Ozha and 2 co-authors reported mid-2016 that optical microscopy, Raman spectroscopy and electron probe microanalyses were conducted on a migmatitic metapelite to investigate the effects of alpha radiation and subsequent alteration at the interface between uraninite inclusions and the host minerals biotite, chlorite and albitic plagioclase. The study reveals:
1) anomalous coloration under the polarizing microscope;
2) pertinent changes in the characteristic Raman spectra of host phases;
3) reaction aureoles, composed of secondary phases (viz. chlorite and K-feldspar) of various sizes (~15–45 μm) within biotite and plagioclase, respectively;
4) K-feldspatization and sericitization of plagioclase at the grain boundaries;
5) agreement between the observed α-penetration depths and the Monte-Carlo simulation results; and
6) dissolution textures within the matrix monazites.
Analysis of the compositions of the reaction aureoles in albitic plagioclase reveals a systematic distribution of K-feldspar-, LREE- and clay-rich zones; while the same in biotite is composed of secondary chlorite.
The growth sequence of the secondary phases indicates an influx of a K-rich fluid, following intense radiation damage, efficiently superimposed by LREE-metasomatism and later acidic alteration. These changes took place under low-temperature (~150°C) conditions, wherein radial cracks (within plagioclase) and cleavages/fractures (within biotite) favored fluid infiltration-circulation into the reaction aureoles.
Depletion of the LREEs from the dissolved matrix monazites and their enrichment as a discrete LREE phase within the damaged aureoles in plagioclase demonstrate micrometer-scale LREE mobility.
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