Significance 146 Sm- 142 Nd radioactive systematics can provide constraints on the timing of early differentiation processes on Earth, Moon, and Mars. The uncertainties related to the initial abundance and half-life of the extinct isotope 146 Sm impede the interpretation of the 146 Sm- 142 Nd systematics of planetary materials. The accurate determinations of Sm, Nd, and Mg isotopic compositions of the oldest “andesitic” achondrite Erg Chech 002 (EC 002) define a crystallization age of 1.8 Myr after the formation of the Solar System and provide the most accurate and reliable initial ratio of 146 Sm/ 144 Sm for the Solar System at 0.00840 ± 0.00032 using a 146 Sm half-life of 103 Ma, making EC 002 an anchor for 146 Sm- 142 Nd systematics for Earth and planetary materials.

The 146 Sm-142 Nd extinct decay scheme (146 Sm half-life of 103 My) is a powerful tool 17 to trace early Earth silicate differentiation. Differences in 142 Nd abundance measured 18 between different chondrite meteorite groups and the modern Earth challenges the 19 interpretation of the 142 Nd isotopic variations found in terrestrial samples because the 20 origin of the Earth and the nature of its building blocks is still an ongoing debate. As 21 bulk meteorites, the enstatite chondrites (EC) have isotope signatures that are the 22 closest to the Earth value with an average small deficit of ~10 ppm in 142 Nd relative 23 to modern terrestrial samples. Here we review all the Nd isotope data measured on EC 24 so far, and present the first measurements on an observed meteorite fall Almahata 25 142 Nd but the dispersion is important (µ 142 Nd=-10 ± 12 (2SD) ppm). This scatter 29 reflects their unique mineralogy associated to their formation in reduced conditions 30 (low fO 2 or high C/O). Rare-earth elements are mainly carried by the sulfide phase 31 oldhamite (CaS) that is more easily altered than silicates by weathering since most of 32 the EC meteorites are desert finds. The EL6 have fractionated rare-earth element 33 patterns with depletion in the most incompatible elements. Deviations in Nd mass 34 independent stable isotope ratios in enstatite chondrites relative to terrestrial standard 35 are not resolved with the level of analytical precision achieved by modern mass 36 spectrometry techniques. Here we show that enstatite chondrites from the EL3 and 37 EL6 subgroups may come from different parent bodies. Samples from the EL3 38 subgroup have Nd (µ 142 Nd=-0.8 ± 7.0, 2SD) and Ru isotope ratios undistinguishable 39 from that of the Bulk Silicate Earth. EL3 samples have never been analyzed for Mo 40 isotopes. Because these enstatite chondrites are relatively small in size and number, 41 they are usually not available for destructive isotopic measurements. Average values 42 based on the measurement of EL6 samples should not be considered as representative 43 of the whole EL group because of melting and thermal metamorphism events 44 affecting the Sm/Nd ratios and prolonged open-system history. The EL3 chondrites 45 are the best candidates as the Earth's building blocks. These new results remove the 46 need to change the composition of refractory incompatible elements early in Earth's 47 history. 48 49 1. Introduction 50 51 Variations in mass independent stable isotope ratios, also called nucleosynthetic 52 anomalies, of refractory elements have become extremely useful to trace the origin of 53 the different planetary bodies and their relative contribution into the building process 54 of terrestrial planets (e.g., Warren, 2011). Enstatite chondrites appear to be the closest 55 in isotope composition to terrestrial samples and carbonaceous chondrites the furthest. 56 Enstatite chondrites and the Earth are undistinguishable for many isotope systems, i.e. 57 Cr, Ti, O, Ni (see Dauphas, 2017 for a recent review), meaning...

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