ACID LEACHING EXPERIMENTS ON THE MANTLE-DERIVED VITIM CLINOPYROXENES : IMPLICATIONS FOR THE ROLE OF CLINOPYROXENES IN THE MANTLE PROCESSES

摘要

Because its crystal structure is unfavorable to the large and highly charged cations, the contribution of clinopyroxene to the highly incompatible trace element budget (such as Rb, Ba, Th, U, Nb, Ta, Pb) of the mantle rocks and melts is often thought to be negligible. Here, we present a complete picture of the incompatible element distribution in clinopyroxenes from a study of the geochemical effects of leaching experiments on the clinopyroxenes concentrated from various Vitim (Siberia, Russia) mantle rock types. It reveals that the clinopyroxene trace elements fit into four categories : 1) those which are generally unmodified (Th, U, Zr, Hf); 2) those which are generally lost (Co, Cu, Zn, Ge, Rb, Nb, Ba, Ta, Pb); 3) those which are slightly enriched (Sc, Y, REE); 4) those which offer a variable behavior (Sr). The acid leaching involved changes are mostly controlled by two major features: a) the existence of some submicroscopic Ge bearing inclusions (probably a Co-bearing sulfide); b) the presence of crystal defects and/ or vacancies within the clinopyroxene structure. The roles of the major components of the clinopyroxene and the nature of the rock types seem to be of secondary importance. There is no evidence for a contamination by fluid or melt inclusions. These leaching experiments bear important constraints on the way the incompatible ratios could behave during a metasomatic event involving the clinopyroxenes. The