Extreme Cr-rich nano-oxides in the CI chondrite Orgueil - Implication for a late supernova injection into the solar system

Systematic variations in ⁵⁴Cr/⁵²Cr ratios between meteorite classes ( Trinquier et al., 2007] and Qin et al., 2010a]) point to large scale spatial and/or temporal isotopic heterogeneity in the solar protoplanetary disk. Two explanations for these variations have been proposed, with important implications for the formation of the Solar System: heterogeneous seeding of the disk with dust from a supernova, or energetic-particle irradiation of dust in the disk. The key to differentiating between them is identification of the carrier(s) of the ⁵⁴Cr anomalies. Here we report the results of our recent NanoSIMS imaging search for the ⁵⁴Cr-rich carrier in the acid-resistant residue of the CI chondrite Orgueil. A total of 10 regions with extreme ⁵⁴Cr-excesses (δ⁵⁴Cr values up to 1500‰) were found. Comparison between SEM, Auger and NanoSIMS analyses showed that these ⁵⁴Cr-rich regions are associated with one or more sub-micron (typically less than 200 nm) Cr oxide grains, most likely spinels. Because the size of the NanoSIMS primary O⁻ ion beam is larger than the typical grain size on the sample mount, the measured anomalies are lower limits, and we estimate that the actual ⁵⁴Cr enrichments in three grains are at least 11 times Solar and in one of these may be as high as 50 times Solar. Such compositions strongly favor a Type II supernova origin. The variability in bulk ⁵⁴Cr/⁵²Cr between meteorite classes argues for a heterogeneous distribution of the ⁵⁴Cr carrier in the solar protoplanetary disk following a late supernova injection event. Such a scenario is also supported by the O-isotopic distribution and variable abundances in different planetary materials of other presolar oxide and silicate grains from supernovae.