  We have performed a coordinated focused ion beam (FIB)-scanning and
transmission electron microscopy (S/TEM), electron probe microanalysis
(EMPA)-synchrotron X-ray fluorescence (SXRF) microprobe study to determine
phase-specific microstructural characteristics and high-resolution in situ
trace element concentrations of primary pyrrhotite, pentlandite, and associated
metal grains from chondrules in CM2 and CR2 carbonaceous chondrites. This work
is the first of its kind to link trace element chemical and microstructural
observations in chondritic sulfides in an attempt to determine formation
mechanisms and conditions of primary sulfides in these meteorite groups. SXRF
microprobe analyses allowed the concentrations of the minor and trace elements,
Co, Cu, Ge, Zn, and Se to be quantified, in addition to Fe and Ni, at a spatial
resolution of 2 microns. The similarity between the CM and CR PPI sulfide trace
element patterns provides evidence for a common formation mechanism for this
type of sulfide grain in both meteorite groups. In addition, the SRM sulfide
and metal have comparable trace element patterns that indicates a genetic
relationship between the two, such as sulfidization of metal. Enrichments in
Ni, Co, Cu, and Se are consistent with the chalcophile/siderophile behavior of
these elements. The observed depletions in Ge suggest that it may have been
lost by evaporation or else was never incorporated into the metal or sulfide
precursor materials. The depletion in Zn may also be attributable to
evaporation, but, being partially lithophile, may also have been preferentially
incorporated into silicates during chondrule formation. Trace element
concentrations support crystallization from an immiscible sulfide melt in
chondrules for formation of the PPI grains and sulfidization of metal for the
origin of the SRM grains.
