Raman study of the surface oxidation in (U, Pu)O2 as a function of Pu content

International audienceThis work presents a systematic Raman study of the matrix oxidation in a variety of (U1-y, Puy)O2 compositions (0 ≤ y ≤ 0.46) at different temperatures, between 250 DC and 400 DC. Our results indicate that the increase in Pu content hinders the oxidation process of the dioxide matrix. Further oxidation of the uranium-plutonium mixed dioxides in air starts between 250 DC and 310 DC, on a time scale of several hours. M4O9 seems to be the most stable intermediate phase formed upon oxidation of all the investigated mixed oxides, before final oxidation to M3O8. In addition, X-ray diffraction measurements and SEM images confirm the trend observed by Raman spectroscopy, i.e. Pu stabilises the fcc structure of the dioxide

Hydrous mantle transition zone indicated by ringwoodite included within diamond

The ultimate origin of water in the Earth’s hydrosphere is in the deep Earth—the mantle. Theory1 and experiments2, 3, 4 have shown that although the water storage capacity of olivine-dominated shallow mantle is limited, the Earth’s transition zone, at depths between 410 and 660 kilometres, could be a major repository for water, owing to the ability of the higher-pressure polymorphs of olivine—wadsleyite and ringwoodite—to host enough water to comprise up to around 2.5 per cent of their weight. A hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics5, 6, 7, yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results8, 9, 10, and the issue of whether the transition zone contains abundant water remains highly controversial11. Here we report X-ray diffraction, Raman and infrared spectroscopic data that provide, to our knowledge, the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine: we find ringwoodite included in a diamond from Juína, Brazil. The water-rich nature of this inclusion, indicated by infrared absorption, along with the preservation of the ringwoodite, is direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The finding also indicates that some kimberlites must have their primary sources in this deep mantle region