The ultimate aim of the project was to help understanding the complex picture of light element recycling in the Earth's mantle by constraining the B and Li concentrations and isotope characteristics of the mantle in three different geodynamic environments: a mantle modified in subduction conditions (e.g. mantle xenoliths from the Cerro del Fraile Volcanic Field, Patagonia), a stable continental lithosphere (e.g. mantle xenoliths from the Paili Aike Volcanic Field, Patagonia), and a mantle reworked in rift conditions (e.g. mantle xenoliths from the Marsabit Volcanic Field, Africa). In a very general context, the project aimed at contributing to a better understanding of the light elements behavior, and therefore of the global mantle isotope geochemistry and evolution. The progress made during the last year of the project has allowed to compile a consequent database on trace element and REE geochemistry of xenoliths samples from a mantle wedge, but most importantly on associated Li concentrations and isotopic compositions, which constitute the first and precise one of that kind. The concordance of the results with a model of diffusion is of major importance in the Li research world, and show that diffusion of Li is the important parameter for Li isotope fractionation even at high temperature, However, due to the late outcome of these conclusions, the details of it are still being processed. The first premise of the fall down of the reign of Li as subduction and mantle processes tracer have arise during the last few years (Elliot et al., 2009), even though many people are still confident in using it as a main mantle processes research tool. This is why the scientific community should henceforth be aware of the difficulties of using Li in such contexts: findings revealed by combining numerical and natural data (i.e. Dohmen et al., in press, and this study) have helped clarify which use should (not) be made of Li. Although the obtained results do not allow to answer the questions about Li placed in the proposal, neither the problematic this element was supposed to help constrain, they allowed to prevent from any ambiguity on the use of this element in this particular geological context for the future, and therefore helped research in isotopic geochemistry in general.