LA-ICP-MS and TEM constraints on the magmatic and post-magmatic processes recorded by the zircon-xenotime intergrowth in pegmatite (Piława Górna, Góry Sowie Block, SW Poland)

Abstract A strongly altered zircon-xenotime intergrowth in pegmatite from the Pilawa Gorna (the Gory Sowie Block, SW Poland) was studied with transmission electron microscopy (TEM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Distinct domains demonstrate features related to fluid-induced coupled dissolution-reprecipitation processes resulting in microscale to nanoscale porosity in the zircon and in xenotime, and diffusion-reaction processes in the metamict core of the zircon. The inclusions of secondary phases include fluorapatite, unknown layered Fe-silicate, fibrous Fe-oxides, solid solutions of coffinite-thorite and uraninite-thorianite, uraninite and secondary U-rich zircon. The fluid had high concentrations of Ca and Fe, transporting these nonformula elements through the zircon and xenotime structure while mobilizing U, Th and Pb even in robust xenotime, thus forming small inclusions in nanopores. Nanoinclusions of galena in the zircon represent major Pb mobilization that may cause disturbance during U Pb geochronological analysis. The trace element composition of zircon matches patterns typical of altered zircon. The differentiation of geochemical characteristics between the metamict core and the altered rim of zircon implies different alteration mechanisms, with a stronger role of fluids in the alteration of the rim. Nanoscale observations complement trace element microanalysis and serve as an important and necessary tool to further understand the alterations of geochronometers in the presence of alkaline-rich fluid, especially in the complex environments of pegmatites.