Ab Initio Study of U-bearing Aluminous CaSiO3 Perovskite
Abstract
The Earth's internal activity is evidenced through earthquakes, volcanic eruptions and continental drifts. These events are energetically related to the heat flux at the Earth surface (44 TW). It is now well accepted that about half of this heat flux comes from the radioactive decay of uranium, thorium and potassium, with 9 TW accounting for U alone. Uranium is expected to be mainly present in the mantle and it is now assumed that 50 wt% of total U in the Earth is stored in the lower mantle. The location of these heat sources in the mantle is essential for a better understanding of the geodynamics and thermal behaviour of the Earth. Because of its large cationic size, Ca is a very good candidate for its substitution by uranium. In the lower mantle, calcium is present in the CaSiO3 perovskite, which is believed to be the third important major phase (about 7 wt% or 5 mol%). Preliminary HP-HT experiments were performed between CaSiO3 perovskite and uranium where we showed that U can be incorporated in the CaSiO3 perovskite. The formed U-bearing aluminous CaSiO3 phase is showed to be stable up to 54 GPa. The substitution of Ca2+ by U^{4+} is found to require electronic charge compensation that can be assessed by the insertion of Al3+ in the Si-site of the CaSiO3 perovskite. The ab initio simulations in this work are based on density functional theory (DFT) within the generalised gradient approximation, and employ the projector-augmented wave method as implemented in the VASP code. We study the structures and energetics of the U-bearing Ca-perovskite at pressures of up to 130 GPa. Calculations were performed on 160- atoms for the U-bearing aluminous CaSiO3 perovskite. The Monkhorst-Pack scheme is used for sampling the Brillouin zone with a 2×2×2 mesh. Here we show that the existence of an aluminous U-rich CaSiO3 perovskite is not incompatible with Earth's mantle mineral data. Uranium is expected to be present in the lower mantle in a deep radiogenic reservoir of primitive materials whose shape is actually discussed as an abyssal deep layer or as 10- kilometers small domains present in the lowermost part of the mantle. The U-bearing CaSiO3 perovskite phase could therefore be the major host of uranium in the Earth as a part of this radiogenic reservoir. The agreement of these results with recent models of mantle structure is discussed.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMMR53C1000G
- Keywords:
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- 3620 Mineral and crystal chemistry (1042);
- 3630 Experimental mineralogy and petrology;
- 3924 High-pressure behavior;
- 3939 Physical thermodynamics;
- 8124 Earth's interior: composition and state (1212;
- 7207;
- 7208;
- 8105)