Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23c1721y&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P23C-1721
Mathematics
Logic
[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology
Scientific paper
CM chondrites have undergone prevailed aqueous alteration in the parent asteroids and the alteration changed mineralogical and isotope signatures of water-bearing C-type asteroids.[1,2] In the previous studies, classification schemes of CM chondrites by the alteration degree were proposed by )[3,4] In the present study, we have performed a mineralogical study of 9 CM chondrites found in Antarctica in order to estimatie the nature and extents of aqueous alteration reactions. At first, small pieces (200 microns in size) of matrix were picked up and analyzed by synchroton radiation X-ray diffraction to identify minerals and relative abundances. The results of the X-ray diffraction analysis and SEM observations indicate that serpentine is present in the matrix of all samples as a major phase, suggesting that all samples have undergone aqueous alteration and have not been heated to temperature enough to dehydrate serpentine. Two out of 9 samples (LEW85311 and LAP03178 hereafter GroupA) contain Fe-Ni metal grains in both chondrule and matrix and anhydrous silicates (olivine and pyroxene) in matrix. Five of 9 samples (GRA06172, MET01072, LEW87022, LAP02269 and GRO95566 hereafter GroupB) contain fine-grained PCPs and anhydrous silicates but no metallic phases in matrix. In addition their chondrules are altered only in glassy parts and in limited parts of anhydrous silicates. The rest two of 9 samples (LAP031166 and MAC88100 hereafter GroupC) don't contain Fe-Ni metal and PCPs and contain little anhydrous silicates in matrix. In addition chondrules are almost completely altered. Since metallic phases are more susceptible to aqueous alteration than anhydrous silicates, the results suggest that the alteration degree is in the order of GroupA< GrouB< GroupC. The EPMA analysis indicates that most data of matrix composition fall within the triangle area defined by [1]: the area is enclosed by the composition of PCPs and two serpentines (the serpentin compositions are defined by those from Murry and Nogoya meteorites) in a Mg-Fe-Si ternary diagram. Nine samples have different compositional trends and PCP/(PCP+serpentine) ratios, suggesting that these 9 samples have suffered various extents of aqueous alteration. The matrix compositions reflect the conditions of aqueous alteration, because of its fine-grained nature. Therefore, it is expected that, for instance, GroupA samples show compositional trends similar within the Group, but different from other Groups. However, GroupA sample (LAP03178) has the same trend as GroupB sample (GRO95566), and GroupB sample (MET01072) has the same trend as GroupC sample (MAC88100).This suggests that aqueous alteration process is very complex: the starting matrix compositions are variable, and the resultant matrix compositions are also different. This indicates that matrix chemical composition alone is not enough to define the alteration degree. [1]McSween (1986), GCA 51, 2469-2477 [2]Clayton and Mayeda (1984) Earth and Planetary Science Letters 67, 151-161 [3]Zolensky et.al (1997) GCA 61, 5099-5115 [4]Rubin et al. (2007) GCA 71, 2361-2382
Fujimaki Hirokazu
Nakamura Takashi
Yoshioka Koichi
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