Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.07.016]
1Department of Earth and Planetary Sciences, MSC-03 2040 1 University of New Mexico, Albuquerque, NM 87131, USA
2Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA
3Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA
We have performed a coordinated focused ion beam (FIB)-scanning and transmission electron microscopy (SEM, 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 and TEM analyses were performed on a small number of FIB sections which act as representative samplesof primary sulfides and associated metal present in pyrrhotite-pentlandite intergrowth (PPI) grains and sulfide-rimmed metal (SRM) grains.
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 µm. 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.