Microprobe analysis of interstitial apatite from a 100-m section through the Merensky reef of the Bushveld Complex reveals that this stratigraphic section marks a transition from Cl-rich to more F-rich compositions in the Critical and Main zones, respectively. Apatite compositions across the section vary by more than 80 mole percent in the chlorapatite component. The trend of decreasing Cl concentration with stratigraphic height is not regular, and Cl contents in apatites from some samples may vary as much as 50 mole percent. La 2 O 3 and Ce 2 O 3 tend to be enriched in the more melanocratic, orthocumulate parts of the section. SrO concentrations of apatite show an increase with stratigraphic position, a trend which is opposite that observed in coexisting cumulus plagioclase.The data are consistent with mineralogic and textural evidence for an influx of Cl-rich vapor into the cumulus section which has affected the Cl/F variations through the lower portion of the Bastard unit. This influx occurred above the crystallization temperature of apatite (1,000 degrees -1,050 degrees C). It is suggested that much of the modal variation seen in the section may be the result of shifts in liquidus field boundaries caused by gain or loss of volatile components from the melt.

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