Globální Sr-Nd-Os izotopická systematika a prvky platinové skupiny v karbonatitech

Polák, Ladislav

Carbonatites are rare igneous rocks formed from carbonate-rich parental melts predominantly derived from the mantle. These melts hold the key to our understanding of deep mantle processes and carbon cycling as well as carbonatite metasomatism in the Earth's mantle, liquid immiscibility, and enrichments in rare earth elements and other critical metals. By examining new and published data from almost 600 samples of globally distributed carbonatites, three principal groups of carbonatites were defined based on their Sr-Nd isotopic compositions. The Group 1 represents carbonatites derived from variably enriched mantle sources. The Group 2 shows evidence of incorporated marine sediments or carbonates into their ultimate mantle sources. The Group 3 carbonatites were derived from a long-term isolated mantle source which has been metasomatized by slab-derived melts during the late Archean. A pioneering study of combined highly siderophile elements (HSE) and Re-Os isotopic compositions to unravel the behavior of these elements in carbonatitic magmas was accomplished for a suite of global carbonatites spanning emplacement ages from ~2060 to <40 Ma. This study reveals overall very low HSE contents in carbonatites attributed to PGE sequestration into sulfide and/or S-rich silicate melts during...

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