Understanding geologic processes with xenotime: Composition, chronology, and a protocol for electron probe microanalysis

Xenotime (YPO₄) is iso-structural with zircon-group minerals and lies at one end of a compositional solid-solution with monazite. As with monazite and zircon, naturally occurring xenotime may accommodate significant concentrations of rare earth elements, as well as Th and U, making it an important repository for these valuable geochemical marker elements and a potential geochronometer. Xenotime grows in sediments during diagenesis, in low- to granulite-grade metamorphic rocks, in migmatites, and is common in peraluminous igneous rocks. Xenotime can be a complement, or indeed an alternative, to monazite and/or zircon in geochemical and geochronological studies. To maximise the petrographic potential of xenotime, a complete understanding of its composition and relationship to rock-forming assemblages is essential. Electron beam techniques provide micron-scale resolution of compositional and textural variation and yield precise dates for individual micro-volumes in multi-domain grains. Methods for collecting trace element data by electron probe microanalysis differ from those routinely employed to collect major element concentrations. Careful characterisation of background curvature and background interferences around the Th, U and Pb peaks is required. To avoid large overlap correction for Y-Lγ_2,3 on Pb-Mα, analysis of Pb is made on the Mβ-peak. A broad array of geochemical and geochronological applications can take advantage of the links between xenotime composition and texture and the evolution of silicate-mineral assemblages in a diversity of environments. One example of metasomatised xenotime from a pegmatite in SW Norway demonstrates a practical application to dating geologic processes, as well as the potential limitations that arise in compositionally heterogeneous xenotime.