Isotopic evidence for chondritic Lu/Hf and Sm/Nd of the Moon.

Sprung P., Kleine T., Scherer E.E.

Zusammenfassung

Refractory lithophile elements are generally considered to occur in chondritic relative abundances in terrestrial planets. This assumption forms the basis for using isotope systems such as 176Lu–176Hf and 146, 147 Sm–142, 143 Nd as tracers of planetary evolution. However, on the basis of high-precision 142 Nd measurements, higher-than-chondritic Sm/Nd and Lu/Hf values have been recently proposed for the Earth, Moon, and Mars. This hypothesis can be tested using the combined 147Sm–143Nd and 176Lu–176Hf isotope systematics of the Moon. Here we show that the Hf isotope compositions of lunar samples are strongly affected by capture of secondary neutrons produced during cosmic ray exposure on the Moon, and present a model to correct these effects. After correction for neutron capture effects, the Lu–Hf model age for the formation of KREEP from a Moon having chondritic Lu/Hf is ca. 4.35–4.43 Ga, in good agreement with other independent constraints on the timing of lunar differentiation. The combined Sm– Nd and Lu–Hf isotope systematics of low- and high-Ti mare basalts and KREEP-rich samples provide powerful evidence for chondritic Sm/Nd and Lu/Hf in the Moon and do not support higher-than- chondritic values for these two ratios. Given the strong genetic link between the Earth and Moon, this finding implies that the bulk Earth is also characterized by chondritic Lu/Hf and Sm/Nd. A chondritic Sm/Nd in the Earth and Moon means that their 142Nd/144Nd may be inferred from the 142 Nd/144 Nd of the lunar mantle 146 Sm–142 Nd isochron at a chondritic Sm/Nd. The resulting 142Nd/144Nd value would be ∼12 ppm higher than that of ordinary chondrites. Because the Earth, Moon, and chondrites share a common Sm/Nd, this difference cannot result from 146Sm decay, but must be nucleosynthetic in origin. The deduced 142Nd/144Nd of the Moon and the bulk silicate Earth is only slightly lower than—and not unequivocally resolved from—that of the modern terrestrial mantle, indicating that any 142Nd difference between the accessible silicate Earth and the bulk silicate Earth is smaller than previously thought. © 2013 Elsevier B.V. All rights reserved.