Neutron-Rich Chromium Isotope Anomalies in Supernova Nanoparticles

Abstract

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.

The Chromium Isotopic Composition of the Ungrouped Carbonaceous Chondrite Tagish Lake

“…Figure 1 illustrates the amplitude of the ε 54 Cr heterogeneity among sequentially dissolved fractions of bulk rock carbonaceous chondrites measured in this study and in Murchison and Allende (Trinquier et al 2007) for comparison. The pattern of isotopic data for Orgueil is similar to those obtained in previous studies (e.g., Dauphas et al 2010;Podosek et al 1997;Qin et al 2009;Rotaru et al 1992;Trinquier et al 2007). The first three leaching steps (L1-L3) exhibit deficits in 54 Cr relative to Earth.…”

“…Hence, the Orgueil CI1 chondrite presents the highest 54 Cr anomalies ever measured in major mineral fractions, which is consistent with this meteorite being the least metamorphosed one (Lodders 2003). Finally, these authors explained the 54 Cr anomalies as reflecting the inhomogeneous distribution of distinct nucleosynthetic components in the solar system (e.g., Dauphas et al 2010;Moynier et al 2010;Podosek et al 1997;Qin et al 2009;Rotaru et al 1992;Trinquier et al 2007). However, the source of 54 Cr in the protoplanetary disk remains unknown since the carrier phase of the 54 Cr anomaly has not yet been identified (e.g., Dauphas et al 2010;Moynier et al 2010;Qin et al 2009).…”

“…Variations of 53 Cr relative to Earth measured during the stepwise dissolution of carbonaceous chondrites are two orders of magnitude smaller than those measured for 54 Cr (e.g., Dauphas et al 2010;Moynier et al 2010;Podosek et al 1997;Qin et al 2009;Rotaru et al 1992;Trinquier et al 2007). They have been interpreted as resulting from 53 Mn decay (Rotaru et al 1992).…”

“…The stepwise dissolution of carbonaceous chondrite whole rocks with increasing acid strengths exhibit deficits and excesses in 54 Cr relative to Earth (e.g., Dauphas et al 2010;Moynier et al 2010;Podosek et al 1997;Qin et al 2009;Rotaru et al 1992;Trinquier et al 2007). Moreover, Rotaru et al (1992) demonstrated that the magnitude of the 54 Cr variations decreases in the sequence CI1 chondrites > CR2 chondrites > CM2 chondrites > CV3 chondrites > CO3 chondrites > CK4 chondrites and correlates with the degree of thermal metamorphism of each carbonaceous chondrite class.…”

“…Finally, these authors explained the 54 Cr anomalies as reflecting the inhomogeneous distribution of distinct nucleosynthetic components in the solar system (e.g., Dauphas et al 2010;Moynier et al 2010;Podosek et al 1997;Qin et al 2009;Rotaru et al 1992;Trinquier et al 2007). However, the source of 54 Cr in the protoplanetary disk remains unknown since the carrier phase of the 54 Cr anomaly has not yet been identified (e.g., Dauphas et al 2010;Moynier et al 2010;Qin et al 2009). This is presumably because it is fine grained and chemically labile.…”