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MicroRaman spectroscopy of diamond and graphite in Almahata Sitta and comparison with other ureilites

Ross, A.J. and Steele, A. and Fries, M. and Kater, L. and Downes, Hilary and Jones, A.P. and Smith, C.L. and Jenniskens, P. and Zolensky, M. and Shaddad, M. (2011) MicroRaman spectroscopy of diamond and graphite in Almahata Sitta and comparison with other ureilites. Meteoritics & Planetary Science 46 (3), pp. 364-378. ISSN 1086-9379.

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This work is the first detailed study of carbon phases in the ureilite Almahata Sitta (sample #7). We present microRaman data for diamond and graphite in Almahata Sitta, seven unbrecciated ureilites, and two brecciated ureilites. Diamond in Almahata Sitta was found to be distinct from that in unbrecciated and brecciated ureilites, although diamond in unbrecciated and brecciated ureilites is indistinguishable. Almahata Sitta diamond shows a peak center range of 1318.5–1330.2 cm−1 and a full width at half maximum (FWHM) range of 6.6–17.4 cm−1, representing a shock pressure of at least 60 kbar. The actual peak shock pressure may be higher than this due to postshock annealing, if shock synthesis is the source of ureilite diamonds. Diamond in unbrecciated and brecciated ureilites have peak center wave numbers closer to terrestrial kimberlite diamond, but show a wider range of FWHM than Almahata Sitta. The larger peak shift observed in Almahata Sitta may indicate the presence of lonsdaleite. Alternatively, the lower values in brecciated ureilites may be evidence of an annealing step either following the initial diamond-generating shock or as a consequence of heating during reconsolidation of the breccia. Graphite in Almahata Sitta shows a G-band peak center range of 1569.1–1577.1 cm−1 and a G-band FWHM range of 24.3–41.6 cm−1 representing a formation temperature of 990 ± 120 °C. Amorphous carbon was also found. We examine the different theories for diamond formation in ureilites, such as chemical vapor deposition and shock origin from graphite, and explore explanations for the differences between Almahata Sitta and other ureilites.

Item Type: Article
School or Research Centre: Birkbeck Schools and Research Centres > School of Science > Earth and Planetary Sciences
Depositing User: Administrator
Date Deposited: 16 Jun 2011 10:48
Last Modified: 17 Apr 2013 12:20

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