Ungrouped achondrite NWA 4518 is an ultramafic breccia with abundant siderophile rich IIA-like metal. Its silicate chemistry is similar to that of WINs, HEDs, and silicate inclusions of IIE irons. Oxygen isotopic composition is nearby IAB-IIICD-WIN

Franchi, I.

Greenwood, R.

Humayun, M.

Ivanova, M. A.

Lorenz, C. A.

Teplyakova, S. N.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsOrigin of the ungrouped achondrite NWA 4518:mineralogy and geochemistry of FeNi-metalConference or Workshop ItemHow to cite:Lorenz, C. A.; Teplyakova, S. N.; Humayun, M.; Ivanova, M. A.; Franchi, I. and Greenwood, R. (2011).Origin of the ungrouped achondrite NWA 4518: mineralogy and geochemistry of FeNi-metal. In: 42nd Lunar andPlanetary Science Conference, 7-11 Mar 2011, Houston, TX, USA.For guidance on citations see FAQs.c© 2011 The AuthorsVersion: Version of RecordLink(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1291.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.ukORIGIN OF THE UNGROUPED ACHONDRITE NWA 4518: MINERALOGY AND GEOCHEMISTRY OF FeNi-METAL.  C. A. Lorenz1, S. N. Teplyakova1, M. Humayun2, M. A. Ivanova1, I. Franchi3 and R. Green-wood3 1Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, 119991, Kosygin St. 19, Moscow, Rus-sia,  c-lorenz@yandex.ru,  2Florida State University,  1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA,  3Open University, Walton Hall, Milton Keynes MK7 6AA, Bucks, UK.Introduction: Ungrouped  achondrite  NWA 4518 is an ultramafic polymict breccia with abundant FeNi-metal. The silicate chemistry of NWA 4518 is similar to  that  of  winonaites,  HEDs  and  silicate  inclusions from IIE irons. However, its oxygen isotopic composi-tion is very different from all known types of meteor-ites.  Two thin polished sections of NWA 4518 were studied  with  optical  microscopy  and  EMPA.  Petro-graphy suggests that breccia probably was formed by impact mixing of molten metal with solid silicates and suffered a high temperature metamorphism and redu-cing metasomatism.  In situ elemental compositions of 13 metal particles were determined by LA-ICP-MS in one thin section of NWA 4518. The metal is enriched in  refractory  siderophile  elements  (RSE)  and  W  by ~100хCI and does not correspond to any other groups of iron meteorites.Results: The  NWA  4518  is  a  recrystallized equigranular  polymict  breccia,  consisting  of  olivine Fo68 (Fe/Mn=36)  and  minor  pyroxenes  En45Wo42 (Fe/Mn=29) and En72Wo2 (Fe/Mn=42) [1]. FeNi-metal (2 vol%) occurs among silicates as polygonal grains of 50-800 µm size. These metal grains are interconnected by a network of thin metal-sulfide veins, which locally contain  metal-silicate  melt  pockets.  The  majority  of metal grains has a mosaic internal texture and consist of several polygonal areas, only slightly different in Ni content  (6.5-7.2  wt%),  but  obviously visible in  BSE images. Tiny Ni-rich inclusions are widespread in the metal grains. Taenite lamellae, up to 7 µm in width, oc-cur on the borders of polygonal areas inside the metal grains  and along the contacts  of  the metal  with sur-rounding silicates.  The lamellae have M-shaped pro-files of Ni concentration from 22.5 wt% on the borders to 15 wt% in the center.  The Ni content in kamacite varies in different grains in the range 4 - 7 wt% (aver-age 6.24 wt%, Ni/Co=8.7). The metal in the veins has similar composition. All of metal phases lack Si and have constant P concentrations ~ 0.2 wt%.Sulfides  in  NWA 4518  occur  among silicates  as polygonal grains of Ni-poor troilite, up to 500  µm in size,  interconnected by thin troilite veins with higher Ni content (0.2-0.3 wt% Ni). The large grains of troil-ite  locally  have  straight  sharp  contacts  with  metal grains.The  numerous  troilite-filled  veins  are  running through the whole rock. The olivine around these veins is everywhere replaced by a fine-grained aggregate of troilite and orthopyroxene.The large fragment of orthopyroxenite consists of FeNi-metal inclusions, up to 500 µm in size, enclosed in pyroxene crystals.  The texture and composition of these metal inclusions are similar to those of other met-al  grains  in  NWA 4518.  The  metal  often  associates with chromite and sulfide. The chromite contains rare small  inclusions  of  Ni-rich  troilite,  pentlandite  and metal (Ni 1.5 wt%; Ni/Co=6). Discussion:  Previous  petrographic  observations showed that  olivine and pyroxene grains from NWA 4518 are fragments of cumulate ultramafic rocks [1], free of feldspathic component. A complicated internal texture of metal grains and their compositions are prob-ably the result of extensive nucleation of taenite during the  relatively quick  crystallization  of  the  metal  with slow cooling from ~700 to 500°C. The quick cooling at high temperatures could take place on the surface of parent body after impact mixing of molten metal with silicate  breccia.  The  metal  inclusions  in  pyroxenite fragment are similar in texture and composition to met-al  grains,  distributed among silicate fragments in the breccia,  and  could  have  originated  from  the  same source. The metal enclosed in pyroxene, appears to be indigenous.  