Chemistry of micas from kimberlites and xenoliths—I. Micaceous kimberlites |
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| Authors: | JV Smith Ruth Brennesholtz JB Dawson |
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| Institution: | Department of the Geophysical Sciences, University of Chicago, Chicago, IL60637 U.S.A.;Department of Geology, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9TS U.K. |
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| Abstract: | Micaceous kimberlites from South Africa and Canada contain two types of groundmass mica less than 1 mm across. Very rare Type I micas are relatively iron-rich with , TiO2 3–6 wt%, Al2O3 14–16wt%, no Fe3+ required in tetrahedral sites, low NiO (~0.02 wt%), and relatively high na Na2O/(Na2O + K2O)] 0.02–0.03. The much more abundant Type II micas are variable in composition, but relative to Type I micas are more magnesium (mg 0.80-0.93), lower in TiO2 (0.7–4.0 wt%) and Al2O3 (6.8–14.2 wt%), have substantial Fe3+ in tetrahedral sites, and have relatively low na. Both types may have rims with compositions indicative of mica-‘serpentine’ mixtures resulting from reaction with a highly aqueous fluid. The petrographically-determined ‘serpentine’ is chemically of two types: Fe-rich serpentine and Fe-rich talc. Associated phases in the ground-mass vary from one kimberlite to another: calcite, dolomite, diopside, chromite, Mg-ilmenite, perovskite, barite, pyrite, pentlandite, millerite?, heazlewoodite?, quartz.Inter-grain variations in composition of Type II micas may result from establishment of local reservoirs on a mm scale, consequent upon mechanical mixing and competition of other phases for minor elements (e.g. chromite for Cr, serpentine for Ni).Type I micas may result from an intrusive precursor (carbonatitic?) to kimberlite, perhaps genetically related, which was incorporated into a later pulse of kimberlite from which the Type II micas crystallized. |
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