Structure and Chemistry of Pre-cursor Troilite in Pallasites

Introduction: Pallasites are widely considered to be poor in sulphide, as such little is known about troilite evolution and processing. Very few examples exist indicating the multiple stages of troilite evolution. Discussed here are two such samples of FeS structures one is a nickel enriched FeS grain in the main group pallasite Hambleton. The other is a series of micron FeS particles within olivine crystals lying along partially annealed fractures in NWA4482. Discussion: Hambleton is a main group pallasite rich in FeS, previous study [1] has illustrated this sulphide as interconnecting veins and sheets contained within these are typically broken fragments of olivine and chromite. Many of these Troilite veins display a nickel enriched exsolution texture on a scale of hundreds of microns. We have now also identified a nickel enriched FeS grain within a later FeS vein, the grain displays a nickel enriched exsolution texture on a scale orders of magnitude smaller than that observed in vein structures. It is possible that the nickel enrichment within the veins is the result of melting of small fragments of nickel rich precursor FeS materials. This may implies that the grain evolved from a chemically distinct nickel rich melt either within the same parent body or seperately and subsequently mixed. NWA4482 is a highly weathered main group pallasite consisting of many fragments of a magnetic metal oxide-olivine meteorite. Only a small quantity of troilite was identified within the sample analysed it was embedded exclusively as particles within olivine crystals. The commonest of these particles are submicron size spheres distributed in equally spaced arrays that appear to lie along annealed fractures. Similar structures have been noted by others in olivine crystals within the Omolon pallasite [2] and were interpreted as indicator of post deformational annealing [3] or due to terrestrial atmospheric entry and impact mechanisms. Arrays of tubular structures were also noted by others [4] in the main group pallasite Fukang they were attributed to exsolution of incompatible elements. Conclusion: The observation of precursor FeS structures may yield evidence to early formation processes in the pallasite parent bodies in relation to distribution of sulphide. The structures identified in these two samples are supportive of Hambleton forming via introduction of a significant sulphide volume under pressure into a metal-olivine mixture with metal approaching solidus temperature, deforming olivine, chromite and precursor FeS during the processes. The arrays of sulphide present in NWA4482 are most probably …