However,  the compositions of  metal  and silicates are  not in equilibrium and crystallized from the different sources. Therefore all of NWA 4518 met-al is exotic. In this case magmatic-like texture of metal in the pyroxenite fragment could be formed due of im-pact  injection  of  metal  into  fine  cracks,  similar  to mechanism  proposed  for  metal-bearing  eucrites  [2]. Occurrence  of  melt  pockets  and  metal-troilite  veins, running along the margins of recrystallized silicates in-dicate that NWA 4518 experienced a shock event after the thermal metamorphism, resulting in the equigranu-lar texture. Observed replacement of olivine by aggreg-ate of pyroxene and troilite is probably result of redu-cing metasomatic reactions of olivine and S-rich gas, that was noted before in lodranites [3].Four analyses of oxygen isotopes were performed on silicate portions of NWA 4518 using techniques de-scribed in [4]. The average oxygen isotopic composi-tion of NWA 4518 is (‰): δ17O 2.62; δ18O 5.38; ∆17O -0.17.  The composition is on the far  prolongation of the HED trend, near, but out of  IAB-IIICD-winonaites field, and indicates unusual oxygen reservoir (Fig. 1).1291.pdf42nd Lunar and Planetary Science Conference (2011)The  LA-ICP-MS  analyses  of  FeNi-metal  grains from the main mass of the NWA 4518 have shown that compositions of analyzed metal grains vary in a narrow range and characterize single homogeneous source of metal. The metal of NWA 4518 is enriched in RSE, up to 100xCI. The RSE are not significantly fractionated from one another, and are present in chondritic relative abundances (Fig. 2). The Ir/Os (1.06) and Re/Os (0.1) ratios  of  the average  metal  are  close  to  those  of  CI (0.98 and 0.075 respectively). The Mo is depleted rel-ative to RSE and probably was partly distributed into sulfide.  The less refractory elements are  strongly de-pleted relative to RSE. Ni/Co ratio (10) is lower then that of CI. The metal is enriched in Au and Ge and de-pleted  in  Ga.  Average  concentrations  of  siderophile elements do not correspond to any known group of iron meteorites:  Ni  (6.44  wt%),  Ir  (26.3  ppm)  Ga  (10.3 ppm) Ge (555 ppm). Chondritic ratios of RSE could correspond  to  fractionation  controlled  by  volatility rather than magmatic fractionation. However nonchon-dritic Ni/Co ratio probably indicate another mechanism of fractionation. The Os is slightly depleted relatively to Re and Ir,  that is sign of some fraction crystalliza-tion.  Therefore,  NWA  4518  metal  could  crystallize from metal liquid after removing of small solid metal fraction in the interior of large differentiated body. The metal of NWA 4518 is unique by HSE enrich-ment. The most RSE-rich group of IVB irons [5] is not related to NWA 4518 metal because IVB irons are Ni-rich and  poor  in  moderately volatile  siderophile  ele-ments. The NWA 4518 metal could have some affinity to  the  Ni-poor,  W,  RSE-rich  iron  meteorites  of  IIA group that were not oxidized during nebular evolution [6]. Concentrations of Re, Ir and Os of the NWA 4518 metal correspond to fractionation trend of IIA irons [7] and indicate that these two types of metal could be ge-netically  linked.  The  NWA 4518  metal  appears  to  be crystallized  on  the  stage  of  fractional  crystallization after the Bennett County but probably before Сoahuila crystallization.The enrichment of metal by W relatively to HRS could be a result of partial reduction of tungsten from silicates and redistribution of W into the metal phase during the high temperature metamorphism [8]. Deple-tion in Ga probably also is result of metamorphism.Conclusion: The most probably, silicate lithology of NWA 4518 was formed on differentiated asteroid, which was not represented by meteorites before, while metal component is originated from metal body, prob-ably genetically linked to IIA iron meteorites.References: [1] Lorenz C. A. et al. (2007) MAPS,  42, 5169.  [2]  Yamagushi A. et  al.  (2006)  LPSC 37, 1678.pdf. [3] Papike J. J. et al. (1995) GCA, 59, 3061-3070.  [4]  Miller  M.  F.  et  al.  (1999)  Rapid.  Comm. Mass. Spectrom.,  13, 1211-1217. [5] Campbell A. J., Humayun M. (2005), GCA, 69, 4733-4744. [6] Kelly W. R., Larimer J. W. (1977) GCA, 41, 93-111. [7] Hu-mayun M., Campbell A. J. (2000), LPSC 31, Abstract 2031.pdf.  [8]  Newsom  H.  E.,  Drake  M.  J.  (1979) LPSC 10, 910-912.  [9] Petaev M. I.,  Jacobsen S. B. (2004)  MAPS, 39, 1685-1697. [10] Wasson J.T. et al. (2007)  GCA,  71,  760-781.  [11]  Cook  D.  L.  et  al. (2004)  GCA,  68,  1413-1431.  [12]  Kong  P.  et  al. (1998)  MAPS,  33, 993-998.  [13]  Clayton  R.  N.  and Mayeda T. K. (1996) GCA, 60, 1999-2018.2 4 6 8 10O ‰012345O ‰ACAP-LODHEDIIETFUREAMIAB-IIICD-WINδ 18δ17NWA 4518Fig. 1.  Oxygen isotopic compositions of NWA 4518 (red circles), the data fields from [13].0.11.010.0100.0W Re Os Ir Ru Pt Rh Mo Co Pd Fe Au As Ga Ge(X/Ni)CI2 - IIA Negrillos3 - IIA Bennett County5 - IIA Filomena8 - H-chondrite metal6 - IVB Warburton Range4 - IIA Coahuila7 - IVB Cape of Good Hope1 - NWA 4518 metal12345 8673Fig. 2. Composition of the NWA 4518 metal in com-parison with IIA [9-11], IVB [5] irons and H-chondrite metal [12] (normalized to Ni and CI).1291.pdf42nd Lunar and Planetary Science Conference (2011)

Origin of the ungrouped achondrite NWA 4518: mineralogy and geochemistry of FeNi-metal

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Origin of the ungrouped achondrite NWA 4518: mineralogy and geochemistry of FeNi-metal

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