Significance of High Field Strength and Rare Earth Element Distributions in Deciphering the Evolution of the Inner Solar System

Kent C. Condie, Charles K. Shearer

Geochimica et Cosmochimica Acta
Available online 28 May 2019


“Major processes affecting high field strength (HFSE) and rare earth (REE) element ratios in planetary basalts are degree of melting, separation of metal-sulfide melt fractions, addition and loss of silicate melt, ilmenite fractionation, and subduction. Fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data: the Moon, Vesta, and the angrite parent body. Thorium mobilization in aqueous fluids may account for decoupling of Th and Nb in planetary systems, and this is especially notable on Earth but also possible on Mars, the Moon and some asteroids. On Earth, HFSE and REE ratios in young basalts characterize hydrated (HM), enriched (EM) and depleted (DM) mantle sources, associated with, respectively, subduction, mantle plumes and ocean ridges. Terrestrial hydrated and depleted mantle were in existence by at least 4 Ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 Ga. Also, removal of Nb in metal-sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on HFSE-REE graphs, thus not requiring hydration. Such an origin is probable for “hydrated” mantle in primitive achondrites and plutonic angrites. The record of all three types of mantle in basalts from other bodies in the Solar System indicates the three mantle reservoirs are not diagnostic of plate tectonics, but can be produced in stagnant lid settings.

Enriched mantle is thus far recognized only in Earth and possibly Mars. There are at least two enriched mantle reservoirs in Earth: a primordial (> 4 Ga) reservoir, perhaps hidden in the D” layer above the core and rarely sampled by basalts, and a recycled plate reservoir (< 3 Ga), perhaps located in the two LLSVPs commonly sampled by oceanic island basalts. Between 3 and 2 Ga, the recycled enriched mantle reservoir became established in Earth, possibly in response to the widespread propagation of subduction. On Mars enriched mantle shows depleted radiogenic isotopic signatures and requires a multistage process to decouple trace element and isotopic systems.

Although there are several processes by which Nb can be fractionated from Ta in planetary bodies, the low Nb/Ta (<15) characteristic of some planetary and asteroid basalts may reflect separation of a metal-sulfide melt enriched in Nb, which may or may not produce a core. This fractionation must occur early during a relatively reduced stage of planetary evolution (IW-3 to IW-5) such that Nb behaves as a chalcophile or siderophile element. If the average Nb/Ta ratio of both primitive and depleted mantle is equal to 15, production of basaltic magma in the terrestrial mantle through time has not fractionated Nb from Ta. On the other hand, if the Nb/Ta in primitive mantle equals 17, Nb must be fractionated from Ta before 4 Ga, perhaps by partitioning into the core during or soon after planetary accretion when reducing conditions may have existed.”