I have to make comments on some plots of geochemical data and I need to get better understanding of how elements get enriched or depleted during crystallization.
Dear Ahmed A. Rashad which kind of rock are you referring to? Because geochemical behaviour of trace elements and Rare Earth Elements (REE) changes according to the petrological system.
Some elements are closely related to the crystallization (or fusion) of some mineral phases depending of their compatibility with the solid phase. So the residual melt (and future rock) could presents enrichments or deplation on that particular element (e.g Eu with plg). Futhermore, the element aboundance is closely controlled by % of partial melting of the source and a lots of secondary processes (e.g. fluid/rock interaction and consequent enrichment).
Your question is more than universal , trace elements and REE. There is only one source where you can find the lithologies and the pertinent average data for them. Based upon Bowen´s crystallization principles from ultrabasic through felsic you will see how the various trace elements, in general, and the REE, in particular , change along with this lithogenetic process:
See:
Wedepohl, K. H (different years). Wedepohl, K.H. (ex. ed.) Handbook of Geochemistry. Springer: Berlin, Heidelberg, New York Catalogue style to be continued.