~146Sm-~142Nd systematics measured in enstatite chondrites reveals a heterogeneous distribution of ~142Nd in the solar nebula

摘要

The short-lived ~146Srn-~142Nd chronometer (7_1/2 = 103 Ma) is used to constrain the early silicate evolution of planetary bodies. The composition of bulk terrestrial planets is then considered to be similar to that of primitive chondrites that represent the building blocks of rocky planets. However for many elements chondrites preserve small isotope differences. In this case it is not always clear to what extent these variations reflect the isotope heterogeneity of the protosolar nebula rather than being produced by the decay of parent isotopes. Here we present Sm-Nd isotopes data measured in a comprehensive suite of enstatite chondrites (EC). The EC preserve ~142Nd/~144Nd ratios that range from those of ordinary chondrites to values similar to terrestrial samples. The EC having terrestrial ~142Nd/~144Nd ratios are also characterized by small ~144Sm excesses, which is a pure p-process nuclide. The correlation between ~144Sm and ~142Nd for chondrites may indicate a heterogeneous distribution in the solar nebula of p-process matter synthesized in supernovae. However to explain the difference in ~142Nd/~144Nd ratios, 20% of the p-process contribution to ~142Nd is required, at odds with the value of 4% currently proposed in stellar models. This study highlights the necessity of obtaining high-precision ~144Sm measurements to interpret properly measured ~142 Nd signatures. Another explanation could be that the chondrites sample material formed in different pulses of the lifetime of asymptotic giant branch stars. Then the isotope signature measured in SiC presolar would not represent the unique s-process signature of the material present in the solar nebula during accretion.