Deciphering the nebular and asteroidal record of silicates and organic material in the matrix of the reduced CV3 chondrite Vigarano

Neyda M. Abreu, Adrian J. Brearley

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Abstract

We have conducted scanning electron microscope (SEM) and transmission electron microscope (TEM) studies of a variety of occurrences of matrix in the reduced CV3 chondrite breccia Vigarano. Matrix, which occurs as clastic interchondrule material and finer-grained rims, is dominated by morphologically variable olivines that host submicron, hercynitic spinel, and carbonaceous inclusions. Clastic matrix and fine-grained rims show significant differences in their olivine morphologies, abundance, and composition of olivine inclusions, and characteristics of the carbonaceous matter. We suggest that these differences are the result of different degrees of alteration of clastic matrix and rims and are not due to variability in their precursor materials. Textural and compositional characteristics of olivine in the matrix are consistent with formation by growth, possibly from an amorphous precursor material during asteroidal metamorphism, in the presence of limited quantities of aqueous fluids. Spinel inclusions in olivine may be nebular condensates that acted as seeds for nucleation of olivine or may have formed during metamorphism and were subsequently overgrown by olivine. Carbonaceous material occurs as nanometer-sized inclusions within olivine in both fine-grained rims and clastic matrix, but is most abundant as 100-200nm grains, interstitial to matrix olivines. Most carbonaceous material is amorphous, but poorly graphitized carbon (PGC) also occurs as a minor component in both olivine inclusions and interstitial C. The widespread occurrence of fine-grained amorphous carbon grains in the interstitial regions between olivine grains may preserve the distribution and grain size of nebular organic material. No clear textural relationships exist between carbonaceous grains and the other mineralogical components of Vigarano matrix that could help constrain the origin of the organic grains (i.e., evidence for Fischer-Tropsch-type reactions). Finally, there are considerable differences between matrix olivines in Vigarano in comparison with those in oxidized CV3 chondrites. In particular, the mineralogy and morphology of the matrix olivines and the nature, composition, and distribution of inclusions in the olivine grains are distinct. Based on these differences, we conclude that matrix in the oxidized CV3 chondrites could not have formed by thermal processing of Vigarano-like material.

Original languageEnglish (US)
Pages (from-to)252-274
Number of pages23
JournalMeteoritics and Planetary Science
Volume46
Issue number2
DOIs
StatePublished - Feb 1 2011

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All Science Journal Classification (ASJC) codes

  • Geophysics
  • Space and Planetary Science

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