Primary and secondary phlogopites and clinopyroxenes in garnet lherzolite xenoliths

Abstract

Microprobe analyses are presented for primary and secondary phlogopites in six garnet lherzolite xenoliths from South African kimberlite pipes, together with the wet chemical analysis of a bulk mineral separate of phlogopite from a similar xenolith. The primary phlogopites form rare large (2−3 × 1 mm) discrete plates which do not show any textural evidence of disequilibrium towards the other primary phases, whilst the secondary phlogopites form narrow rims around garnets. The primary phlogopites have lower TiO 2, Cr 2O 3 and Al 2O 3 contents, lower Fe 2+ Mg 2+ and Na + K + ratios, and higher SiO 2 contents than the secondary phlogopites. They also have much lower TiO 2 contents and Fe 2+ Mg 2+ ratios than the previously postulated TiO 2 rich primary phlogopites from a Lashaine garnet Iherzolite xenolith ( Dawson et al., 1970) and from Jan Mayen alkali basalts ( Flower, 1971). They are thought more likely to represent a primary phase in equilibrium with the four phase garnet Iherzolite assemblage in the upper mantle than the Lashaine and Jan Mayen phlogopites. The presence of on average 1% by volume of primary phlogopite in these xenoliths, thought to have originated from depths of 100–150 km, suggests that the upper mantle material at such depths may contain on average about 0.09 wt.% K 2O and 0.03 wt.% H 2O + in phlogopite. The clinopyroxenes in the xenoliths examined commonly have marginal zones which are significantly depleted in jadeite molecule compared with the primary cores. As no felspar or other Na 2O and Al 2O 3 rich phase could be detected associated with the marginal zones, it appears that the jadeite released has probably been taken up in secondary phlogopite and amphibole.