AAS 207th Meeting, 8-12 January 2006
Session 4 Solar System
Poster, Monday, 9:20am-7:00pm, January 9, 2006, Exhibit Hall

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[4.16] Using AOAs to Document Aqueous Alteration in the CO3 Parent Asteroid

C. Bendersky (IFA, Mount Holyoke College), L. Chizmadia (IfA)

Chondritic meteorites are considered to be among the most primitive materials remaining from the early solar system. Chondritic meteorites have experienced few changes since their formation; only subtle changes are observed, due to aqueous and thermal alteration. CO3 carbonaceous chondrites are of particular interest for studying incipient alteration because they show a clear metamorphic sequence, designated 3.0 (least altered) to 3.8 (most altered). Amoeboid olivine aggregates (AOAs) are fine grained, irregularly shaped objects composed dominantly of olivine with minor amounts of anorthite, diopside and spinel. The fine grained and porous nature of AOAs make them very sensitive to hydrothermal alteration.

We used established procedures of measuring the size, shape and distribution of relict Mg-rich olivine patches, and the Fe content and distribution of the olivine to assign petrologic subtypes to seven CO3 chondrites which had not previously been assigned a petrologic subtype (metamorphic stage).

ALH85003 and ALH82101 are 3.5. There is an even volume of Mg-rich olivine and Fe-rich olivine. Their Fe-rich olivine veins have widened into 10-15 \mum channels. \~0.8 /mum halos around relic Mg-rich olivine cores separate the cores from the Fe-rich veins.

A-881632 is a 3.6. The inclusions are dominated by Fe-rich olivine. Veins are no longer apparent; instead Mg-rich olivine cores remain. These Mg-rich cores have 2--3 \mum halos.

DaG 055 is a 3.7. There is no remnant Mg-rich olivine. The Fe contents of the AOAs and matrix have not equilibrated.

Finally, Y-790992, EET92126 and Y-791717 are 3.8. Only Fe-rich olivine remains in the AOAs. The Fe content of the matrix and the AOAs has fully equilibrated.

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