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Initial Solar System Composition and Early Planetary Differentiation

Final Report Summary - ISOSYC (Initial Solar System Composition and Early Planetary Differentiation)

For refractory and lithophile elements such as Nd, the Earth was expected to display the same composition as chondrites. However, the discrepancy between the Earth and chondrites for 142Nd systematics has been well documented since 2005 (Boyet and Carlson, 2005). On the other hand, understanding the origin of this discrepancy has proven to be more complicated. The identification of small variations in nucleosynthetic processes (rapid or slow) within Nd isotopes has been recently advocated for this difference (Bouvier and Boyet, 2016; Burkhardt et al., 2016), and after correction of those nucleosynthetic excess or deficit, the Earth would have a chondritic composition in 142Nd. Amongst chondrites, enstatite chondrites (EC) are particularly interesting as they share many isotope characteristics with Earth, even though their composition in major elements cannot be reconciled with the bulk composition of the Earth.
During the project, we have measured the Nd isotope systematic of more than forty Antarctic enstatite and ordinary chondrites for high-precision multi-static Nd measurements to further investigate this issue. No systematic variation has been observed between groups or petrological types. As previously observed, ordinary chondrites (OC) are statistically different from Earth in µ145Nd and µ148Nd, while EC are indistinguishable from terrestrial values for those isotopes. Analytical error on µ150Nd precludes firm interpretation. On the other hand, both EC and OC share a similar µ142Nd value of -15 ± 5 and -15 ± 12 respectively. Reconciling a discrepancy in 145Nd and 148Nd between OC and Earth-EC on one side, with a discrepancy in 142Nd between EC-OC and Earth on the other side, cannot solely be related to s-process nucleosynthetic anomaly. This implies that the present-day accessible Earth is not chondritic anymore, hence requiring the presence of a hidden reservoir, either deep in the Earth but never sampled, or lost at some point during the evolution of the Earth.