Ni Isotopes in the Early Solar System: an Overview

The short-lived 60Fe-60Ni chronometer (t1/2=1.49 Myrs) can theoretically provide strong constraints on the exact chronology of the earliest events of the solar system, whereas the mass-dependent isotopic fractionation of Ni stable isotopes helps us to better understand the formation processes of meteorites. Nickel is also an interesting element when looking at nucleosynthetic anomalies and thus stellar sources in a close vicinity to the nascent Sun as 62Ni and 64Ni are over-produced in a neutron-rich environment, e.g. in giant stars like supernovae. With this threefold goal, we analyzed Ni isotopes in different objects: Ca-Al-rich inclusions, chondrules, iron meteorites, eucrites and angrites. The chemical extraction of Ni from the samples is mainly based on ion-exchange, coupled with a liquid-liquid extraction in some cases for silicate samples. All nickel isotope measurements have been performed using the large geometry high-resolution MC-ICPMS Nu 1700 at ETH Zurich, at a mass resolution of ~2600 to get rid of gas-related interferences. The CAIs display correlated excesses of 60Ni and 62Ni possibly coupled with effects on 96Zr. These nucleosynthetic anomalies are the signature of a neutron-burst in a massive star that produced 60Fe, 62Ni, 96Zr among other radionuclides shortly before the start of the solar system. Nucleosynthetic anomalies are detected in sulfides from iron meteorites as well (61Ni excesses correlated to small 60Ni-deficits). There is also evidence for the presence of life 60Fe at the time when some CAIs formed. Allende and most Tieschitz chondrules have the same 60Ni/58Ni as the standard: the Fe-Ni system must have re-equilibrated during metamorphism. However, two chondrules from Tieschitz show a slight excess of radiogenic 60Ni, indicating that they formed about 2.3 Myrs after CAIs. Iron meteorites have no resolvable anomalies in radiogenic 60Ni, consistent with their age and their Fe/Ni ratios. Eucrites experienced a complex history. It is nonetheless clear that 60Ni was present at the time the core formed in their parent body but also at the time of their crystallization. In contrast, there was no or very little live 60Fe when angrites formed, even if their absolute age is extremely old at 4562-4563 Myrs. In addition, different approaches to estimate the initial 60Fe/56Fe ratio of the solar system yield quite different values. These observations tend to show that the 60Fe-60Ni system cannot be used yet as a reliable ubiquitous chronometer in the early solar system. Finally, the mass-dependent fractionation of Ni provides strong constraints on the formation processes of iron meteorites. More specifically, the isotopic fractionation observed between kamacite and taenite is attributed to kinetic fractionation and results from the diffusion of Ni during exsolution of kamacite and cooling of the meteorite. This kamacite-taenite fractionation is a powerful tool to determine the cooling rate of iron meteorites.