Element diffusion ability in metasomatic agents and its effect on chemical characteristics of metasomatized peridotites

Detailed in situ LM-ICPMS researches on the composite xenoliths from Yingfengling volcano of Leizhou Peninsula, South China, indicate that most incompatible trace elements of clinopyroxenes in composite xenoliths decrease spatially from pyroxenites to distal lherzolites, and compatible elements and HREE increase steeply. The increasing and decreasing rate is distinct for different trace elements, which give rise to element chromatographic fractionation within metasomatized lherzolites. The element chromatographic fractionation result actually from the difference in element diffusive rate in melts or fluids percolating through wallrock lherzolites. Based on element variation profiles in composite xenoliths this study indicates that Sr, Nb, La and Ce have the highest diffusive mobility, MREE-HREE are moderate, and Zr, Hf, Ti, Ga and Sc are very low in most cases. Higher diffusive rates of LREE than HREE would enlarge the REE fractionation of metasomatized peridotites, and lower diffusive rate of Zr, Hf and Ti relative to neighbor REEs with similar incompatibility would cause the relative depletion of these elements in metasomatized peridotites. Trace elements commonly have a higher diffusive rate in fluid-rich infiltrating melt, which will weaken element chromatographic fractionation during the metasomatism. The range of mantle metasomatism caused by silicate melt intrusion is very limited, generally within tens of centimeters wide. The width of metasomatized wallrock peridotites near the pyroxenite or horblendite veins was strictly controlled by both melt volume and chemical characteristics.