Trace element partitioning between mantle minerals and silico-carbonate melts at 6-12 GPa and applications to mantle metasomatism and kimberlite genesis

摘要

The partitioning of a number of trace elements (ULE, HFSE, REE, Cu, Pb, Co, Ni) between mantle minerals (olivine, pyroxenes and garnet) and silico-carbonate melts was experimentally studied at 6-12 GPa and 1300-1700 °C. The starting compositions were model kimberlitic with ～30 wt% SiO2, which differentiated to carbonatite-like melts with <10 wt% SiO2 depending on the degree of crystallization. The melts were rich in CO2 (up to ～30 wt%) and contained 0 to 30 wt% H2O. Trace elements were added to the starting mixtures to levels of ～100 ppm. They were analyzed by LA ICP MS in the products of 18 experiments. The partition coefficients of Ba, La, Ta, and Nb are very low for all phases (<0.01). These elements are especially susceptible to contamination in the experimental products. We suspect that some relatively high values obtained for these elements in previous studies are overestimated. Generally, the partition coefficients (D~(s/1)) of the moderately incompatible and compatible elements increase in the sequence olivine-low-Ca pyroxene-high-Ca pyroxene. Garnet shows maximum fractionation of the trace elements, such that D values for the highly incompatible elements are lower than those for pyroxenes and highest for the HREE, Sc, and V. The partition coefficients of a number of incompatible elements are rather insensitive to pressure, temperature, and melt composition. There is no correlation between the partition coefficients and the content of H2O, CO2 and the overall content of trace elements. They are predominantly controlled by the composition of the crystalline phases. In particular, garnet-liquid partitioning is very sensitive to Ca in garnet and pyroxene-liquid partitioning to Al in pyroxene. The comparison of the obtained partition coefficients with the compositions of likely kimberlite magmas shows that depleted and refertilized harzburgite from the continental lithospheric mantle is a viable candidate as the source material of kimberlites. The estimation of trace element characteristics of near-solidus mantle melts indicates that the most reliable indicator of carbonate melt metasomatism is depletion of Zr and Hf relative to Sm and Nd. Criteria based on the ratios of more incompatible elements (e.g. Nb/La) are very sensitive to the melting conditions and can be misleading.