Oxygen isotopic data argues that HEDs and mesosiderites may be from the same parent body. A spectral survey of mesosiderites was done to determine their spectral properties in the visible and near-infrared and compare to HEDs

Buchanan, P.C.

Burbine, T.H.

Franchi, I.A.

Greenwood, R.C.

Smith, C.L.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsReflectance spectra of Mesosiderites: Implications forasteroid 4 VestaConference or Workshop ItemHow to cite:Burbine, T.H.; Greenwood, R.C.; Buchanan, P.C.; Franchi, I.A. and Smith, C.L. (2007). Reflectance spectraof Mesosiderites: Implications for asteroid 4 Vesta. In: 38th Lunar and Planetary Science Conference, 12-16 Mar2007, League City, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2119.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukREFLECTANCE SPECTRA OF MESOSIDERITES:  IMPLICATIONS FOR ASTEROID 4 VESTA.  T. H. Burbine1, R. C. Greenwood2, P. C. Buchanan3, I. A. Franchi2, and C. L. Smith4, 1Department of Astronomy, Mount Holyoke College, South Hadley, MA 02139, USA (tburbine@mtholyoke.edu), 2Planetary and Space Sciences Re-search Institute, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK. 32401 Elgem Street, Tyler, Texas 75701-4901, USA, 4Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK.   Introduction:  Mesosiderites are mixtures of sub-equal proportions of silicates and FeNi metal [1,2].  Silicates commonly occur as mineral fragments and lithic clasts and exist in a matrix composed of FeNi metal.  The mesosiderite silicate fraction bears a close resemblance to the HED (howardite, eucrite, and dio-genite) suite of basaltic and gabbroic achondrites, commonly thought to be from the asteroid 4 Vesta [3].  Mineral and lithic clasts in the mesosiderites are pre-dominantly composed of pyroxene (either magnesian orthopyroxene or exsolved mixtures of augite and pi-geonite) and Ca-rich plagioclase [1,2].  In contrast to HED meteorites, however, mesosiderite silicates also include significant proportions (generally <10 vol%) of olivine as individual mineral fragments and dunitic clasts [1,2].  FeNi metal that makes up the matrix of mesosiderites is most similar to that found in IIIAB iron meteorites [1]. Mesosiderites appear to be a mixture of material from the crust and core of a differentiated body and contain very little material from the region in-between, the olivine-rich mantle. As a consequence, the origin of the mesosiderites has always been somewhat con-troversial.  One theory for the origin of the mesoside-rites is that they resulted from the impact of the molten metallic core of an asteroid iron core with the crust of a differentiated body [5,6].  Haack et al. [6] date the impact as occurring ~4.4 Gy ago.  Using Mn and Cr isotopes, Wadhwa et al. [2] find a time difference of ~2 Ma in the establishment of the mantle source reser-voirs on the HED and mesosiderite parent bodies, which they argue implies distinct but compositionally similar parent bodies.   Recently, Greenwood et al. [7] presented high-precision oxygen isotope measurements of mesoside-rites and compared their values to HEDs.  They found that the silicate fraction of the mesosiderites and HEDs are isotopically identical.  This is consistent with HEDs and mesosiderites being derived from the same parent body, or parent bodies, with virtually identical oxygen isotopic compositions [7].  Reflectance spectra of the HEDs in the visible and near-infrared are similar to the spectra of the ~500-km diameter asteroid 4 Vesta [3,8]; however, Vesta is usually thought to be the parent body of the HEDs but not the source of the mesosiderites.  HEDs do not usually contain signifi-cant abundance of metallic iron as would be expected if a “large” metallic iron body had impacted into the surface layers of the HED-parent body. Rosing and Haack [9] have identified the first mesosiderite-clast in a howardite (Dar al Gani 779).  The clast is a centimeter-sized metal-rich inclusion with fragments of olivine, anorthite, and orthopyrox-ene plus minor amounts of chromite, tridymite, and troilite.  The presence of this metal-rich clast could imply that mesosiderites and HEDs originate on the same parent body.  