COMMON PB IN MONAZITE

Monazite is a rare earth phosphate mineral that occurs in a wide array of metamorphic, igneous and sedimentary rocks. Monazite contains a range of elements in its structure, including Pb. A general assumption about the presence of Pb in monazite is that radiogenic Pb is stable in natural monazite but the mineral excludes common Pb during crystallization. Geochronologic studies report varying amounts of common Pb in monazite grains, with the highest amounts occurring in grains extracted from Llallagua tin ore deposits in Bolivia and the Amelia monazite from Virginia. Since most experimental studies have indicated that common Pb is difficult to practically impossible to incorporate into the mineral structure, it is speculated that instead Pb resides in locations other than the mineral itself, such as on the surface or within cracks. To evaluate how and if common Pb possibly substitutes itself into the monazite structure, we chemically analyzed the Llallagua and Amelia monazite grains using an electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The Llallagua monazite has no detectable Pb, suggesting its high common Pb reports are due to instrumental detection limits. The Amelia grain appears to contain common 204Pb within zones of alteration and cracks (>50000 ppm 204Pb). The grain itself varies in 204Pb content from 5 +/-2 ppm to 3792 +/- 172 ppm. Results from this study suggests that when common Pb is present in monazite, it is likely to occur along grain boundaries, cracks or alteration zones. These are regions typically avoided during electron microprobe analyses.