© Lloyd, Charles Chandler

 

HEAT FROM RADIOACTIVE DECAY

"Volatiles, including noble gases and heat producing elements, were sweated out of the materials that are now in the deep interior and concentrated into the outer shells."2

"The heat-producing elements K, Th, and U... would have been highly concentrated in the exterior shell of the early Earth."7

"There is little U, Th and K in the deep mantle. The concentration of heat production in the shallow mantle has not been allowed for in any model investigated by isotope geochemists or convection modelers".2

"The only viable heating today appears to stem from radioactive decay." If the Earth was actually over 4 billion years old, "there must be significant radioactive heating of the deep Earth" and very slow transport of this heat to the surface. That would "necessitate chemical stratification of the mantle. This situation must also have been sustained over the entire period during which this heat was stored in the deeper mantle, and hence throughout Earth history."7

"The BSE could be depleted by as much as 50% in U by preferential collisional erosion. The problem with such a low BSE abundance of UThK is, of course, the concomitant low radiogenic heat production in the mantle. The heat produced by 3 ppb UThK... is 3 TW, which is only a small fraction (approximately 10%) of the heat flow from the ocean basins of 31 TW." "It has led to models of a layered mantle separated by a boundary impermeable to 4He but not to heat, although how this might be accomplished is not apparent. An alternative explanation is simply that the BSE is not chondritic due to collisional erosion."12

"Radioactive decay of U and Th produces both heat and 4He." "There should be a simple relationship between the radiogenic heat and the 4He coming out of the mantle."12

"The inference from the 4He flux is that the present Earth's convecting mantle is thoroughly depleted from top to bottom."12

"There is no plausible mechanism for creating a uniform distribution of U, Th and K in the mantle, or for having the mantle cool uniformly or for retaining undegassed reservoirs."2

"In the uppermost parts of the mantle,... the amount of primordial heat that has been retained to the present-day is negligible."7

Once the mantle was depleted of heat-producing elements, it's "intrinsic heat production has probably been less than 10% of the average global heat production for most of Earth history, including the earliest Archean."7

"Concentration of the heat-producing elements K, Th, and U near the surface of the Earth considerably increases the efficiency of heat loss."7

"About twice as much heat is being lost from the Earth as is being generated today. This 'extra' heat lost today represents ancient heat produced and stored within the Earth."7

"Planets with magmas at the surface cool extremely quickly, even in the presence of an atmosphere."5

For magma oceans with depths ranging from 400 to 1500 km, cooling times are 40 to 165 years.15