Micro- to nanoscale constraints on metasomatic alterations of xenotime, inclusions of Th-, U- and Pb-phases and their geochronological implications (Ås pegmatite, Evje and Hornnes, S Norway)

Five crystals of pegmatitic xenotime from angstrom s II feldspar quarry (Evje, S Norway) were studied with compre-hensive analytical methods from microscale to nanoscale with respect to fluid-mediated alterations and their geochronological implications. Xenotime crystals are strongly altered during dissolution-reprecipitation pro-cesses that resulted in the formation of (Th, U)-depleted xenotime subdomains with numerous microinclusions of (Th, U)-silicate, uraninite and minor galena. Remains of a primary xenotime (Xtm1) yielded LA-ICPMS U-Pb data characterized by a reverse discordance (from-10.5 to-2.6%) with a 208Pb/232Th mean age 988.4 +/- 5.9 Ma (95% conf., MSWD = 0.75, n = 71), which is within uncertainty with a 207Pb/235U mean age 979.1 +/- 5.0 Ma (95% conf., MSWD = 1.4, n = 22) yielded by data filtered to below +/- 5% discordance (i.e., from-5.0 to-2.6%). The altered xenotime domains (Xtm2) provided highly scattered dates, including 208Pb/232Th dates from 200 +/- 14 to 2135 +/- 120 Ma (n = 135). Discordant U-Pb data yielded upper intercept age 909 +/- 16 Ma (MSWD = 7.9). EPMA Th-U-total Pb measurements and compositional characteristics of uraninite inclusions indicate three age populations of ca. 852-983, 594-687 and 37-101 Ma.TEM investigations revealed initial alterations within domains Xtm1, representing a primary xenotime, which progressed along parallel submicron-to nanoscale fractures. These fractures represent partially open grain boundaries, which are empty or are filled with secondary inclusions of (Th, U)-silicates, uraninite or coffinite. The submicron-sized inclusions are accompanied by subdomains of (Th, U)-depleted xenotime. Altered xenotime (Xtm2) is well crystalline in TEM imaging and electron diffraction patterns in contrast to primary unaltered xenotime domains (Xtm1) that demonstrate, in Raman spectra, moderate degree of radiation damage caused by U and Th decay. Secondary inclusions of Th and U phases are nanocrystalline or amorphous, which increases their potential for Pb-loss or accumulation of Pb in excess. Some of them contain nanoinclusions of Pb3O4 or metallic Pb, whereas <100 nm-sized inclusions of (Pb, Sb)-oxide formed in the altered xenotime.To summarize, this study provides important insights for our understanding of coupled dissolution-reprecipitation processes that affect xenotime and mobilization of released U, Th and Pb. Removal of highly mobile U in a fluid, and the presence of nanoinclusions of Pb3O4, (Pb, Sb)-oxide and metallic Pb have particular importance for xenotime dating and explain disturbance towards older ages. Nevertheless, both removal of U, Th and Pb from altered and recrystallized xenotime as well as presence of submicron-to nanoinclusions can result in age disturbance resulting in spread of dates along a concordia curve.

Automated summary

This study examines the fluid-mediated alterations and geochronological implications of pegmatitic xenotime crystals from a quarry in Norway. The crystals undergo dissolution-reprecipitation processes that result in the formation of depleted xenotime subdomains with microinclusions of (Th, U)-silicate, uraninite, and minor galena. Primary xenotime had a mean age of 988.4 +/- 5.9 Ma, while altered xenotime crystals produced highly scattered dates ranging from 200 +/- 14 to 2135 +/- 120 Ma. TEM analysis revealed the progression of alterations in primary xenotime and the presence of submicron-sized inclusions. These findings provide important insights into the mobilization of U, Th, and Pb in xenotime.