Major and trace element composition of the high He-3/He-4 mantle: Implications for the composition of a nonchonditic Earth

The bulk composition of the silicate portion of the Earth (BSE) has long been assumed to be tied to chondrites, in which refractory, lithophile elements like Sm and Nd exist in chondritic relative abundances. However, the Nd-142/Nd-144 ratios of modern terrestrial samples are 185 ppm higher than the ordinary-chondrite reservoir, and this challenges the traditional BSE model. Here we investigate a hypothesis that this terrestrial Nd-142 excess is related to a Sm/Nd ratio 6% higher than chondritic. This Sm/Nd ratio yields a superchondritic Nd-143/Nd-144 (approximate to 0.5130) similar to that identified in the highest He-3/He-4 mantle reservoir, and we argue that this reservoir represents the BSE composition for lithophile elements. We develop a compositional model for BSE in which the elevated Sm/Nd requires a shift of Nd-143/Nd-144 from 0.51263 (chondritic) to 0.51300. The new BSE composition is depleted in highly incompatible elements, including K, relative to the chondrite-based BSE, and offers a solution the missing Ar-40 paradox. This BSE compositional model requires that >83% of the mantle is depleted to form continental crust. It also implies a approximate to 30% reduction in BSE U, Th and K, and therefore in the current rate of radiogenic heating and, thus, a proportional increase in the heat flow delivered to surface by plate tectonics. We explore thermal history models including effects related to a newly recognized evolution in the style of plate tectonics over Earth history: The lower radiogenic heat production may delay the onset of core convection and dynamo action to as late as 3.5 Gyr.