Oxygen and carbon isotope patterns in the Dicker Willem carbonatite complex, southern Namibia

Carbon and oxygen isotope data are presented for carbonatites and associated alkali silicate rocks from the Tertiary (49 Ma) Dicker Willem complex in southern Namibia. Carbonatites are grouped into: (1) coarse-grained sövites and associated silicate-oxide-phosphate-rich cumulates; (2) finer-grained alvikite intrusions, showing porphyritic and spinifex textures, comb layering and gravity-settled layers; (3) late-stage dykes, pipe breccias, veins and druses. The early sövites carry many inclusions of silicate rocks (ijolites, syenites). The most primitive carbon and oxygen isotope compositions are found in phenocrysts from calcite-phyric microsövite, bulk s6vites and interstitial carbonate in the ijolites, with δ¹³C (−5‰ vs. PDB) and δ¹⁸O ( +7 to +9‰ vs. SMOW ). Oxygen isotope fractionation between cumulus pyroxene, magnetite and biotite in the sövites yields near magmatic temperatures of 600–900°C. Carbonates in some cumulates yield magmatic temperatures, but commonly show evidence of secondary alteration. Phenocrysts in dolomite-phyric alvikite are slightly enriched in ¹³C (average $\text{δ}{}^{13}\text{C}\text{=−3.6‰}$) and ¹⁸0 (average $\text{δ}{}^{18}\text{O}\text{=+9.9‰}$) relative to primitive ratios, but taken together with data for phyric calcite define a linear trend of increasing δ¹³C with δ¹⁸O and can be modelled as being the product of combined carbonate-silicate-oxide-phosphate fractionation of a parent sövite. Groundmass carbonate in the porphyritic alvikites, as well as the bulk alvikites, all show variable degrees of ¹⁸O enrichment relative to the phenocrysts, and reflect partial recrystallization of carbonate in the presence of low-temperature hydrous fluids.