of strontium isotopes with time. The shaded held mdicates the evolution of the bulk earth om 4.6 Ga to the present-day value of between 0.7045 and 0.7052, reflecting Rb/Sr rados of between 0.03 and 0.032. The depleted mande curve (1) is nun Bell et al. (1982) and indicates the separation of a depleted mantle reservoir from the primitive mande at ca 2.8 Ga ben th die Superior Province, Canada. Curve (2) is from Ben Othman et a . (1984) and is based upon the data from MORB, ophiolites, komadites and meteorites. It represents the equation ( Sr/ Sr) - + Bt + C where A = —1.54985776 x 10 , B -...

---