  Systematic variations in 54Cr/52Cr ratios between meteorite classes (Qin et
al., 2010a; Trinquier et al., 2007) 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
({\delta}54Cr 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.
