Scanning electron microscopy characterization of iron, nickel and sulfur in chondrules from the Allende meteorite – further evidence for between-chondrules major compositional differences

Abstract

We analyze the relative contents and internal spatial elemental distribution of Fe, Ni and S in minerals within selected individual chondrules of a fragment of the Allende chondrite, as derived from scanning electron microscopy (SEM) - energy dispersive X-ray spectroscopy (EDS) analyses. Chondrules were selected to cover the range of micromagnetic properties in terms of hysteresis coercivity and magnetization ratios. Variation in micromagnetic properties appear correlated to the external and internal morphologies and mineralogical assemblages. In order to identify the distribution of mineral assemblages characterized by Fe, Ni and S, a mathematical technique for image analysis was applied to the SEM-EDS images. The resulting compositional images display the complexity in composition and microstructural morphologies of individual chondrules. Chondrule c27b is characterized by Fe-rich minerals and is Ni and S poor, although Ni and S appear concentrated within a Fe-rich rim. Chondrule c40b, formed by barred olivine, is Fe poor and has low contents of Ni and S; Fe is homogeneously distributed within the olivine crystals. Chondrule c53b has a spherical shape, except for a partly preserved Fe-rich rim that forms a bulge; minerals with higher contents of Fe, Ni and S are observed in the chondrule interior. Chondrule c55b, of porphyritic olivine type, has a more complex morphology, characterized by variable Fe, Ni and S compositional distribution and characteristic mineral assemblages. Results confirm that closely spaced chondrules within given sectors of Allende meteorite have major differences in elemental content and distribution, in addition to differences in shape, size, texture and mineralogy, which support the idea that chondrules underwent distinct thermal, shock, alteration and evolutionary histories, related to chondrule formation and alteration processes, in the early stages of evolution of the planetary system.