Corin Jorgenson

B.Sc. (Honours) Thesis

Sulphur solubility of carbonatites as a mass transfer agent in the mantle

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Carbonatites are rare, mantle-derived igneous rocks with >50 wt% carbon, compared to more typical SiO2-rich compositions. Whereas the solubility of sulfur for mafic silicate melts has been extensively studied, equivalent data for carbonate-rich compositions has not been obtained. This research will determine the sulfur solubility in molten carbonate to assess the potential for such melts as a mass transfer agent for sulfur, along with precious metals, in the mantle. The goal is to determine the importance of carbonatite metasomatism to establish precious-metal-rich source regions for magmatic ore deposits. The concentration of sulfur at sulfide saturation in molten carbonate was measured as a function of several variables, including melt composition and pressure to assess the sulfur solubility mechanism. Experiments are done using piston cylinder apparatus at the Dalhousie Laboratory for High Pressure Geological Research. Run-products are analyzed using the electron microprobe analyzer using wavelength dispersive spectroscopy and analyzed by laser ablation mass spectroscopy. A synthetic carbonate melt modeled after phase equilibrium experiments with a mantle peridotite assemblage was used. This material is mixed with a similar mass of FeS, doped with 1 wt% each of Ni and Cu, and additional Fe2O3, then loaded, along with ~ 5 wt% H2O, into a graphite-lined Pt capsule. Capsules are placed into a pressure cell comprised of crushable MgO, with an outer graphite furnace, pyrex sleeve, and NaCl sleeve. Run-products consist of a monosulfide solution (MSS) or sulfide liquid coexisting with quenched carbonate melt, represented by a fine-grained intergrowth of carbonate phase and sulfide dendrites. Experiments have been completed up to Fe-oxide saturation by adding up to 30 wt % Fe2O3 and the iron content in the carbonate remains quite low, ~4% FeO. The solubility of sulfur in carbonatite melts range from 845-5448 ppm for experiments at 1 GPa, and 107-1502 ppm for the experiments done at 2 GPa. Results suggest slightly higher solubility for carbonate versus silicate melts. The S content shows an exponential dependence on the FeO content of the melt, similar to that of silicate melts and is invariant with temperature and time. The sulfide-carbonatite partitioning of precious metals of Cu and platinum group elements was determined and shows to be similar to the silicate melts, aside from Pt which shows partitioning 2 orders of magnitude higher than the MSS-silicate values. Ru and Ir vary from 2-1 orders of magnitude lower than the MSS-silicate values, and Os has values partitioning one order of magnitude lower than the sulfide-silicate partitioning from the literature. Cu partitioning does not show significant variation with pressure, time, SCSS or iron content.

Keywords: sulfur – solubility – carbonatites – experimental petrology- platinum group elements
Pages: 65
Supervisor: James Brenan