Ankerite carbonatite from Swartbooisdrif, Namibia: the first evidence for magmatic ferrocarbonatite

Thompson, R.N; Smith, P.M; Gibson, S.A; Mattey, D.P; Dickin, A.P.

Abstract

Although general accounts of carbonatites usually envisage Ca-Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe2+)(CO3)2 cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe3O4. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420-460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (87Sr/86Sr=0.7033; )Nd=0.2-1.0; 206Pb/204Pb=16.37; 207Pb/204Pb=15.42; 208Pb/204Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate '18O=8.0 and '13C=-7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.

Citation

Thompson, R., Smith, P., Gibson, S., Mattey, D., & Dickin, A. (2002). Ankerite carbonatite from Swartbooisdrif, Namibia: the first evidence for magmatic ferrocarbonatite. Contributions to Mineralogy and Petrology, 146(3), 377-395. https://doi.org/10.1007/s00410-002-0350-0

• Journal Article Type: Article
• Publication Date: 2002-06
• Deposit Date: Feb 16, 2007
• Journal: Contributions to Mineralogy and Petrology
• Print ISSN: 0010-7999
• Electronic ISSN: 1432-0967
• Publisher: Springer
• Peer Reviewed: Peer Reviewed
• Volume: 146
• Issue: 3
• Pages: 377-395
• DOI: https://doi.org/10.1007/s00410-002-0350-0
• Public URL: https://durham-repository.worktribe.com/output/1596795