MANTLE PHLOGOPITE - A SIGNIFICANT LEAD REPOSITORY

Pb concentrations of > 1.5 ppm have been measured on leached phlogopit es from mantle xenoliths from both alkali basalt and kimberlite. Micas from four different xenolith lithologies have a range in Pb concentra tion from 160 to 1600 ppb. U concentrations of the micas are independe nt of those of Pb ranging from 10 to 400 ppb. The resulting U-238/Pb-2 04 ratios of the micas vary from 0.9 to 48. Pb isotopic compositions r eflect this variation. Altered rims of mica from a kimberlite-hosted p hlogopite clinopyroxenite show Pb isotopic evidence of high Th concent rations (> 4 ppm) possibly due to an as yet unidentified included phas e. Interaction of the micas with host magma has produced a decrease in their Pb, Na and Cr contents. Phlogopites with high Pb concentrations have high Na and Cr contents and high magnesium numbers suggesting th ey may be examples of primitive, high-pressure mantle micas. This type of phlogopite represents a potentially important reservoir of easily mobilized Pb of variable isotopic composition in the mantle.