Non-chondritic Sm/Nd ratio in the terrestrial planets: Consequences for the geochemical evolution of the mantle–crust system

Abstract

Super-chondritic 142Nd signatures are ubiquitous in terrestrial, Martian and lunar samples, and indicate that the terrestrial planets may have accreted from material with Sm/Nd ratio higher than chondritic. This contradicts the long-held view that chondrites represent a reference composition for the 147Sm– 143Nd system. Using coupled 146Sm– 142Nd and 147Sm– 143Nd systematics in planetary samples, we have proposed a new set of values for the 147Sm/ 144Nd and 143Nd/ 144Nd ratios of the bulk silicate Earth ( Caro et al., 2008). Here, we revise the Bulk Silicate Earth estimates for the 87Rb– 87Sr and 176Lu– 176Hf systems using coupled Sr–Nd–Hf systematics in terrestrial rocks. These estimates are consistent with Hf–Nd systematics in lunar samples. The implications of a slightly non-chondritic silicate Earth with respect to the geochemical evolution of the mantle–crust system are then examined. We show that the Archean mantle has evolved with a composition indistinguishable from that of the primitive mantle until about 2 Gyr. Positive ε 143Nd and ε 176Hf values ubiquitous in the Archean mantle are thus accounted for by the non-chondritic Sm/Nd and Lu/Hf composition of the primitive mantle rather than by massive early crustal formation, which solves the paradox that early Archean domains only have a limited extension in the present-day continents. The Sm–Nd and Lu–Hf evolution of the depleted mantle for the past 3.5 Gyr can be entirely explained by continuous extraction of the continents from a well-mixed mantle. Thus, in contrast to the chondritic Earth model, Sm–Nd mass balance relationships can be satisfied without the need to call upon hidden reservoirs or layered mantle convection. This new Sm–Nd mass balance yields a scenario of mantle evolution consistent with trace element and noble gas systematics. The high 3He/ 4He mantle component is associated with 143Nd/ 144Nd compositions indistinguishable from the bulk silicate Earth, suggesting that the less degassed mantle sources did not experience significant fractionation for moderately incompatible elements.