  We report trace element analyses by laser ablation inductively coupled plasma
mass spectrometry of metal grains from 9 different CR chondrites,
distinguishing grains from chondrule interior ("interior grains"), chondrule
surficial shells ("margin grains") and the matrix ("isolated grains"). Save for
a few anomalous grains, Ni-normalized trace element patterns are similar for
all three petrographical settings, with largely unfractionated refractory
siderophile elements and depleted volatile Au, Cu, Ag, S. All types of grains
are interpreted to derive from a common precursor approximated by the least
melted, fine-grained objects in CR chondrites. This also excludes
recondensation of metal vapor as the origin of the bulk of margin grains. The
metal precursors presumably formed by incomplete condensation, with evidence
for high-temperature isolation of refractory platinum-group-element (PGE)-rich
condensates before mixing with lower temperature PGE-depleted condensates. The
rounded shape of the Ni-rich, interior grains indicates melting and
equilibration with silicates upon slow cooling (1-100 K/h), largely by
oxidation/evaporation of Fe. We propose that Ni-poorer, amoeboid margin grains,
often included in the pyroxene-rich periphery common to type I chondrules,
result from less intense processing of a rim accreted onto the chondrule
subsequent to the melting event recorded by the interior. This means either
that there were two separate heating events, which formed olivine/interior
grains and pyroxene/margin grains, respectively, between which dust was
accreted around the chondrule, or there was a single high-temperature event, of
which the chondrule margin record a late "quenching phase", in which case dust
accreted onto chondrules while they were molten. In the latter case, high dust
concentrations in the chondrule-forming region are indicated.
