Polymict ureilite NWA 6356 has not suffered an intensive metamorphism and keeps the evidence of multistage carbon injections, reducing sulphuric metasomatism, and consists of feldspathic clasts and best preserved CM-like chondrite fragments

Franchi, I. A.

Greenwood, R.

Kononkova, N. N.

Lorenz, C. A.

Stehlik, H.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsNWA 6356: unequilibrated polymict ureiliteConference or Workshop ItemHow to cite:Lorenz, C. A.; Kononkova, N. N.; Stehlik, H.; Franchi, I. A. and Greenwood, R. (2011). NWA 6356: unequilibratedpolymict ureilite. In: 42nd Lunar and Planetary 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/1293.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.ukNWA 6356: UNEQUILIBRATED POLYMICT UREILITE.  C. A. Lorenz1, N. N. Kononkova1, H. Stehlik2, I. A. Franchi3 and R. Greenwood3 1Vernadsky institute of geochemistry and analytical chemistry RAS, 119991, Kosygin St. 19, Moscow, Russia, c-lorenz@yandex.ru, 2Hagedornweg 2/12, 1220 Wien Austria, hstehlik@aon.at, 3Open Uni-versity, Walton Hall, Milton Keynes MK7 6AA, Bucks, U. K.Introduction: The polymict ureilites are suggested to be a regolith of ureilite parent body (UPB) [1]. Poly-mict ureilite NWA 6356 is unequilibrated clastic mat-rix  breccia,  consisting  of  rock  fragments,  breccias, melts, carbonaceous chondrite clasts (CCs) and chon-drules. Four sections of the meteorite with total area of 3.6  cm2 were  studied  with  optical  microscopy  and EPMA. Oxygen  isotopic composition of  NWA 6356 was determined by laser fluorination. Unusual features of the breccia are: 1) sulphur reduction of olivine; 2) occurrence of two highly reduced forsterite lithologies; 3) presence of several textural types of graphite; 4) oc-currence of medium-grained gabbroic rocks;  5) pres-ence  of xenolithes in microlithic feldspatic  clasts;  6) presence of the best preserved CC fragment.Results: NWA 6356 is a coarse-grained polymict breccia, consisting of angular fragments of olivine (Ol) and minor pyroxene (Px), up to 5 mm of size. Graphite is  a  minor phase,  plagioclase  (Pl),  chromite,  troilite, Ca-phosphate,  FeNi  metal,  diamond, Fe-silicides and silica are rare.  The Ol is  Fo78 (Fe/Mn=42; 0.61 wt% Cr2O3; 0.32 wt% CaO) and rarely contains round inclu-sions of Px (En78.2Wo3.4; Fe/Mn=32.5 at; Cr2O3 0.81 wt%), FeNi-metal  and sulfides.  The Ol is  getting more MgO-rich (up to Fo95, Fe/Mn=10) towards graphite in-clusions. This zoning is interrupted by the clast mar-gins. The most MgO-rich areas of Ol grains contains many tiny FeNi-metal inclusions.Thin veins of troilite (0.8 Ni, 0.3 Cr, wt%), running through  the  Ol,  are  accompanied  by  replacement  of surrounded Ol by fine-grained aggregate of troilite and Px (En92.3Wo1.2,  Fe/Mn=11.5).  The Ol varies in com-position from Fo78 (Fe/Mn=40) to Fo86 (Fe/Mn=34) to-wards the areas of Ol replacement, interrupted by the margins of Ol clasts. Minor and accessory minerals. Two populations of olivine, different from dominant lithology were found. The  population  I  includes  large  grains  of  Fo91-96 (Fe/Mn=9-16), that are free of fractures and any traces of  secondary  alterations.  The  population  II  includes grains  of  forsterite  Fo99 with  network  of  feldspatic veins  (An73Ab17Or10).  This  Ol  also  has  no  reduction features and contains microelements in concentrations close to detection limits of EPMA.Large Px clasts have average composition En77Wo4 (Fe/Mn=30)  varying  in  a  narrow range.  Px  contains fine lamellae of Ca-pyroxene. Troilite, associated with interstitial carbon, contains 1.9 wt% of  Cr.   Troilite inreducing veins contains less Cr – 0.3 wt%. FeNi-metal contains 3.4 Si, 0.25 Cr, 2.5 Ni (wt%). Thin FeNi-met-al veins crossing the large OL clasts are Si,Cr- poor. Chromite, occurs in the matrix, and has composition in ranges:  Fe/(Fe+Mg) 0.66-0.77; Cr/(Cr+Al)  0.78-0.88. Silicides  have  a  wide  range  of  compositions.  