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2007 Annual Science Report

University of Hawaii, Manoa Reporting  |  JUL 2006 – JUN 2007

Chondritic Meteorites as Records of Aqueous Activity on Asteroidal Parent Bodies

Project Summary

The Fe-content in amoeboid olivine aggregates (AOAs) is a sensitive indicator of parent body hydrothermal alteration (Komatsu et al. 2001; Chizmadia et al. 2002; Krot et al. 2004). Initially, AOA are composed dominantly of forsteritic olivine (Mg2SiO4) with minor amounts of diopsitic pyroxene (CaMgSi2O6) and anorthitic plagioclase (CaAl2Si2O8).

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Project Progress

The Fe-content in amoeboid olivine aggregates (AOAs) is a sensitive indicator of parent body hydrothermal alteration (Komatsu et al. 2001; Chizmadia et al. 2002; Krot et al. 2004). Initially, AOA are composed dominantly of forsteritic olivine (Mg2SiO4) with minor amounts of diopsitic pyroxene (CaMgSi2O6) and anorthitic plagioclase (CaAl2Si2O8). Occasionally, spinel (MgAl2O4) is also present. With increasing amounts of alteration, the forsteritic olivine is replaced by an Fe-rich olivine, with ~40 at% Fe (Fig. 1). Eventually, all of the olivine in the AOAs is converted to Fe-rich olivine and no Mg-rich olivine can be detected. On the basis of the amount of the olivine converted to Fe-rich olivine and the distribution of the olivine compositions, the metamorphic stage can be assessed and a petrologic subtype assigned to the meteorite.

As her REU summer project (2006), Anne Sweet, used the scanning electron microscopes (SEM) in the School of Ocean and Earth Science and Technology (SOEST) and the Biological Electron Microscopy Facility (BEMF) to image up to 10 AOAs in each optical thin section of six CO3 chondrites which have not previously assigned a petrologic subtype. These CO3 chondrites are from the Hawai’i Institute of Geophysics and Planetology (HIGP) collection and the Smithsonian Antarctic Meteorite Collection (courtesy of Dr. Ed Scott and Dr. Tim McCoy, respectively). She performed a qualitative analysis, using the backscattered electron (BSE) images she collected. She compared her images to those of Chizmadia et al. (2002) and made predictions as to the metamorphic degree of the six chondrites (ALH83108, A-82094, MET00694, MET00711, MET00737, QUE97416). She also analyzed the quantitative Fe/(Mg+Fe) ratio of the olivine in the six CO3 chondrites in addition to two “control” CO3 chondrites; Lance and Y-82050 which were analyzed and reported by Chizmadia et al. 2002. The distribution of the Fe-rich olivine is shown in Figure 1 and the assigned subtypes are shown in Table 1. We found that A-82094 contains AOIs which have sustained different levels of alteration and is therefore probably a breccia. We were able to confirm that MET00694, MET00711 and MET00737 demonstrate the same level of alteration (subtype 3.8) which is consistent with their being paired.

She presented her findings to both the IfA and through a national telecom to participants of the REU program at NAI institutions. She reported the results at last year’s AAS (American Astronomical Society; Sweet and Chizmadia 2007). This manuscript is current in preparation.

In addition, I am using thermodynamic modeling to describe how Mg-rich olivine can change into Fe-rich olivine without the formation of hydrous phases commonly associated with the aqueous alteration of olivine, such as serpentine, talc and chlorite. I have performed an initial analysis but need to update my spreadsheet with current thermodynamic values. My initial results indicate that at a high chemical activity of Fe in the aqueous solution, the phase fields of Mg-rich olivine and Fe-rich olivine are in contact and therefore a direct dissolution/precipitation is possible without the formation of intermediate hydrous phases.

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  • PROJECT INVESTIGATORS:
    Lysa Chizmadia Lysa Chizmadia
    Project Investigator
  • PROJECT MEMBERS:
    Anne Sweet
    Co-Investigator

    Klaus Keil
    Collaborator

    Sasha Krot
    Collaborator

    Ed Scott
    Collaborator

  • RELATED OBJECTIVES:
    Objective 1.1
    Models of formation and evolution of habitable planets