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

Inner Solar System The neodymium–samarium system boasts two useful isotopic clocks. Decay of samarium–147 to neodymium–143 is a mainstay of studies of ancient volcanic rocks. And decay of samarium–146, with a relatively short half life of 103 million years, to neodymium–142, is a high–precision tool used for dating mantle differentiation in planetary bodies. The chronology assumes, however, that the composition of the total planet is identical to that of primitive undifferentiated meteorites called chondrites. The difference in neodymium isotope ratios between chondrites and terrestrial samples may therefore indicate the early isolation of the upper mantle, or a non–chondritic bulk Earth composition. Caro et al . present high–precision neodymium isotope data for 16 martian meteorites and show that Mars also has a non–chondritic composition. This suggests that the Earth, Moon and Mars all accreted in part of the inner Solar System with an Sm/Nd ratio about 5% higher than that of material accreted in the asteroid belt, the presumed source of chondrites. High precision neodymium isotope data for 16 martian meteorites is presented, and shows that Mars has a non-chondritic composition. The Earth, Moon and Mars therefore seem to have all accreted in a portion of the inner solar system with ∼5 percent higher Sm/Nd ratio compared with material accreted in the asteroid belt. Small isotopic differences in the atomic abundance of neodymium-142 ( 142 Nd) in silicate rocks represent the time-averaged effect of decay of formerly live samarium-146 ( 146 Sm) and provide constraints on the timescales and mechanisms by which planetary mantles first differentiated 1 , 2 , 3 , 4 . 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 142 Nd/ 144 Nd ratio between chondrites and terrestrial samples may therefore indicate very early isolation (