MSA AWARD LECTURE: HETEROGENEOUS MAGNESIUM ISOTOPIC COMPOSITION OF THE MANTLE AND ITS IMPLICATIONS

Our studies show that wehrlites, pyroxenites and cratonic eclogites display much wider variation in Mg isotopic composition than lherzolite and harzburgite xenoliths. Large Mg isotope fractionation is also found between coexisting garnet and omphacite in eclogites, as well as among coexisting spinel, garnet, clinopyroxene, orthopyroxene and olivine in mantle peridotites and pyroxenites. These significant inter-mineral isotope fractionations reflect both equilibrium partitioning and disequilibrium processes. The noticeable Mg isotopic variations at both mineral and whole-rock scales have either resulted from the difference in Mg coordination of major rock-forming minerals, or mantle metasomatism, or been inherited from subducted isotopically light oceanic crust due to surface processes. Our results demonstrate that considerable Mg isotope fractionation can occur at mantle temperatures and Mg isotopes have great potential applications in tracing a range of crustal and mantle processes.