Meteorites are privileged witnesses of solar system accretion processes and early planetary evolution. Short-lived radioactive chronometers are particularly adapted in dating and understanding these early differentiation processes. This proposal is dedicated to two main questions: (1) what is the initial composition of the solar system and terrestrial planets?; (2) having refined these parameters, how and when silicate bodies differentiated?
Among short-lived chronometers, the system 146Sm-142Nd is particularly adapted to solve these questions. While it is generally assumed that the global bulk composition of Earth and other terrestrial planets is chondritic for refractory elements such as Sm and Nd, it has recently been shown that the 142Nd/144Nd values display a systematic and reproducible bias between all the chondrites and the average composition of the Earth, and also possibly of other planets. Several hypotheses have been proposed: (i) there is an enriched reservoir hidden deep in Earth, with a composition balancing the currently observed terrestrial composition in order to get a global chondritic composition for the Earth. (ii) The Earth and other terrestrial planets are non-chondritic for their composition in refractory elements. (iii) Nucleosynthetic anomalies have modified the isotopic composition measured in chondrites. (iv) The starting parameters of the 146Sm-142Nd system are not well defined. However, this last point has never been carefully evaluated.
The main scientific strategy of this proposal is based on reinvestigating with the best precision ever achieved the starting parameters of the 146Sm-142Nd systematic using the oldest objects of the solar system: Ca-Al inclusions and chondrules. The final goal of the present proposal is to determine if Earth and other planets are chondritic or not, and to understand the implications of their refined starting composition on their geological evolution in terms of early planetary differentiation.
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