Research project funded by National Science Centre 2017/27/B/ST10/00813
29.06.2018 – 28.12.2023
“Mineral transformations and REE, Th, U, Pb distribution during alteration processes of monazite, xenotime and allanite in nano- and microscale, and analytical improvements of electron probe microanalysis and Raman micro-spectroscopy for these phases”
Primary Investigator: Bartosz Budzyń
Monazite, xenotime and allanite represent accessory minerals, mainly of metamorphic and igneous rocks, which are the main hosts of rare earth elements (REE) in the Earth’s crust, and due to their high contents of Th and U they are used in U-(Th-)Pb geochronology. Petrochronology of these minerals in petrogenetic reconstructions allows, among others, to determine conditions of igneous or metamorphic processes and their age. Monazite is an important geochronological tool in provenance studies – from the scale of local outcrops to large-scale geotectonic reconstructions. Considering all these, it is important to understand stability and behavior of these minerals during igneous, post-igneous or metamorphic processes, which can result in compositional and structural alterations. The project aimed to solve numerous research problems, which included reconstructions of monazite, xenotime and allanite transformation processes using modern analytical techniques on a microscale (scanning electron microscopy SEM; electron probe microanalysis EPMA; laser ablation with inductively coupled plasma mass spectrometry LA-ICPMS –U-Th-Pb dating and trace element measurements TE; Raman microspectroscopy) and on a nanoscale (transmission electron microscopy TEM). The methodological part of the project was focused on the improvement of EPMA analytical protocols for REE measurements, and improvement of Raman microspectroscopy to monazite and xenotime studies. The research material included samples of metamorphic and igneous rocks from the Śnieżnik Massif and the Góry Sowie Block (Sudetes, SW Poland), the Čistá pluton (Czech Republic), the Ås region (Evje and Hornnes, S Norway), as well as the products of earlier laboratory experiments of the PI, which were focused on stabilities of monazite, xenotime and allanite in the presence of fluids, in a broad range of P-T conditions (200–1000 MPa, 250–750°C).
The most important achievements of the project include: (1) Characterization, on microscale and nanoscale, of monazite, xenotime, allanite and zircon transformation processes induced by fluids during igneous and metamorphic processes. The research results are important for petrogenetic reconstructions using these phases. (2) Characterization of alteration processes induced by alkaline fluids, radiation damage due to radioactive decay and their impact on the U-Pb and Th-Pb age record in experimentally metasomatized monazite under conditions of 250–750°C. The research results determine the conditions inducing transformations that affect the isotopic record in monazite, which may lead to misinterpretations in the reconstructions of geological processes. (3) Determination of the conditions (550–650°C) under which partial resetting of xenotime U-Pb ages occurs, as a result of fluid-induced alterations during igneous or metamorphic processes. (4) Discovery of Th and U segregation in dislocations (crystal lattice defects) of xenotime, ca. 15–20 nm wide and up to several hundred nanometers long. The results involve selective transport and removal of U and, consequently, the presence of inverse discordance in the 207Pb/235U vs. 206Pb/238U diagrams. In other words, processes occurring at the nanoscale affect the isotopic record, and the obtained U-Pb geochronology results may not reflect the true age of geological processes recorded by the xenotime. (5) Raman microspectroscopy of monazite and xenotime, including the characterization of transformations induced by fluids and the improvement of the Raman database for these phases. (6) Development of analytical protocols for the measurement of REE (including monazite, xenotime and allanite) using the two most commonly used electron microprobe models. (7) Characterization of the new reference material (RM) monazite TS-Mnz for U-Th-Pb geochronological analyzes using, among others, LA-ICPMS, SIMS and EPMA.
A wide range of research conducted using modern analytical methods has provided data that significantly expand our knowledge of phase transformations during alteration of monazite, xenotime and allanite, which are important for the reconstructions of igneous, post-magmatic and metamorphic processes, as well as in the interpretations of U-(Th-)Pb geochronology. The research results provide implications for widely used petrochronology in the research conducted both on the scale of local outcrops and large-scale geotectonic reconstructions. Moreover, the collected wide range of Raman spectra dataset for unaltered and altered, due to fluid-induced processes, monazite and xenotime is an excellent reference for future research using monazite and xenotime petrochronology. The developed protocols for EPMA measurements of REE are crucial for trace element analysis in other laboratories and, together with the new monazite reference material TS-Mnz, also in the context of analytical procedures of dating monazite and xenotime. To summarize, the project provided key and even fundamental data for a wide range of studies using the petrochronology of monazite, xenotime and allanite, both in the interpretation of the obtained results, especially in geochronology, and in the context of EPMA measurements and applications of Raman microspectroscopy, which represent the basic analytical methods.