Hofmann et al. have shown in an important paper that the Ce/Pb- and Nd/U- ratios are "logarithmically constant" in basalts from ridges as well as in basalts from oceanic islands.
These ratios are symmetric to continental crust when normalized to chondrites. This fractionation that statistically predates the establishment of different isotopic signatures in MORB and OIB, presents an important constraint for the understanding of the structure and evolution of the Earth's mantle.
We further explored the concept of "Isotope like Trace Element Ratios" (ISOTER) and used 10 000 samples analysis obtained on basalts from different origins. Our aim is to better constrain the chemical evolution of the Earth.
We applied the following ratios: Ta/Th, Sm/Hf, Ce/U, Ba/Rb. All satisfy the criterion defined by Hofmann: homogeneity within basalts independent of their origin as MORB and OIB and symmetry with continental materials relative to carbonaceous chondrites.
A global mass balance inversion was calculated for the different reservoirs of the Earth and the global exchange mass between upper and lower mantle was established. It is certainly considerably lower than the present day subduction flux.
A careful inspection of the variance of MORB and OIB indicates that relative variance of MORB is larger than OIB, this is in contrast to the isotopic ratios and confirms the model of Allègre et al. (1994). We studied the reason for this discrepancy by using direct modeling of partial melting, heterogeneity of the source but also variability in the melting conditions are implied.
The studies of subduction zone volcanics show very distinct ISOTER signature that lie outside of the continental crust domain in contrast to such simple model of continental crust extraction. This places serious constraints on the genesis of the continental crust but also on the genesis of the island arc basalts.
The continental materials like granites and granulites have also been studied for the set of ISOTER. The results show that continental crust can not be identified to subduction zones volcanics.