Causes of 142Nd Variation in Earth

Abstract:

Variability of the ¹⁴²Nd/¹⁴⁴Nd ratio can reflect Sm/Nd fractionation during the lifetime of ¹⁴⁶Sm, i.e. the first 500 Ma of Solar System history¹ and nucleosynthetic heterogeneity inherited from the solar nebula. Deciphering the message carried by ¹⁴²Nd variability requires a detailed examination of the data for Earth and meteorites. The elevated ¹⁴²Nd/¹⁴⁴Nd in terrestrial samples relative to average chondrites suggests that all terrestrial rocks sampled by volcanism over the Earth’s history come from a geochemical reservoir characterized by a superchondritic Sm/Nd ratio. The chemical compliment to this reservoir, however, has never been seen, so it either was lost during Earth’s accretion^2,3, or is preserved in a deep hidden reservoir ^1,4. These models are based on a comparison of Earth rocks and O-chondrites because they do not show any variation in stable Sm and Nd isotopic composition compared to Earth^6-8. The first analyzed E-chondrites with terrestrial ¹⁴²Nd/¹⁴⁴Nd showed ¹⁴⁴Sm excesses that reflect an excess p-process contribution. Although ¹⁴²Nd is mainly produced by s-process, there is a direct p-process component estimated to be lower than 4 %.

We will present new Sm and Nd isotopic data on meteoritic materials. CAIs show deficits in both r- and p-process isotopes that would lead to elevated ¹⁴²Nd, yet the bulk C-chondrites in which they are contained show excesses in r-process isotopes and hence ¹⁴²Nd/¹⁴⁴Nd lower than terrestrial. The new E-chondrites data do not confirm the ¹⁴²Nd-¹⁴⁴Sm correlation observed in bulk chondrites In light of these results and using ¹⁴⁶Sm-¹⁴²Nd isochrons for constraining the bulk ¹⁴²Nd/¹⁴⁴Nd ratio of planetary bodies, we will discuss the ¹⁴²Nd signature of terrestrial samples (from Hadean to present).

¹Boyet & Carlson, Science 2005; ²O’Neill & Palme, Phil. Trans. R. Soc 2008; ³Caro et al. Nature 2008; ⁴Andreasen et al. EPSL 2008; ⁶Andreasen & Sharma, Science 2006; ⁷Carlson et al., Science 2007; ⁸Gannoun et al. PNAS 2011.