Fluid inclusion geochemistry and rare earth element distributions in the Oka carbonatite, Quebec.

摘要

Two types of primary, aqueous fluid inclusions are present in monticellite, apatite and calcite from the Oka carbonatite: (1) liquid-vapour (LV) inclusions, and (2) liquid-vapour-halite (LVH) inclusions. LV inclusions have salinities of 4.3 to 24.7 wt.% and densities of 0.75 to 1.04 g/cm$sp3,$ and homogenize to the liquid phase between 95 and 435$spcirc$C. The observed crystallization sequence of the host minerals was monticellite-apatite-calcite. Systematic variation in homogenization temperatures, salinities and densities of LV inclusions, combined with the crystallization sequence, suggests that type 1 fluids evolved from high to low salinity and probably from high to low temperature. The minimum pressure at which monticellite, apatite and calcite crystallized is estimated to have been about 7 to 10 kb. The nature of the trapped minerals in the inclusions and data from leachate and decrepitate analyses indicate that the fluids in the inclusions are principally composed of Na, Cl and S (sulphate?), and contain Ca, Mg, K, Fe, Si (silicate?) in subordinate amounts. LV fluid have high Ca/Na, K/Na, and Mg/Na ratios and lower Fe/Na ratios than LVH fluid. REE concentrations and La/Lu ratios of apatite increase with decreasing inclusion abundance in apatite and LV inclusion-rich apatites have higher REE concentrations but lower La/Lu ratios than LVH inclusion-rich apatites. This suggests that hydrothermal fluids may have removed LREE from the magma, and that the chemistry of fluids may have had an influence on REE distribution in the magma. The rare earth elements substitute for Ca in the apatite structure through the coupled substitution: $m REEsp{3+}+Sisp{4+}o Casp{2+}+Psp{5+}.$ Saline, aqueous, hydrothermal fluids were present during crystallization of monticellite, apatite and calcite from some of the carbonatite magmas. (Abstract shortened by UMI.)Dept. of Geology and Geological Engineering. Paper copy at Leddy Library: Theses u26 Major Papers - Basement, West Bldg. / Call Number: Thesis1993 .L588. Source: Masters Abstracts International, Volume: 32-02, page: 0564. Adviser: I. M. Samson. Thesis (M.Sc.)--University of Windsor (Canada), 1993.