Yamaguchi et al. [10] have also found that the platinum group element (PGE) abun-dances of a number of eucrites are similar to the PGE abundances in mesosiderites and IIIAB irons.  They argue that this observation implies the same parent body for the HEDs and mesosiderites.       Vesta is currently one of the target objects for the upcoming Dawn mission [11], which will map this asteroid with both a visible-IR mapping spectrometer and a gamma-ray and neutron spectrometer.  Even though a number of spectral studies have been done on HEDs [12,13], very few spectral measurements have previously been done on mesosiderites [12].  To de-termine their spectral characteristics, we have done a spectral survey of mesosiderites.   Data:  Reflectance spectra (Figure 1) in the visible and near-infrared were obtained at RELAB on four mesosiderite samples from the Natural History Mu-seum, London.  The mesosiderites included Esther-ville, Lamont, Pinnaroo, and Vaca Muerta.  Only Es-therville is an observed fall.  The silicate fractions of the samples were ground to a powder, but the samples were not sieved.  Beside the powdered samples, a large metallic grain in Pinnaroo was also measured spec-trally. 00.020.040.060.080.10.120.140.160.180 0.5 1 1.5 2 2.5Wavelength (μm)ReflectanceEsthervilleLamontPinnarooPinnaroo metalVaca MuertaFigure 1. Reflectance spectra of mesosiderites.   Lunar and Planetary Science XXXVIII (2007) 2119.pdf The reflectance spectra of Estherville and Lamont are consistent with pyroxene-dominated assemblages since they have the distinctive pyroxene absorption features that are centered near 0.9 μm (Band I) and 1.9 μm (Band II).  The spectra of Estherville and Lamont are similar to the spectra of HEDs [12].  The subdued absorption features of Pinnaroo and Vaca Muerta are consistent with mixtures of metallic iron and pyroxene.  The spectrum of Pinnaroo metal is relatively red-sloped, but does appear to have a slight spectral con-tribution due to pyroxene as seen by the shallow ab-sorption centered near 0.9 μm.  All the spectra appear to have been affected by some terrestrial weathering as seen by the weak absorption features in the spectra shortwards of 0.6 μm. Implications for Dawn:  It may be possible to de-termine if Vesta could be the parent body of the mesosiderites using instruments on Dawn.  If a metal-lic iron-rich region is present on the surface of Vesta, this area would be expected to have weaker pyroxene absorption features than other regions on the surface.  The gamma-ray and neutron spectrometer can deter-mine the abundances of major elements such as O, Si, Fe, Ti, Mg, Al, and Ca [11].  A metallic iron-rich re-gion on Vesta should have enriched concentrations of Fe, but depleted concentrations of O, Si, Ti, Mg, Al, and Ca compared to other areas. References:  [1] Mittlefehldt D. W. et al. (1998) Reviews in Mineralogy, Vol. 36, Planetary Materials, 4-1-4-195. [2] Wadhwa M. et al. (2003) Geochim. Cosmochim. Acta, 67, 5047-5069. [3] McCord T. B. et al. (1970) Science, 168, 1445-1447. [4] Perez F. R. and Martinez-Frias J. (2003) J. Raman Spectroscopy, 34, 367-370. [5] Wasson J. T. and Rubin A. E. (1985) Nature, 318, 168-170. [6] Haack H. et al. (1996) Geo-chim. Cosmochim. Acta, 60, 2609-2619. [7] Green-wood R. C. et al. (2006) Science, 313, 1763-1765. [8] Hiroi T. et al. (1994) Meteoritics, 29, 394-396. [9] Rosing M. T. and Haack H. (2004) LPS XXXV, Abstract #1487. [10] Yamaguchi A. et al. (2006) LPS XXXVII, Abstract #1678. [11] Russell C. T. et al.  (2004) Planetary and Space Science, 52, 465-489. [12] Gaffey M. J. (1976) JGR, 81, 905-920. [13] Hiroi T. et al. (1995) Icarus, 115, 374-386.  Acknowledgements:  The authors would like to thank the Natural History Museum for loaning the mesosiderite samples.  Brown University's Keck/NASA Reflectance Experiment Laboratory (RELAB) is a multi-user facility supported by NASA grant NAG5-3871. Lunar and Planetary Science XXXVIII (2007) 2119.pdf

Reflectance spectra of Mesosiderites: Implications for asteroid 4 Vesta

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Reflectance spectra of Mesosiderites: Implications for asteroid 4 Vesta

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