Relationship between noble gases and radiogenic isotopes

Relationship between noble gases and radiogenic isotopes 

Although many models call upon a mantle with undepleted and undegassed (i.e., primitive) mantle characteristics, such as with a chondritic Th/U ratio (Galer and O Nions 1985 Turcotte et al. 2001) or undegassed noble gas concentrations (e.g., Hart et al. 1979 Kurz et al. 1982 Farley et al. 1992 Kellogg et al. 1999), there is no direct evidence that a component with all the expected associated geochemical characteristics exists. For example, OIB and MORE generally contain Nb/U and Ce/Pb ratios that are both non-chondritic and uniform. The observed ratios can only be produced by arc subduction or crust formation processes, and it appears that there are no mantle reservoirs that have not been impacted by these processes (Hofmann et al. 1986 Newsom et al. 1986 Hofmann 

Further evidence for widespread processing of the mantle comes from Nd-Hf isotope studies (Blichert-Toft and Albarede 1997). In a plot of vs Hf/ Hf, 

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