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

Abstract

[1] 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 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 '' 40 Ar paradox. This BSE compositional model requires that >83% of the mantle is depleted to form continental crust. It also implies a $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.

Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province

“…7). Using the most recently reported trace element compositions for the peridotite components, we modeled the partial melting of mixtures consisting of DMM (Workman and Hart, 2005) or non-chondritic primitive mantle (Jackson and Jellinek, 2013) and subduction-modified MORB (sm-MORB 2; cf. Fig.…”

“…Fig. 6), DMM (Workman and Hart, 2005), non-chondritic primitive mantle (NCPM; Jackson and Jellinek, 2013), peridotite component mixtures with sm-MORB 2 (cf. Fig.…”

Probing the Earth’s Deep Interior Through Geochemistry

“…Others have calculated somewhat lower, but still very significant depletions. Jackson and Jellinek (2013) calculated concentrations for the most incompatible elements, including Th and U, that are a little more than 30 % lower than the McDonough and Sun values (Fig. 9.2).…”

“…After accounting for the amount likely to be in the continental crust or the depleted mantle reservoir, they concluded that 25 % or more of the Earth's 40 Ar inventory was stored in an undegassed or primitive deep mantle. However, using K abundances estimated by Lyubetskaya and Korenaga (2007), (White, 2013), or Jackson and Jellinek (2013), all or nearly all of the Earth's 40 Ar can be accounted for by the atmospheric, crustal, and depleted mantle reservoirs.…”

“…heat flowing out of the mantle) of approximately 36.5±4.5 TW (Korenaga, 2008) and hence an apparent "mantle Urey ratio" (ratio of the convective heat flux to the terrestrial heat production corrected for the continental heat production -I say apparent because it includes heat flowing out of the core, but there is no heat production in the core) of about 0.40. The compositional models of Jackson and Jellinek (2013) and Luybetskya and Korenaga (2007) reduce heat production to 14 to 16 TW and the Urey ratio to 0.29 to 0.34. More dramatically, mantle heat production drops to 5 to 7 TW and the apparent mantle Urey ratio decreases to 0.20 to 0.28.…”