Geochemie silně siderofilních prvků a Re-Os izotopů plášťových pyroxenitů: důsledky pro obohacení svrchního pláště
Mantle pyroxenites represent the second most common mantle rock found in maficultramafic series. They usually form layers or veins of variable size and shapes and their composition ranges from orthopyroxenites through websterites to clinopyroxenites, usually with variable amounts of olivine. In spite of their common abundance (especially in large orogenic peridotite bodies), their origin is still matter of debate. Early work of Dick and Sinton (1979) and also Chen et al. (2001) suggested in-situ melting/dissolution of pyroxenes and their consequent precipitation in pyroxenite layers. In contrast, prevalent studies explained pyroxenites as rocks formed by crystal precipitation from silicate melts passing through the lithospheric mantle (e.g., Frey, 1980; Loubet and Allegre et al., 1982). Finally, Davies et al. (1993), Pearson et al. (1993) or Medaris et al. (1995) suggested that mantle pyroxenites represent high-pressure cumulates from subduction-related melts and therefore, they can provide insights to crustal recycling into the upper mantle. Highly siderophile element (HSE - Os, Ir, Ru, Rh, Pd, Pt, Re, Au) studies provide unique and complex view on mantle refertilization. This is because these elements behave in a different way during partial melting and they are concentrated in different phases (e.g., sulphides, alloys) to different degrees. All these features lead to HSE fractionation during upper mantle processes such as magma generation/differentiation or metasomatism. The Re- Os isotopic system provides another unique tool for understanding mantle refertilization due to the Re fractionation from Os during melt generation. This is because Re behave incompatibly during melting whereas Os is strongly compatible and retain in the source (Carlson, 2005 and references therein). As a consequence, melting products of upper mantle (basalts) are characterized by much higher Re/Os ratios and more radiogenic 187Os/188Os then upper mantle.
Pyroxenites plays very significant role in processes of mantle refertilization and enrichment. Therefore, their study provide direct insights into these processes, which are, in turn, very important for genesis of numerous mantle-derived melts. This project is focused on distribution and fractionation of highly siderophile elements and Re-Os geochemistry of upper mantle pyroxenites representing different mantle sources (oceanic vs. subcontinental) and on study of peridotite-pyroxenite interaction (mixing).