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

Abstract

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 sövites and interstitial carbonate in the ijolites, with δ 13C ( −5‰ vs. PDB) and δ 18O ( + 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 δ 13C= − 3.6‰) and 18O (average δ 18O= + 9.9‰) relative to primitive ratios, but taken together with data for phyric calcite define a linear trend of increasing δ 13C with δ 18O 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.