Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon

Abstract

Small isotopic differences in the atomic abundance of neodymium-142 (142Nd) in silicate rocks represent the time-averaged effect of decay of formerly live samarium-146 (146Sm) and provide constraints on the timescales and mechanisms by which planetary mantles first differentiated. This chronology, however, assumes that the composition of the total planet is identical to that of primitive undifferentiated meteorites called chondrites. The difference in the 142Nd/144Nd ratio between chondrites and terrestrial samples may therefore indicate very early isolation (<30 Myr from the formation of the Solar System) of the upper mantle or a slightly non-chondritic bulk Earth composition. Here we present high-precision 142Nd data for 16 martian meteorites and show that Mars also has a non-chondritic composition. Meteorites belonging to the shergottite subgroup define a planetary isochron yielding an age of differentiation of 40 +/- 18 Myr for the martian mantle. This isochron does not pass through the chondritic reference value (100 x epsilon(142)Nd = -21 +/- 3; 147Sm/144Nd = 0.1966). The Earth, Moon and Mars all seem to have accreted in a portion of the inner Solar System with approximately 5 per cent higher Sm/Nd ratios than material accreted in the asteroid belt. Such chemical heterogeneities may have arisen from sorting of nebular solids or from impact erosion of crustal reservoirs in planetary precursors. The 143Nd composition of the primitive mantle so defined by 142Nd is strikingly similar to the putative endmember component 'FOZO' characterized by high 3He/4He ratios.

Early differentiation of the Earth and the Moon

“…Caro et al (2008) have argued that this is an unlikely possibility. To make this case more convincing, we explore more systematically scenarios whereby the common point for terrestrial planets shown in figure 7 can be explained by an early differentiation event as argued for the Earth by Boyet & Carlson (2005).…”

“…Thus, one may conclude that the observations of Caro et al (2008) imply that the Sm-Nd fractionation observed in Martian meteorites must have taken place either before or immediately after accretion. However, if one calls upon the segregation of a hidden reservoir, the Sm-Nd fractionation must have been strikingly identical to that observed in the Earth.…”

“…In this section, we examine whether one can assume that the bulk composition of planets in Sm-Nd is chondritic on the basis of the new Nd-isotope data for Martian meteorites (Caro et al 2008). The new data of Caro et al (2008) shown in an 3 142 Nd versus 147 Sm/ 144 Nd diagram in figure 7 clearly show that the Martian bulk composition is probably not chondritic.…”

“…Kleine et al (2002) and Nimmo & Kleine (2007). This putative event would also have to pre-date the large-scale mantle differentiation event dated at 40 Myr (Caro et al 2008), which seems unreasonable. If the bulk composition of Mars is below the terrestrial composition, then the fractionation in Sm-Nd must be older than the Solar System, which is impossible.…”

“…Possible scenarios for obtaining planetary isochrons in Mars starting from a chondritic composition are shown. We assume that the bulk Mars composition lies on the planetary isochron formed by shergottites (Caro et al 2008). If the bulk Sm/Nd of Mars is smaller than the intersection with the Earth (line A), then the age of Mars-chondrite fractionation is earlier than the age of the Solar System.…”