International Geologiical Congress - Oslo 2008


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EID-02 Properties and dynamics of mantle and core


Potassium in the lower mantle and core and implications for energetics of the Earth


Eiji Ohtani, Tohoku University (Japan)
Seiji Kamada, Tohoku University (Japan)
Naohisa Hirao, Japan Synchrotron Radiation Institute (Japan)
Takeshi Sakai, Tohoku University (Japan)


Potassium is one of the most important radiogenic elements as the source of the radiogenic energy. Existence of potassium in the core has been a matter of great debate. If some radiogenic elements exist in the core, the heat generated by the elements may affect the cooling history of the core, and thus the timing of generation of Earth's magnetic field. In spite of its importance, there are few experiments on potassium behavior at the core-mantle boundaries and the core. We have conducted experiments on chemical reactions between the Fe-S melt and K-silicate, and investigated the partitioning behavior of potassium between metallic liquid and silicate under the lower mantle and CMB conditions. We also clarified the phase transformation of K-feldspar under the lower mantle conditions in order to clarify the host minerals of potassium in the deep lower mantle.
Experiments were conducted by diamond anvil cell. Thin foils of metallic iron or mixture of Fe and FeS were sandwiched by K-feldspar KAlSI3O8 and placed in the hole of the rhenium gasket. Experiments were conducted in the pressure range from 50 to 135 GPa, and the temperature range from 1500 to 3500 K. The high pressure phase of K-feldspar was identified by using in situ X-ray diffraction experiments at Photon Factory. The reaction products recovered from DAC experiments were observed by SEM and TEM. The chemical compositions were determined by analytical TEM and EPMA.
We observed very low dissolution of potassium in molten iron and molten Fe-S. Existence of sulfur in molten iron enhance entry of potassium in the metallic melt, and the partition coefficient between S-bearing metallic melt and silicate shows a negative pressure effect and a positive temperature effect, i.e., the amount of potassium in molten metal decreases with pressure, whereas it increases with increasing temperature. Based on these partitioning experiments, we can estimate only a small amount of potassium can be dissolved into the core which can generate heat corresponding to 2-10% of the core heat flux from the core-mantle boundary. We also investigated a high pressure polymorph of K-feldspar. Hollandite II phase is stable up to the CMB conditions of 130 GPa and about 2000-3000 K. This phase can be an important host mineral of potassium at the base of the lower mantle.


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