Analysis and geochemistry of siderophile elements (osmium, iridium, ruthenium, platinum, rhenium) and oxygen isotopic compositions of kimberlites and their mantle inclusions: Relation to evolution, diamond growth and metasomatism in the mantle.

摘要

A simplified analytical method has been developed for the direct determination of trace amounts of the noble metals Os, Ir, Ru, Pt and Re in kimberlitic and peridotitic rocks from small sample aliquots, using a double focusing sector field mass spectrometer (Finnigan MAT Element). The concentrations of the noble metals Ir, Ru, Pt and Re are determined in high resolution mode using standard addition. Highly volatile Os is spiked with an isotopic tracer, separated from the matrix and isotopic ratios are measured in low resolution mode. The analytical method was used to determine the concentrations of the noble metals in kimberlitic rocks and kimberlite inclusions, such as peridotites, MARIDs and garnet and ilmenite megacrysts. Results show that kimberlites and related rocks inherit their whole rock PGE abundances through the disaggregation of entrained high PGE-content mantle peridotites. This enables one to discriminate between magmas that have erupted through fertile off-craton, depleted on-craton or metasomatised on-craton lithosphere. Noble metal ratios and abundances in diamondiferous kimberlites from Southern Africa and China are correlated with diamond grade, and are interpreted to reflect both the diamond potential of the mantle and the carrying capacity of a particular magma. Trace siderophile, chalcophile, lithophile element and oxygen isotopic compositions of a suite of coarse, low temperature and sheared, high temperature harzburgites from Southern Africa were determined in order to characterize a variety of metasomatic enrichments (i.e. high and low temperature, lithophile and siderophile) preserved in these samples, and to elucidate possible genetic links with kimberlites and megacrysts included in kimberlites. Sheared, high temperature harzburgites are compatible with being the metasomatic end-products after interaction with siderophile-type (megacrystic) melts. Similar to the bulk of the coarse, low-temperature harzburgites in this study, they retain typical mantle-like oxygen isotopic compositions. A second distinct suite of megacrysts, with the highest deltaO 18 values (12 permil) reported for any mantle derived material to date, has been identified. These high values are consistent with a source in the mantle derived from subducted oceanic crust. The first oxygen isotopic compositions for phenocrysts from Group 1 kimberlites reported in this thesis, range from 5 to 8.1 permil, and lends support to the trace elemental data which suggests that there may be a link between the aberrantly high deltaO 18 value megacrysts in this study and Group 1 kimberlites.