Silica contains of 0.55 FeO and 0.14 Al2O3 (wt%).Graphite in NWA 6356 occurs in veins within Ol, irregular aggregates in the breccia matrix, which prob-ably are fragments of veins. One fragment of olivine rocks  contains  abundant  thick  vermicular  veins  of graphite. The prismatic crystals of graphite occur in Ol and in the matrix. Isometric grains, of diamond,  0.5-50 µm in size, occurs in graphite aggregates. No any evid-ences  of  reduction  of  the  matrix  silicates  were  ob-served at the contacts with carbon.Feldspar-pyroxene  rocks.  Minor  populations  of crystalline  (CFP)  and  microlithic  (MFP)  feld-spar-pyroxene rocks were  found in the breccia.  CFP clasts  consist  of  200-400  µm grains  of  PX En75Wo9 (Fe/Mn=25, 1 wt% Cr2O3) and partly isotropic PL (An-84Ab16-An92Ab8).The MFPs are presented by three groups. MFPs of group I are composed of tiny co-oriented skeletal crys-tals in glassy mesostasis.  These rocks are varying in composition  from  PX  (En52Wo40 (MG#87.5; Fe/Mn=11.1) + GL (An86Ab13 – An95Ab4) to PX (En50-Wo21; MG#62; Fe/Mn=39) + GL (An70Ab28).MFPs of group II are composed of 10-30 mm eu-hedral  crystals  of  olivine  Fo89 (Fe/Mn=26)  normally zoned  to  Fo81 (Fe/Mn=35),  which  embedded  into  a glassy  mesostasis  An80Ab18.5Or1.5 with  tiny  skeletal crystals of pyroxene.The  group  III  includes  two  large  fragments  of MFPs, consisting of xenolithes. The main mass of clast #10  composes  of  elongated  microlithes  of  Px  (En60-Wo32) and feldspatic glass (An47Ab52).Xenolithes  of  medium-grained  rock  consists  of prismatic  crystals  of  Pl  (An50Ab50)  with  interstitial minor  pyroxene  (En70Wo24).  The  rock  joints  the  sur-rounded breccia fragments and most likely it is an im-pact melt.Fragment #74 consists of pyroxene microlithes En52-Wo31 (MG#=77,  Fe/Mn=12)  with  interstitial  glass Ab65An2Or14  and  200-400  µm  xenocrysts  of  Pl Ab70An27Or3 and Px En36Wo36 (MG#=56; Fe/Mn=17) with pyroxene composition in the reaction  rim En50-Wo36 on the contact with the host rock.1293.pdf42nd Lunar and Planetary Science Conference (2011)Carbonaceous  chondritic  fragments.  CCs  are widely distributed in NWA 6356. The largest observed CC is 2x0.5 mm of size, and has a best preserved chon-drite texture. It consists of chondrules, glass fragments, zoned olivine grains, pyroxene and troilite, embedded into fine-grained phillosilicate matrix (60 vol %). The texture and composition of the CC are corresponding to CM2-like chondrite [2]. The smaller fragments have no chondrules and widely vary in matrix/objects ratio, contents of opaque phases and Ni content in sulfide.Several  objects  in  the  breccia  were  identified  as fragments  of barred and granular  olivine chondrules. BO chondrules  consist  of  normally zoned (Fo74-Fo66, Fe/Mn=30)  Ol  and  minor interstitial  SiO2-rich  glass. Some of chondrules contain K2O-rich glass (up to 3 wt%).  Granular  chondrules  consist  of  homogeneous  Ol (Fo75-82; Fe/Mn=54-74).Oxygen  isotopic  composition of  the  meteorite  is δ17O 3.32 ‰;  δ18O 7.84 ‰;  ∆17O -0.764 ‰, that cer-tainly  consistent  with  ureilite  classification  of  NWA 6356. Discussion. Mineral  chemistry  data  and  oxygen isotopic  composition  suggest  that  NWA  6356  is  an ureilitic polymict breccia [1, 3], mostly composed by fragments  of  coarse-grained  dunite-like  rocks.  