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Johnson, Diane and Grady, Monica
(2009).
URL: http://digitalcommons.library.arizona.edu/objectvi...
Abstract
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 frag-ments of olivine and chromite. Many of these Troilite veins dis-play 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 en-richment within the veins is the result of melting of small frag-ments 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 subse-quently mixed. NWA4482- Is a highly weathered main group pallasite consisting of many fragments of a magnetic metal oxide-olivine meteorite. Most metal in this sample was weathered into iron hydroxides it was also found to contain small quantities of schreibersite and chromite grains. Only a small quantity of troilite was identified within the sample analysed it was embedded ex-clusively 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 en-try 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 pre-cursor FeS structures may yield evidence to early formation processes in the pallasite parent bodies in relation to distribution of sulphide. The struc-tures 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 pre-cursor FeS during the processes. The arrays of sulphide present in NWA4482 are most probably due to exsolution during formation. Further studies of both these structures could yield new insights into the formation process.
References
[1] Johnson D et al 2008 Meteoritics and Plane-tary Science, 43, A67
[2] Sharygin V.V et al. 2006 LPS XXVII A1235
[3] Buseck P R 1977, Geochim Cosmochim. Acta, 41,711
[4] Stevens M, Buseck P R, 2008 LPS XXXIX, A2157.
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