A Radioactive Heat Source in Planetary Cores: Experimental Evidence for Potassium

Physics

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1015 Composition Of The Core, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1507 Core Processes (8115), 1749 Volcanology, Geochemistry, And Petrology, 8115 Core Processes (1507)

Scientific paper

The idea and the extent to which potassium is a radioactive heat source in the Earth's core has been highly controversial for the past thirty years because of ambiguous and contradictory experimental results, presumably due to unsuspected experimental difficulties. We present here results of studies free of such difficulties to show conclusively that K is soluble in Fe-S melts. In synthetic systems composed of K-silicate, Fe-metal, and FeS, potassium is readily enters the Fe-S melt at 2 GPa and magmatic temperatures, at fO2 1.5 log units below the iron-wustite (IW) buffer. The data show a precise loglinear relationship between the partition coefficient DK (concentration of K in sulfide/concentration of K in silicate) and inverse temperature, indicating that the solubility of K in the sulfide melt shows a strong positive correlation with temperature (T). If the Earth's core formed by segregation of metallic liquids in the Fe-FeS system, these observations suggests the presence of a significant amount of potassium in the core with consequent radiogenic heat production. The effects of pressure and composition on the partitioning of K in to Fe-S melt are not well constrained at this time. Our preliminary data show no effect of pressure in the limited range of our experiments but a significant effect of silicate melt composition. Until the effects of these parameters are defined better, only a heuristic estimate of the core radiogenic heat production is possible. For a range of 3000-4000 K core mantle equilibration temperature, the K content of the core is 60-130 ppm with a present-day heat production at 0.4-0.8x 1012 Watts and exponentially more in the past. A similar analysis suggests the radioactive heat production in Mars core to be 3x 1010 Watts. This additional heat source in the cores of Earth and Mars has major implications for a number of global processes and the early history of these planets. Among these are, the early but now extinct global magnetic field of Mars, the longevity of 3.5 b.y. old terrestrial magnetic field, the age of the Earth's inner core, and the internal dynamics of the solid planet.

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