Origin of phlogopiteorthopyroxene inclusions in c

Authors: Chusi Li Edward M Ripley Arindam Sarkar Dongbok Shin Wolfgang D Maier

Publish Date: 2005/07/12

Volume: 150, Issue: 1, Pages: 119-130

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Abstract

About 30 of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains Naphlogopite and orthopyroxene are most abundant in the inclusions Edenitic hornblende Kphlogopite oligoclase and quartz are less abundant Clrich apatite rutile zircon and chalcopyrite are present at trace levels Naphlogopite is unique to the inclusions it has not been found elsewhere in the Bushveld Complex Other minerals in the inclusions are also present in the matrix of the chromitite seam but their compositions are different The Mg/Mg+Fe2+ ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix Kphlogopite in the inclusions contains more Na than in the matrix The average compositions of the inclusions are characterized by high MgO 26 wt Na2O 24 wt and H2O 26 wt and low CaO 11 wt and FeO 44 wt The δ18O value of the trapped melt estimated by analysis of inclusionrich and inclusionpoor chromites is ∼7‰ This value is consistent with the previous estimates for the Bushveld magma and with the δ18O values of silicate minerals throughout the reef The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatilerich melts during chromite crystallization The volatilerich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and NaKrich fluids expelled from the underlying crystal pile The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene leaving the system saturated only in chromite Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions The Na and Krich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatilerich melts

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