The Gross Brukkaros (Namibia) reflects a broad dome structure showing a crater-shaped depression with numerous peripheral beforsitic carbonatite dykes. These dykes frequently contain an extreme load of basement (Nama-group) xenoliths (> 60 vol.%) including shales, quartzites, granites and gneisses. While xenoliths of exposed country rocks (mainly shales) show an angular habit, a pronounced rounding of xenoliths from other lithologies proves a wide transport and strong abrasion. This consumption of xenolithic material may result in remarkable contamination of the carbonatitic magma. MicroXRF mapping and optical microscopy provides first evidence that the corrosion and alteration of crustal xenoliths is controlled primarily by the xenoliths´ mineralogy and geochemistry. While some xenoliths exhibit distinct zoning reflecting a progressive leaching, others appear to be relatively inert. This proves the Gross Brukkaros of being an ideal natural laboratory to study the influence of crustal contamination in the carbonatitic system, particularly at the subvolcanic-volcanic depth transition. On the other hand, cross-cutting carbonatite dykes generated diatremes almost completely composed of quartz. A closer proximity to the diatreme yields an increase of the Si content in the dykes. In some cases, dykes occur with an extremely high proportion of microscopic xenolith fragments (>95 vol.%) and only subordinate proportions of carbonate. This indicates evaporation of the carbonatite melt during eruption, while the inherent Si remains as a residue along with the xenolith fragments and is precipitated in the diatreme breccia. Combined with C and O isotope systematics, carbonate crystallization is suspected to have proceeded under super-cooled conditions at ~150 °C.
Rebecca Ruwe1, R. Johannes Giebel2,3, Benjamin F. Walter1
1Karlsruher Institut für Technologie, Germany; 2Technische Universität Berlin, Germany; 3University of the Free State, Bloemfontein, South Africa