Low-temperature alteration of monazite: Fluid mediated coupled dissolution-precipitation, irradiation damage, and disturbance of the U-Pb and Th-Pb chronometers
Résumé
Low-temperature alteration of monazite is documented in three centimeter-sized monazite crystals from Norway (Arendal), Madagascar (Ambato), and Sri Lanka. The three crystals have different chemical compositions, especially in their U, Th, Y and Pb contents and have 208Pb/232Th ages ranging from 491 to 900 Ma. Optical microscope (OM), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) images and electron microprobe analyses (EPMA) show that all three preserve a similar patchy texture. This texture is interpreted as resulting from an alteration reaction in which unaltered monazite (Mnz1) reacts to form a secondary, Th-U(Y)-depleted, high-Th/U, monazite (Mnz2) accompanied by thorite/huttonite (ThSiO4), thorianite (ThO2) and xenotime (YPO4), the proportions of which are dependent upon the initial composition of the monazite (Mnz1). Images reveal variably intense internal fracturing, with cracks filled with Th-rich ± Fe-rich phases. Monazite-xenotime thermometry demonstrates that the pristine monazites (Mnz1) interacted with a low-temperature fluid. The alteration process is interpreted to follow a mechanism of fluid-mediated coupled dissolution-precipitation. Chemical dating with the electron microprobe shows no Th-U-Pb age differences between primary and secondary monazites, except in the case of the Ambato monazite, in which altered domains yield older (750 Ma) apparent ages than the pristine Mnz1 domains. U-Pb and Th-Pb isotope dating using LA-ICP-MS yields ages consistent with electron probe dates for pristine Mnz1 zones. However, disturbance of these systems in the altered monazite domains leads to variable age results for these, depending on sample. In the case of Sri Lanka and Arendal, only 208Pb/232Th dates provide a reasonable estimate of the age of alteration, which are constrained to be 450 and 864 Ma, respectively. U/Pb systems are disturbed due to common Pb contamination (up to 40%) and U fractionation relative to Th during alteration, responsible for depletion of U in altered monazites (and increase of Th/U). In contrast, for the Ambato monazite, both the U-Pb and Th-Pb systems were affected and yield inconsistent older dates for altered zones. This is attributed to significant common Pb contamination (up to 80%), which affects all Pb isotopes and explains why electron probe ages are erroneous. Th-U-silicate contamination during measurement, resulting from the presence of a numerous nano-phases and nano-fractures filled with Th-U-silicates that are visible only under TEM, also contributes to the anomalously old ages for these disturbed (Mnz2) domains. These results demonstrate the important role of radiation damage effects, in particular swelling-induced fracturing, and the essential role of porosity and cracks, which allow fluid (charged with elements) migration through monazite during low-temperature alteration.
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