Bland, PA

Burbine, TH

Capitani, G

Folco, L

McCoy, TJ

Menzies, ON

Spiral - Imperial College Digital Repository

IDENTIFICATION OF NANOPHASE IRON IN AN H-CHONDRITE IMPACT MELT.  T. H. Burbine1, L. Folco2, G. Capitani3, P. A. Bland4, O. N. Menzies5, and T. J. McCoy6, 1Laboratory for Extraterrestrial Physics, NASA-Goddard, Greenbelt, MD 20771, USA, tburbine@lepvax.gsfc.nasa.gov, 2Museo Nazion-ale dell’Antartide, 53100 Siena, Italy, 3Dipartimento di Scienze della Terra, Università degli Studi di Siena, 53100 Siena, Italy, 4Department of Earth Science and Engineering, Royal School of Mines, Imperial College London, South Kensington Campus, Lon-don SW7 2AZ, UK, 5Planetary and Space Sciences Research Insti-tute, The Open University, Milton Keynes MK7 6AA, UK, 6Department of Mineral Sciences, National Museum of Natural His-tory, Smithsonian Institution, Washington, DC 20560-0119, USA.  Introduction:  One of the perplexing questions in planetary sci-ence is why there are so few asteroids that spectrally “match” mete-orites.  For example, many asteroids have interpreted mineralogies consistent with ordinary chondrites; however, the asteroids tend to have reflectance spectra with steeper spectral slopes (redder spectra) than measured meteorites in the visible and near-infrared.  This slope difference is believed to be due to the production of nanophase iron in asteroidal regoliths.  Nanophase iron particles are formed [1] by the reduction of FeO to metallic iron during impact heating and during implantation of hydrogen by the solar wind. We have used transmission and analytical electron microscopy (TEM-AEM) to identify nanophase iron particles in an H-chondrite impact melt (Dar al Gani 896 or DaG 896).  We have also obtained a reflectance and a Mössbauer spectrum of this meteorite.  We believe this meteorite has important implications for petrologic and spectro-scopic studies of asteroids. Petrology:  DaG 896 has a microporphyritic texture with abun-dant (~64 vol.%) olivine (Fo82.5±2.5) crystals set in a groundmass consisting mainly of Si-rich glass plus quench microlites of pi-geonite (En59±5Wo9±3).  There are blebs of opaque phases dissemi-nated within the glass.  Chondritic relics (~10 vol.%) are spread throughout the igneous groundmass.  DaG 896 is interpreted to have formed as the result of whole-rock melting of H-chondrite material, nearly complete loss of the metal plus sulfide component, and crys-tallization without significant igneous fractionation.  Opaque Phases:  TEM-AEM investigations allow us to identify and characterize the submicron-sized opaque phases.  These opaque phases consist mainly of metal (kamacite, 1.5% Ni) and troilite blebs.  Metal blebs are typically found in contact with pyroxene microlites.  The metal particles are 150 to 350 nm in size and have an occurrence frequency of ~1 per µm2.  Troilite particles typically free-float within the glass, are usually a few tens of microns in size, and have an occurrence frequency of ~10 per µm2.  The metal and sulfide blebs are interpreted as residues from the removal of FeNi-FeS melt from the silicate melt.   Spectroscopy:  A reflectance spectrum (0.32-25 µm) for a sam-ple of DaG 896 was obtained at the NASA/Keck RELAB facility.  Its reflectance spectrum in the visible and near-infrared is signifi-cantly reddened compared to ordinary chondrites.  The analysis of a room-temperature Mössbauer spectrum of the sample finds no de-tectable Fe3+, arguing that its reddened reflectance spectrum is not due to terrestrial weathering.   Conclusions:  Dar al Gani 896 appears to be evidence that the formation of impact melts on the surfaces of asteroids can produce nanophase iron particles that can redden the reflectance spectra of asteroids.  Studies of impact melts may give us the best insight on the processes that are occurring in asteroidal regoliths. References:  [1] Hapke B. et al. (1975) Moon, 13, 339-353.  66th Annual Meteoritical Society Meeting (2003) 5209.pdf

Identification of nanophase iron in an H-chondrite impact melt.

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