The MG# of low-Ca Px is similar to that of Ol and Px in-clusions in the Ol, indicating a single igneous source of these phases. Small degree of thermal metamorphism indicated by zoning of olivine and a low abundance of impact melt show an insignificant maturity of breccia. High-MgO olivine, associated with inclusions of car-bon,  FeNi-metal  and  Fe  silicides  is  a  result  of  high temperature  reduction similar  to established mechan-ism proposed for other ureilites [1]. Increasing of MgO content in Ol and the replacement of Ol around troilite-filled veins are probably result of partial reduction of olivine by S, observed before in several achondrites [4-6]. Well- preserved zoning in C- and S-reduced oliv-ines suggest that these alterations were formed during the same thermal event. Brecciation of reduced olivine and absence of reduction features in the matrix of brec-cia show that reduction reactions took place in unbrec-ciated rock.Several texturally different types of graphite prob-ably  were  formed  by  multiple  carbon  injections. Graphite  crystals  enclosed  in  olivine  seem to  be  ig-neous [3]. However, vermicular veins of graphite looks like later injections. The olivine fragment enriched in graphite  veins,  could  represent  a  lithology  different from the dominant  lithology of NWA 6356. The di-versity of carbon sources in polymict ureilites was sug-gested based on isotopic composition of C in DaG 319 [7].  Unusual  forsteritic  clasts  of  population-I  could have affinity to forsterite fragment found in polymict ureilite DaG 999 [8]. Absence of features of solid re-duction allows to propose  that  these  forsterites  were crystallized from highly reduced  melt.  The nature of forsteritic population II remains unclear, but it is obvi-ously igneous. However, the low content of Cr in the olivine does not correspond to the ureilite source. The texture of feldspatic veins in forsterite could be a result of filling of cracks by melt, and had never been ob-served in ureilites before.Crystalline feldspar-pyroxene rocks are gabbroic in composition. The CFPs containing of An-rich feldspar have  not  been  observed  before  in  polymict  ureilites. The CFP pyroxene is similar to pyroxene fragments in the breccia, so CFPs could have the similar  source as the dominant  ultramaphic lithology.  The CFPs could probably be a source of MFPs of group I. Texturally all of MFPs are similar to those described in [9]. How-ever, most of MFPs are differed from each other, sev-eral of them contain xenolithes and join the surround-ing breccia  fragments.  Probably,  several  of MFP are not pristine and were formed by assimilation of differ-ent  lithologies  by melt  during impact  or  endogenous processing.The NWA 6356 polymict  ureilite contains one of the best  preserved  carbonaceous  chondrite  fragments that ever found in polymict ureilites. Compositions of CCs matrices mostly correspond to that of CM-chon-drites. However,  some differences in mineralogy and chemistry of CCs point to several CC impactors.. The origin  of  chondrule  fragments  is  not  clear  based  on their mineral chemistry. The chondrules could origin-ate  from CCs fragments,  or  from disrupted  ordinary chondrite impactors, as was showed for chondrules in DaG 319 [I]. However oxygen isotopic composition of NWA 6356 does  not  reflect  a  presence  of  exogenic component into the breccia and could support the CCs origin of the chondrule fragments.References:  [1]  Mittlefehldt  D.  W.  et  al.  (1998) Rev. Mineral., 36, 4-73. [2] Brearly A. J., Jones R. H. (1988)  Rev.  Mineral.,  36, 3-07–3-12.  [3]  Clayton  R. N., Mayeda T. K. (1996)  Geochimica et Cosmochim.  Acta,  60,  1999-2017. [4]  Papike  J.  J.  et  al.  (1995) Geochimica  et  Cosmochim.  Acta,  59,  3061-3070. [5] Norman  M.  D.  et  al.  (1991)  Geoph.  Res.  Lett.,  18,  2081-2084. [6] Rosing M.T.,  Haack H. (2004)   LPS 35, Abstract #1487. [7] Smith C. L. et al. (2003) Met-eoritics & Planet. Sci., 38, Abstract #5224. [8] Downes H. et al. (2008) Geochimica et Cosmochim. Acta, 72, 4825-4844. [9] Cohen B. A. et al. (2004) Geochimica  et Cosmochim. Acta, 68, 4249-4266. 1293.pdf42nd Lunar and Planetary Science Conference (2011)

NWA 6356: unequilibrated polymict ureilite

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NWA 6356: unequilibrated polymict ureilite

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