COMPRES 2009 abstract

Compatibility of K and Rb with Fe at high pressure and implications for the deep Earth

S. ARA Whitaker (1)
D. M. Reaman (1)
J. E. Kabbes (1)
J. S. Piggott (1)
G. L. Hovis (2)
A. J. Campbell (3)
E. Cottrell (4)
H. P. Scott (5)
W. R. Panero (1)

(1) The Ohio State University, School of Earth Sciences
(2) Lafayette College, Department of Geology and Environmental Geosciences
(3) University of Maryland, Department of Geology
(4) Smithsonian Institution, Department of Mineral Sciences
(5) Indiana University, South Bend, Department of Physics and Astronomy

Long-lived radionuclides such as potassium, uranium, and thorium are sources of heat generation in the interior of the Earth. ⁴⁰K decays to ⁴⁰Ca and ⁴⁰Ar with a half-life of about 1.25 Gy, responsible for ∼ 10% of the Earth's current heat production. While not a major component of heat production, ⁸⁷Rb decays to ⁸⁷Sr with a half-life of 48 Gy, and the ⁸⁷Sr/⁸⁶Sr ratio is taken as a tracer for geochemical reservoirs throughout Earth history. Previous experiments on potassium solubility in iron at high pressures show varying results as to the possible concentration of potassium in the core. While there have not been any previous studies on rubidium, it often substitutes for potassium in minerals due to its similar ionic charge and radius and therefore similar results are expected. This study evaluates the effect of sample preparation on the effect of solubility of Rb or K in iron. All samples after quench showed ∼ 2% expansion of the iron lattice when analyzed by X-ray diffraction, for a distribution coefficient of 0.033 at 2500 K. Samples loaded with powder in air varied more than those loaded in a nitrogen environment or with foil. Complimentary TEM and nano-SEM studies found the amount of Rb alloyed with iron to be below the detection limit (3000 ppm), placing an upper bound on the distribution coefficient of 0.012, broadly consistent with the results from XRD. Therefore, the upper bound for K and Rb in the core is inferred to be 8 ppm and 17 ppb, respectively. This implies an upper bound of 5 X 10¹⁰ W for the power in the core due to radioactive decay of K. The possibility of Rb in the core could impact the cycling and distribution of Rb with respect to Sr, affecting the interpretation of ⁸⁷Sr/⁸⁶Sr with respect to mantle geochemical processes.