Gujba is a new Bencubbin-like meteorite fall enriched in N-15 and consisting (in vol.%) of 41% metal nodules, 20% large light-colored silicate nodules and 39% dark-colored, C- and silicate-rich matrix

Clayton, R.N.

Grady, M.M.

Kallemeyn, G.W.

Mayeda, T.K.

Rubin, A.E.

Verchovsky, A.B.

Wasson, J.T.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsGujba: A new Bencubbin-like meteorite fall fromNigeriaConference or Workshop ItemHow to cite:Rubin, A.E.; Kallemeyn, G.W.; Wasson, J.T.; Clayton, R.N.; Mayeda, T.K.; Grady, M.M. and Verchovsky,A.B. (2001). Gujba: A new Bencubbin-like meteorite fall from Nigeria. In: 32nd Lunar and Planetary ScienceConference, 12-16 Mar 2001, Houston, 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/lpsc2001/pdf/1779.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.ukGUJBA: A NEW BENCUBBIN-LIKE METEORITE FALL FROM NIGERIA.Alan E. Rubin1, Gregory W. Kallemeyn1, John T. Wasson1, Robert N. Clayton2, Toshiko K. Mayeda2,Monica M. Grady3 and Alexander B. Verchovsky4.  1Institute of Geophysics and Planetary Physics, Uni-versity of California, Los Angeles, CA  90095-1567, USA (aerubin@ucla.edu); 2Enrico Fermi Institute,University of Chicago, Chicago, IL  60637, USA; 3Natural History Museum, Cromwell Road, LondonSW7 5BD, UK; 4PSSRI, Open University, Walton Hall, Milton Keynes MK7 6AA, UK.The Gujba, Nigeria meteorite is the first ob-served fall (3 April 1984) of a Bencubbin-likemeteorite [1].  Gujba texturally resembles Ben-cubbin and QUE94411 except that, in Gujba,the coarse metal and silicate nodules are roundand generally unfragmented.  The compositionsof the metal and silicates in Gujba are similar tothose of other Bencubbin-like meteorites.  Gu-jba consists of three major components: (1) 41vol.% metal nodules, (2) 20 vol.% large light-colored silicate nodules, and (3) 39 vol.% dark-colored, silicate-rich matrix.The O-isotopic composition of Gujba sili-cates is very similar to those of Bencubbin,Weatherford and QUE94411.  The respectiveδ18O, δ17O and ∆17O values (in ‰ relative toSMOW) are: Gujba light-colored silicates(+0.53, -2.19, -2.47); Gujba dark-colored sili-cates (+0.98, -1.78, -2.29); Bencubbin (+0.90,-1.80, -2.27) [2]; Weatherford (+1.69, -1.65,-2.53) [2]; QUE94411 (+1.50, -1.47, -2.25) [3].Bencubbin-like meteorites are characterizedby enrichments in 15N [4], with δ15N reachingup to +800‰.  Dark-colored Gujba silicateshave a total nitrogen content of 144 µg/g, withδ15N ~+685‰.  The heaviest nitrogen was re-leased at temperatures characteristic of carbideor metal decomposition, with δ15N reaching+790‰.  The dark-colored silicates are alsocarbon-rich, with ~0.5 wt.% C.Gujba metal nodules are ellipsoidal tospheroidal and range in maximum dimensionfrom 400 µm to 7 mm.  Because the metal nod-ules are so undistorted, it is evident that theoriginal whole-rock depositional texture is bet-ter preserved in Gujba than in other Bencubbin-like meteorites.  The metal nodules containvariable amounts of troilite (<0.01 to ~1vol.%); no graphite, carbide or phosphidephases were identified.  Neutron-activationanalyses of six 42-390-mg metal nodules reveala correlation between troilite-texture/troilite-abundance and the concentrations (in µg/g) ofvolatile siderophile elements.  The highest con-centrations of these elements are in nodules 6, 5and 4 (0.470-0.505 Au; 4.36-5.18 As; 115-207Cu; 2.17-3.74 Ga); intermediate concentrationsare in nodules 1 and 2 (0.342-0.356 Au; 3.00-3.40 As; 32-56 Cu; 1.10-1.55 Ga); and the low-est concentrations are in nodule 3 (0.135 Au;1.43 As; 10 Cu; 0.61 Ga).  Nodule 6 consists of200-600-µm-wide patches of kamacite sepa-rated and, in some cases, partially surrounded,by arcuate patches of troilite.  This appears tobe a texture produced by rapid cooling; a simi-lar but much coarser texture is present in theIAB-an iron Mundrabilla.  Nodules 2, 4 and 5have fewer arcuate patches of troilite and gen-erally smaller kamacite domains; they containnumerous troilite-free kamacite patches adja-cent to ones containing 10-20 vol.% 5-10-µm-size round troilite blebs.  (Metal nodules withsimilar blebby troilite textures occur in Ben-cubbin [5]).  Nodule 1 contains no arcuatetroilite but consists of troilite-free kamacite re-gions adjacent to ones containing 5-10 vol.% 5-10-µm-size round troilite blebs.  Nodule 3 isessentially troilite free; there is a single 15×50µm troilite grain in the plane of the section.We propose two distinct models to accountfor the correlation between texture and volatile-siderophile concentrations in the metal nodules.Model 1: All of the metal nodules quenchedfrom high temperatures and initially resemblednodule 6.  