Mineralogy, aqueous alteration, and primitive textural characteristics of fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002

Lysa J. Chizmadia, Adrian J. Brearley

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Abstract

Transmission electron microscopy (TEM) was used to study the microtextural and mineralogical characteristics of fine-grained rims in the unbrecciated CM2 chondrites, Y-791198 and ALHA81002, in an effort to provide constraints on the origins of the rims themselves. Our TEM observations show that the rims in Y-791198 are composed of two distinct types of region, sulfide-poor and sulfide-rich, that are intermixed in a complex manner at the micron to submicron level. The sulfide-poor regions are largely composed of amorphous silicate material or nanocrystalline serpentine, but rare fibrous and coarse-grained serpentine grains have also been identified. No fine-grained cronstedtite or tochilinite were observed, although coarse-grained lumps of tochilinite are present in the rims. In contrast, the sulfide-rich regions are characterized by the presence of a myriad, nanometer-sized Fe, Ni sulfide grains (pentlandite with some Ni-rich pyrrhotite) embedded within an amorphous silicate similar in composition to that of the sulfide-poor regions. The sulfide-rich regions also contain rare phases such as olivine, and Fe, Ni metal grains with grain sizes that are always >100 nm in size. Z-contrast scanning transmission electron microscopy (STEM) reveals that the fine-grained rims consist of a mosaic of irregularly-shaped sulfide-poor and sulfide-rich-regions with sizes of about 0.2-0.5 μm, that have been compacted together during parent body lithification. Despite aqueous alteration, the distinct mineralogical characteristics of these different regions are preserved on a fine-scale and probably represent primitive heterogeneity in the dust from which these rims formed. Serpentine is much better developed and more widespread in the fine-grained rims of ALHA81002 than Y-791198. Complex mats of serpentine fibers are commonly found and cronstedtite and tochilinite are plentiful. Anhydrous minerals such as olivine are rare and have usually been replaced by serpentine. Like Y-791198, all the fine-grained rims studied in ALHA81002 show the same mineral assemblages and textural characteristics throughout and between rims. The homogeneity of the mineralogy, textural relationships and degree of hydration in the rims of these two chondrites is more consistent with parent-body alteration than with pre-accretionary alteration.

Original languageEnglish (US)
Pages (from-to)602-625
Number of pages24
JournalGeochimica et Cosmochimica Acta
Volume72
Issue number2
DOIs
StatePublished - Jan 15 2008
Externally publishedYes

Bibliographical note

Funding Information:
We thank Dr. Ansgar Greshake, Dr. Takaaki Noguchi, an anonymous reviewer and associate editor Dr Hiroko Nagahara for their thoughtful, critical reviews, which helped improve the paper substantially. Electron microscopy and electron microprobe analyses were carried out in the Electron Microbeam Analysis Facility, Department of Earth and Planetary Sciences, University of New Mexico, a facility supported by the State of New Mexico and the NSF National Nanosciences Infrastructure Network (NNIN). We are particularly grateful to Mike Spilde for his assistance with the SEM and electron microprobe work. We also thank Youren Xu and Chris Schwappach at Intel Corporation, Rio Rancho, New Mexico for FIB sample preparation. Financial support from the National Aeronautics and Space Administration through the UNM NASA Space Grant to LJC, the NASA Astrobiology Institute under Cooperative Agreement #NNA04CC08A issued through the Office of Space Science (Meech P. I.) and through NASA Cosmochemistry Grant #651160 (Keil P. I.) and NASA Cosmochemistry grant NAG5-13046 to A.J.B. is gratefully acknowledged.

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