Evidence from achondrites for a temporal change in Nd nucleosynthetic anomalies within the first 1.5 million years of the inner solar system formation

Paul Frossard, Zhiguo Guo, Mary Spencer, Maud Boyet, Audrey Bouvier

Earth and Planetary Science Letters
Volume 566, 15 July 2021



• Nd isotope composition of planetesimals-derived achondrites is different than planetary bodies (Earth, Mars and the Moon).
• Nd isotope composition of achondrites is not related to the heliocentric distance of formation of their parent bodies.
• Thermal processing of dust is likely to have produce a change in the Nd isotope compositions in the protoplanetary disc.
• Chondrule formation may be related to this thermal processing event.
• The nature of the building blocks of terrestrial planets needs to be reassessed in the light of these results.”

“Heterogeneity in isotopic compositions within the protoplanetary disc has been demonstrated for a number of elements measured in extra-terrestrial materials, mostly based on chondrite meteorite analyses. However, precise 182Hf-182W and 26Al-26Mg ages of iron meteorites, achondrites, and chondrules show that chondrites accreted later than achondrites and therefore do not strictly represent the early (<2 Ma) solar system composition. Here we present the Nd mass-independent stable isotopic compositions of a suite of diverse achondrites to better constrain the Nd isotope evolution of the early solar system. Carbonaceous (C) achondrites are indistinguishable from their chondritic counterpart. However, early formed planetesimals as sampled by silicate-rich non-carbonaceous (NC) achondrite meteorites have higher 145Nd/144Nd and 148Nd/144Nd ratios (3.9 <μ145 Nd < 11.0 and 9.1 <μ148Nd < 17.9 in part per million deviation, or μiNd) compared to NC chondrites (2.7 <μ145 Nd < 3.3 and 2.2 <μ148 Nd < 8.1). Moreover, the three terrestrial planets for which we have samples available (Earth, Mars, and the Moon) as well as the silicate inclusions from the non-magmatic IIE iron meteorite Miles present a systematic deficit in μ145Nd and μ148Nd compared to early-formed NC achondrites. Unlike chondrites, the Nd anomalies in achondrites are not correlated to the heliocentric distance of accretion of their respective parent bodies as inferred from redox conditions. Chronological constraints on planetesimal accretion suggest that Nd (and other elements such as Mo and Zr) nucleosynthetic compositions of the inner part of the protoplanetary disc significantly changed around 1.5 Ma after Solar System formation due to thermal processing of dust in the protoplanetary disc. This relatively late event coincides with the beginning of chondrule formation or at least their preservation. Terrestrial planets formed subsequently by a complex accretion regime during several million years. Therefore, two scenarios are envisioned considering the reported Nd isotope composition of early planetesimals: 1) Terrestrial planets accreted mostly chondritic material similar in composition to enstatite chondrites, or 2) early planetesimals constitute substantial parts of terrestrial planets building blocks mixed with highly thermally processed material enriched in s-process, still unsampled by meteorites.”