Other
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.1979s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 10, pp.1979-1989
Other
Scientific paper
Apatite and calcite from calcite carbonatites at Oka, Quebec, Canada, contain primary, liquid-vapour and liquid-vapour-halite fluid inclusions. Leachate analysis of these inclusions indicate that they contain significant concentrations of Na, K, and Cl and lower concentrations of Fe, Mg, and probably Ca. Energy-dispersive X-ray (SEM) analysis of decrepitates from these inclusions confirm that Na, K, and Cl are abundant. In addition, significant S was detected in the decrepitates. A correction was applied to the spectra from decrepitates on apatite in order to subtract the contribution from the substrate (apatite). The reliability of a decrepitate analysis depends on the concentration of an element in the decrepitate relative to that in the substrate and to the relative contributions to the spectrum of decrepitate and substrate. Each elemental analysis was screened for its reliability based on the concentration of the element in the mixed analysis and its concentration in the substrate. The only reliable results were for Na, K, Cl, and S. Both the leachate and decrepitate analyses indicate that Na > K. K/Na + K ratios fall within a narrow range, from 0.01-0.11. Potassium was a more important constituent in the LV than in LVS inclusions. Coupled with high salinities indicated from microthermometic data, these data confirm that aqueous fluids with high concentrations of alkalis may have evolved from low-alkali carbonatitic magmas. In contrast to the K/Na + K ratios, Cl/Cl + S ratios cover a wide range, from 0.17 to 0.93. The presence of sulphate minerals indicates that the sulphur existed as SO;- (and associated species) in the fluids. Chlorinities based on microthermometric data range from 2.5-11.4 m, which indicates SO;- molalities of 1-22.7. Charge imbalances indicate that other anions, probably HCO 3 - and CO;-, are important constituents of the fluids. Fenites around the Oka complex are characterized by the replacement of quartz-alkali feldspar gneisses by aegirine, nepheline, and K-feldspar. The dominance of Na and lack of Si in the fluid inclusions is consistent with the precipitation of predominantly nepheline and aegirine in the fenites. Phase equilibria in the K 2 O---Na 2 O---Al 2 O 3 ---SiO 2 system indicate that the development of K-feldspar in the fenites requires lower NaCl/KCl ratios and higher silica activities in the fluids that can only be explained by fluid-rock interaction at low fluid/rock ratios. The nature of the fluid-rock reactions that result in fenitization may play a more important role than has previously been recognized in controlling the development of potassic versus sodic fenites and on the spatial and temporal evolution of fenites.
Liu Weining
Samson Iain M.
Williams-Jones Anthony E.
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