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Volume 5

Rapid decrease of MgAlO2.5 component in bridgmanite with pressure

Abstract:
The solubility of the MgAlO2.5 component in bridgmanite was measured at pressures of 27, 35 and 40 GPa and a temperature of 2000 oK using an ultra-high pressure multi-anvil press. Compositional analysis of recovered samples demonstrated that the MgAlO2.5 component decreases with increasing pressure, and approaches virtually zero at 40 GPa. Above this pressure, the MgAlO2.5 component, i.e. the oxygen-vacancy substitution, becomes negligible, and Al is incorporated in bridgmanite by the charge-coupled substitution only. These results are supported by the volume change associated with the change from the oxygen-vacancy substitution to charge-coupled substitution. The present result may explain the seismically observed slab stagnation in the mid-lower mantle. Although bridgmanite has been put forward as a potential host for water and argon in the lower mantle by trapping them in oxygen vacancies, such capabilities will rapidly decrease with depth and be lost in regions deeper than 1000 km.


Z. Liu, T. Ishii, T. Katsura
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Geochem. Persp. Let. (2017) 5, 12–18 | doi: 10.7185/geochemlet.1739 | Published 12 October 2017

The solubility of heat-producing elements in Earth’s core

Abstract:
The long term thermal and dynamic evolution of Earth’s core depends on its energy budget, and models have shown that radioactive decay due to K and U disintegration can contribute significantly to core dynamics and thermal evolution if substantial amounts of heat-producing elements are dissolved in the core during differentiation. Here we performed laser-heated diamond anvil cell experiments and measured K and U solubility in molten iron alloy at core formation conditions. Pyrolitic and basaltic silicate melts were equilibrated with metallic S–Si–O-bearing iron alloys at pressures of 49 to 81 GPa and temperatures of 3500 to 4100 K. We found that the metal-silicate partitioning of K is independent of silicate or metal composition and increases with pressure. Conversely, U partitioning is independent of pressure and silicate composition but it strongly increases with temperature and oxygen concentration in the metal. We subsequently modelled U and K concentration in the core during core formation, and found a maximum of 26 ppm K and 3.5 ppb U dissolved in the core, producing up to 7.5 TW of heat 4.5 Gyr ago. While higher than previous estimates, this is insufficient to power an early geodynamo, appreciably reduce initial core temperature, or significantly alter its thermal evolution and the (apparently young) age of the inner core.


I. Blanchard, J. Siebert, S. Borensztajn J. Badro
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Geochem. Persp. Let. (2017) 5, 1-5 | doi: 10.7185/geochemlet.1737 | Published 4 October 2017