Distribution of major and minor constituents between mica and host ultrabasic rocks,and between zoned mica and zoned spinel |
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| Authors: | J Rimsaite |
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| Institution: | (1) Geological Survey of Canada, Ottawa, Canada |
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| Abstract: | Ultrabasic rocks with Niggli values for si between 26 and 100, and k between 0.6 and 1 have been chosen for the study of the distribution of 23 major and minor constituents between phlogopite and the host rock. The rocks include kimberlites, carbonatites and lamprophyres which contain abundant zoned micas and a few zoned spinels. Chemical variations in zoned mica and spinel were determined by electron probe microanalysis, and distribution coefficients for Al, Ti, Fe, Cr, Mg, Mn, and Ni were calculated between mica and spinel for the average composition of the mica and spinel, and for chemically-related and adjacent zones. The spinel changes from chromite in the centre to chromian ulvöspinel in the rim, and the mica from chromian phlogopite, through titaniferous biotite to chromium-poor phlogopite. Distribution coefficients for Cr and Fe between adjacent zones in spinel (0.8 and 0.02 for Cr; 1.1 and 3.1 for Fe) and in mica (0.4 and 0.1 for Cr; 2 and 0.3 for Fe) show more variation than the distribution coefficients between the cogenetic phlogopite and chromite (0.04 for Cr, 0.2 for Fe), and biotite and ulvöspinel (0.08 for Cr; 0.2 for Fe). It is concluded that distribution coefficients for major and minor constituents between related zones are more meaningful than those calculated from the average chemical composition of zoned minerals.The results indicate that phlogopite is the principal carrier of potassium, rubidium, aluminium, fluorine and primary water in kimberlite and carbonatite. Minor elements, such as chlorine, lithium, barium, nickel, chromium, titanium and zinc are present mainly in the mica, whereas sodium, strontium, calcium, carbon and manganese are more abundant in other minerals. |
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