Chemistry of micas from kimberlites and xenoliths—II. Primary- and secondary-textured micas from peridotite xenoliths

Abstract

Micas from coarse granular Iherzolites in S. African kimberlites may be separated into two groups; those showing primary textural relationships with coexisting silicates and those with secondary, alteration relationships with other silicates. Primary-textured micas form a tight cluster with a mean composition from 10 coarse garnet lherzolites of: SiO 2 41.0, TiO 2 0.18, Al 2O 3 13.5, Cr 2O 3 0.82, total Fe as FeO 2.60, MnO 0.02, MgO 26.0, NiO 0.22, CaO 0.01, BaO 0.29, Na 2O 0.31, K 2O 10.0, Rb 2O 0.028, Cl 0.08, F 0.43 wt%. Primary-textured micas in aggregates with clinopyroxene have higher TiO 2 and four specimens which look similar to the primary group but have textural ambiguities have still higher TiO 2 Micas with secondary textures have wide ranges of composition which may be correlated with details of the textural parageneses. Micas from kelyphitic rims around garnets tend to be Cr-rich while those from veinlets are Cr-poor. Both groups tend to have higher FeO and TiO 2 than the primary group. Micas produced by alteration of, or filling veinlets through, orthopyroxene have a wide compositional range which overlaps that of the primary-textured micas, especially for harzburgite specimens. The primary-textured micas show a positive correlation with coexisting pyroxenes for MgO/(MgO + FeO) and TiO 2, but not for Cr 2O 3. Secondary-textured micas do not show correlations with coexisting pyroxenes for any elements. The ‘primary-metasomatic’ micas described by Harte and Gurney (1975) and metasomatic and other micas described by Boettcher et al. (1979) and Boettcher and O'neil (1979) are richer in FeO and TiO 2 than the present primary-textured micas, and are attributed to crystallization from fractionated fluids.