Neutron-rich isotopes with masses near that of iron are produced in type Ia and II supernovae. Traces of such nucleosynthesis are found in primitive meteorites in the form of variations in the isotopic abundance of 54 Cr, the most neutron-rich stable isotope of chromium.The hosts of these isotopic anomalies must be presolar grains that condensed in the outflows of supernovae, offering the opportunity to study the nucleosynthesis of iron-peak nuclei in ways that complement spectroscopic observations and can inform models of stellar evolution. However, despite almost two decades of extensive search, the carrier of 54 Cr anomalies is still unknown, presumably because it is fine-grained and is chemically labile. Here we identify in the primitive meteorite Orgueil the carrier of 54 Cr-anomalies as nanoparticles, most likely spinels that show large enrichments in 54 Cr relative to solar composition ( 54 Cr/ 52 Cr ratio >3.6×solar). Such large enrichments in 54 Cr can only be produced in supernovae. The mineralogy of the grains supports condensation in the O/Ne-O/C zones of a type II supernova, although a type Ia origin cannot beexcluded. We suggest that planetary materials incorporated different amounts of these nanoparticles, possibly due to late injection by a nearby supernova that also delivered 26 Al and 60 Fe to the solar system. This idea explains why the relative abundance of 54 Cr and other neutronrich isotopes vary between planets and meteorites. We anticipate that future isotopic studies of the grains identified here will shed new light on the birth of the solar system and the conditions in supernovae.

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In order to investigate the effect of pressure on periclase (MgO) dislocation slip-system activities, creep experiments have been carried out on MgO single crystals, at T and P, respectively, ranging from 1000 °C to 1200 °C and 4 to 9 GPa, in a deformation-DIA apparatus coupled with x-ray synchrotron radiation. Crystals were deformed in compression along either [100], [100], or [111] directions. These orientations were chosen to activate, respectively, either 1/2〈1-10〉{110} dislocation slip systems, 1/2〈1-10〉{100} systems, or simultaneously 1/2〈1-10〉{110} and 1/2〈1-10〉{100} systems. Experiments are carried out in a temperature range of 1000 °C to –1200 °C and a pressure range up to 8 GPa. Experimental results indicate that pressure influences differently the activities of these slip systems, which should yield a transition of dominant slip systems from 1/2〈1-10〉{110} at low pressure to 1/2〈1-10〉{100}. This pressure induced transition is expected to occur at 23 GPa, which would correspond to a pressure in the top portion of the lower mantle.

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