Radiation damage in diopside and calcite crystals from uranothorianite inclusions

Combining observation and simulation, radiohalos formed around uranothorianite (UTh) from the Tranomaro granulitic skarns (SE-Madagascar) were studied. These structures consist of UTh grains surrounded by both aluminous diopside (Cpx) and calcite (Cc1) crystals. Optical microscope and Scanning Electron Microscope (SEM) images revealed (1) the presence of radiating cracks around the UTh probably due to swelling of the metamict UTh, (2) a diffuse optical halo at the Cc1/UTh interface, and (3) a wide “reaction zone” at the Cpx/UTh interface, composed of “secondary calcite” (Cc2) with low temperature sheet silicate from the smectite (φ) group. Samples prepared across various interfaces using Focused Ion Beam (FIB) were investigated by Transmission Electron Microscope (TEM). In contrast to SEM observations, there is no direct contact between Cc1 and UTh. From Cc1 to UTh, we found: (1) a large (~ 200–300 nm) amorphous zone (A), enriched in U, Th and Ca, but without Si; (2) a chain (B) of very small (~ 20 nm) ThO2 crystals; (3) another amorphous zone (C), which, in contrast to zone A is enriched in Si; and (4) another zone (D) made of small amorphous Si-rich “bubbles”. The organization is similar for the UTh–Cc2 interface. The presence of hydrous minerals (smectite) and carbonate (calcite) in reaction zone and in cracks, the presence of Pb-rich inclusions in secondary calcite, the abundance of fluid inclusions in the porous layer in calcite, the dissociation of U and Th in the calcite–uranothorianite layer, and the ThO2 chains along interfaces, are strong indications that low-temperature crystallization was promoted by a fluid phase. SRIM simulation was used to calculate the effect of α and recoil particles of the three decay chains, in Cpx, Cc and UTh. The thickness of the damaged area calculated for α in Cpx and Cc are similar to the widths of the recrystallized areas observed in thin section (~ 30 μm). Corrected with the “wandering recoil effect”, the size of the damaged area calculated for recoil nuclei in Cc (~ 50–60 nm) is ~ multiplied by 3 and is in rather good agreement with the thickness of the totally amorphous layer at the Cc–UTh interface (~ 200 nm). Finally, it is emphasized that radiohalos are a point of chemical and physical weakness in a rock and probably a starting point for alteration.