The correlation between troilite tex-ture (and, to a lesser extent, abundance) andvolatile-siderophile concentration among thenodules indicates that the nodules were subse-quently heated to very high temperatures (per-haps >2000 K) and that troilite and volatile sid-Lunar and Planetary Science XXXII (2001) 1779.pdfGUJBA:  A NEW BENCUBBIN-LIKE METEORITE:  A. E. Rubin et al.erophile elements were partially boiled off.Troilite blebs formed from melting arcuatetroilite patches.  Nodule 3 (with essentially notroilite) experienced the greatest degree ofvolatilization.  Model 2: Nodule 3 formed bycondensation at high temperatures and nodule 6at lower temperatures, thus accounting for thedifferences in their concentrations of S andvolatile siderophiles.  Other nodules condensedat intermediate temperatures.  Arcuate troiliteformed from a late-stage S-rich melt that crys-tallized around solidified metal.Light-colored silicate nodules are ellipsoi-dal to spheroidal quench-textured objectsranging in maximum dimension from ~800 µmto 10 mm.  Large silicate nodules have crypto-crystalline textures with 50-400-µm-size ex-tinction domains in random orientations.  Eachindividual domain consists of bundles or fan-like arrays of elongated pyroxene crystals (Fs1-2Wo1-3) with interadjacent feldspathic glass.Smaller silicate nodule fragments (50-600 µm)exhibit barred pyroxene (BP) or cryptocrystal-line (C) textures.  In the BP objects, the indi-vidual pyroxene bars average ~6 µm in width;in the C-textured objects, the individual barsaverage 2-4 µm in width.  Rare silicate nodulescontain olivine (Fa2.6).  Metallic Fe-Ni is ab-sent from the silicate nodules.Dark-colored silicate-rich matrix containssmall irregular metal nodules (~100 µm) andsmaller (2-50 µm-size) irregular metal grains,blebs and veins.  Silicates consist mainly ofsmall nodule fragments.  In some cases, smallmetal particles surround silicate nodules; inother cases, small metal veins in the matrix areconnected to large metal nodules.Some shock effects are evident in Gujba.Olivine and low-Ca pyroxene exhibit unduloseextinction, characteristic of shock stage S2.  Insome places, there is a fine-grained dispersionof metal and silicate that closely resembles thetexture of shock veins in ordinary chondritesexcept that, in Gujba, troilite is essentially ab-sent from the opaque-mineral dispersion.  Somelarge metal nodules contain patches of cellularmetal-troilite intergrowths indicative of post-lithification shock. A few percent of the patchesof arcuate troilite in the metal nodules consistsof pure troilite cores surrounded by mixtures of~40 vol.% troilite and ~60 vol.% tiny wormykamacite grains.  The inside edges of somemetal nodules contain long (≤2 mm) stringersof fine-grained silicates mixed with troilite.Inside some 40-100-µm-thick metal veins in thematrix there are 10-30-µm-size silicate frag-ments surrounded by 5-12-µm-thick rinds offine-grained silicates intimately mixed withmetal.  The sides of the silicate fragments weretorn off and, in some cases, melted.Although the O-isotopic compositions ofGujba and the other Bencubbin-like meteorites(Bencubbin, Weatherford, QUE94411, Ham-madah al Hamra 237) are near those of CRchondrites [3], the Bencubbin-like meteorites(and the CH chondrites) differ from normalcarbonaceous chondrites in containing very few(if any) chondrule-like objects with metalgrains or porphyritic textures.  This mitigatesagainst an origin of Bencubbin-like meteoritesand CH chondrites similar to that of normalchondrites.  Suggestions for their origin includeformation at extremely high temperatures indust-free regions of the protoplanetary disk [6]and formation by condensation within impactplumes on a chondritic asteroid [7].  The pris-tine, unweathered nature of Gujba makes it theideal sample for potentially distinguishing be-tween such nebular and non-nebular models.After formation, the Bencubbin-like meteoriteswere shocked; some metal and silicate noduleswere fragmented, some troilite and metal weremelted, and some shock veins were produced.References: [1] Islam and Ostaficzuk(1988) Annals of Borno 5, 110-124; [2] Weis-berg et al. (1995) Proc. NIPR Symp. Ant. Met.8, 11-32; [3] Weisberg et al. (2001) MPS 36, inpress; [4] Prombo and Clayton (1985) Science230, 935-937; [5] Newsom and Drake (1979)GCA 43, 689-707; [6] Krot et al. (2001) Sci-ence, submitted; [7] Wasson and Kallemeyn(1990) EPSL 101, 148-161.Lunar and Planetary Science XXXII (2001) 1779.pdf

Gujba: A new Bencubbin-like meteorite fall from Nigeria

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Gujba: A new Bencubbin-like meteorite fall from Nigeria

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