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Project Progress

Using Amoeboid Olivine Inclusions to Quantify the Degree of Metamorphism in the CO₃ Carbonaceous Chondrites

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.

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As her REU summer project (2005), Claire Bendersky, 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 (courtesy of Dr. Ed Scott). Due to mechanical breakdown on the electron microprobe in G&G, she was not able to analyze the olivine in the AOAs. Therefore, 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. She presented her findings to both the IfA and through a national telecom to participants of the REU program at NAI institutions. Since, Claire’s departure to continue her studies at Mount Holyoke College, the electron microprobe has been repaired and I have analyzed six CO₃ chondrites (ALH82101, ALH85003, A-881632, EET92126, Y-790992 and Y-791717). I have reduced the data and assigned subtypes (Table 1). She reported the results at last year’s AAS (American Astronomical Society; Bendersky and Chizmadia 2006) and LPSC (Lunar and Planetary Science Conference; Chizmadia and Bendersky 2006). This manuscript is current in preparation. My current REU student, Anne Sweet, has spent this summer working on six additional CO₃ chondrites and two control samples (previously-analyzed CO₃ chondrites). She has presented her findings by telecom to REU-NAI participants, will present to the IfA REU program and will present this study at next year’s AAS and LPSC meetings. This manuscript will be prepared in the spring of next year.

I have also compiled the recovered weights of the meteorites and have plotted the weights versus the subtypes to better constrain the heat budgets of the parent body asteroid (Fig, 2). At the upcoming Computational Astrobiology Summer School (CASS), I will get assistance in building a numerical model in which I will iterate the conditions (e.g. porosity, water-to-rock ratio, time of alteration, activity of Fe, etc.) which result in the production of the metamorphic levels similar to that observed in the meteorites. This model will help us better understand the heat and water budgets of carbonaceous asteroids which has implications for the delivery of water and organic materials to early by these types of meteorites.

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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.

Identification and Characterization of the First CO3.1 Carbonaceous Chondrites

In the process of Claire’s project, we came across a meteorite, A-881632, which showed the predicted traits of incipient aqueous alteration predicted by Chizmadia et al. 2002. No such rock has before been reported. Another research group also found such a rock, DOM03238. Both sets of results were presented at last year’s LPSC. We have begun a collaboration to analyze the remaining three possibilities and then we will co-write the papers. I have analyzed the two above rocks and am currently waiting for three other possible 3.1 chondrites, which should hopefully arrive before the end of the summer. I will then use the electron microprobe to analyze the Mg/Fe in the olivine and the SEM to take high-resolution images of the incipient alteration. I will also use the element mapping capability of the SEM to document the incipient alteration in terms of element present only in the secondary phases, e.g. Fe, S and Na. We should have the final results before the end of the fall semester. This project will help us better understand how the alteration begins in the CO₃ chondrites and give us insights into incipient aqueous alteration of carbonaceous chondrites in general. In addition, the alteration of CO₃ chondrites shares many similarities with ordinary chondrites (e.g. Grossman and Brearley 2005), so understanding the initial phase of alteration of the CO₃s may help us better understand the aqueous alteration of the most abundant group of meteorites.

Comparison of chemical compositions of fine-grained rims

The coarse grained objects in carbonaceous chondrites, such as chondrules and CAIs, are surrounded by a rim of fine-grained materials. There are many competing hypotheses concerning the formation of these rims, but most commonly they are thought to have been accreted after the formation of the central object and some time before the accretion of the final parent asteroid (e.g. Metzler et al. 1992; Cuzzi 2004). CAIs are thought to have formed 2-3 Ma before chondrules, based on 26Al systematics. If the fine-grained rims formed as a consequence of free-floating through the nebula (Cuzzi and Hogan 2003; Cuzzi et al. 2003; Cuzzi 2004), then those around CAIs probably accreted before those around chondrules or at least should contain materials from before the formation of chondrules. In addition, there are many types of chondrules which are though to have formed at different times. Therefore, we were interested in the rims around these different objects and whether they would shed light on the evolution of dust in the solar nebula.

Aqueous alteration acts to mobilize elements according to their geochemical affinities. Therefore, it was crucial for me to use two of the most primitive (unaltered) carbonaceous chondrites, ALHA77307 and Y-81020, for this study. I also analyzed the rims in Tagish Lake, an anomalous carbonaceous chondrite which has been speculated to possibly be cometary in origin. In order to comment on the formation of the fine-grained rims. I analyzed the fine-grained rims around three types of chondrules (Mg-rich, Fe-rich and Al-rich) and two types of refractory inclusions (CAIs and AOAs). I used the electron microprobe in G&G for the qualitative analyses. In addition, I used the SEM in HIGP for cursory examination of the rims and the SEM at BEMF for detailed examination of the rim textures.

My textural examination reveled that the rim materials are heterogeneous in the average atomic number and their sizes and shapes (Fig. 3). I found no evidence of any fabric, implying that the accretion of the rim materials did not involve a significant amount of impact-related shock. I have determined that the fine-grained rims are texturally and compositionally similar (Fig. 3 and 4), regardless of the composition of the central object, and therefore must have formed at same time and/or place in the solar nebula. In addition, the striking similarity between the composition of the rims and the interchondrule matrix materials (Brearley 1993) implies that the rims and the matrix must be of similar genesis. Therefore, I suspect that the formation of the rims must have happened just before or during the accretion of the final parent asteroid, similar to the run-away accretion models suggested by radioactive dating of differentiated bodies. In addition, I can confirm the elemental depletions and excesses predicted by Alexander (1995).

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I presented the results of this study last year at the Meteoritics Society Meeting (Chizmadia et al. 2005). Currently, I am writing up the results for ALHA77307, Y-81020 and Tagish Lake for publication in Meteoritics and Planetary Science. This manuscript has been delivered to the coauthors and their comments are being accommodated. I expect to have the manuscript submitted this fall.