NWA 5790 is a recently discovered nakhlite. Its mineralogy, petrology and geochemistry suggest that it is the topmost sequence of the nakhlite lava pile

Badia, D.

Barrat, J. A.

Bollinger, C.

Boudouma, O.

Franchi, I.

Greenwood, R. C.

Jambon, A

Sautter, V.

English

Open Research Online (OU)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsNorthwest Africa 5790. Top Sequence of the NakhlitePileConference or Workshop ItemHow to cite:Jambon, A; Barrat, J. A.; Bollinger, C.; Sautter, V.; Boudouma, O.; Greenwood, R. C.; Franchi, I. and Badia, D.(2010). Northwest Africa 5790. Top Sequence of the Nakhlite Pile. In: 41st Lunar and Planetary Science Conference,1-5 Mar 2010, Houston, Texas.For guidance on citations see FAQs.c© 2010 The AuthorsVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2010/Copyright 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.uk#1696 NORTHWEST AFRICA 5790. TOP SEQUENCE OF THE NAKHLITE PILE. A. Jambon1 J-A. Barrat2,3, C. Bollinger2,3, V. Sautter4, O. Boudouma1 , R.C. Greenwood5, I.A. Franchi5 and D. Badia1, 1UPMC Univ Paris 06, ISTEP CNRS-UMR 7193, Paris France albert.jambon@upmc.fr , 2Université Européenne de Bretagne, 3Université de Brest CNRS-UMR 6538 IUEM, 29280 Plouzané France, 4MNHN Paris, Laboratoire de Minéralogie et Cosmo-chimie, CNRS-UMR 7202, Paris France, 5PSSRI, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.  Introduction: Northwest Africa (NWA) 5790 is the eighth nakhlite, found in the Sahara (Mauritania?) by nomads and purchased in Erfoud (Morocco). As will be seen below it is not paired with any previous find. Dating is reported elsewhere [1] Petrography: The texture is one of a lava (Fig.1) with dominant millimetric euhedral augite (51%) irregularly zoned in their cores and sharply rimmed. Other phe-nocrysts are subsidiary euhedral olivine (9%) of slightly larger size, steadily zoned to their core and rare titanomagnetite (<1%). Abundant mesostasis (40%) contains dendritic crystals of oxides, calcic pyroxene, fayalitic olivine, two feldspars, silica, merrilite, Cl-apatite and glass. Minor Cl-amphibole is trapped in melt inclusions. The texture (Fig 1) strongly recalls NWA 817 or MIL 03346 [2-4].   Fig. 1: BSE picture. Scale bar 0.5 mm  Mineral compositions: augite phenocrysts exhibit a MgO depleted core (En35–Fs24 Wo40 and Fe/Mn = 32), patchy zoning and a sharp rim (En20Fs43Wo36  and Fe/Mn= 42) (Fig 1-3) on the order of 20 microme-ters. Olivine are steadily zoned from their core (Fa65 with Fe/Mn = 47) to their rim (Fa80 with Fe/Mn = 40) (Fig 2). Ti-magnetite phenocrysts Uv32Mt56-Sp7Ct4 exhibit fine ilmenite exsolutions. In the mesostasis dendritic crystals of Ti-magnetite are Al and Cr free Uv52-74Mt38-19, acicular pyroxene En7Fs47Wo46 and Fe/Mn = 55, enriched in alumi-num (Tschermak component) and olivine Fa78-88 are observed. Plagioclase An20Ab71Or9 and K-feldspar An2Ab17Or81 on average, are strongly zoned. The major elements of mesostasis were analyzed by EPM in scanning mode over areas of 0.20x0.15 mm2 and  Figure 2. Zoning of olivine and augite.   Fig. 3. Pyroxene compositions compared to MIL’s [3]    NWA 5790 error MIL 03346 Error SiO2 54.80 0.20 52.49 0.97 Al2O3 14.33 0.09 13.89 0.31 MgO 0.44 0.01 0.42 0.04 FeO 17.13 0.27 16.73 1.17 MnO 0.26 0.01 0.24 0.02 CaO 4.25 0.07 4.00 0.16 Na2O 5.40 0.04 5.22 0.11 K2O 1.10 0.02 1.03 0.05 TiO2 0.81 0.01 0.82 0.14 Cr2O3 0.00 0.00 0.00 0.00 P2O5 0.96 0.05 0.83 0.04 Total 99.50 0.07 95.67 0.67 Table 1 : Mesostasis composition compared to MIL’s  compared to the mesostasis of MIL 03346 measured under similar conditions. 1696.pdf41st Lunar and Planetary Science Conference (2010)#1696 Geochemistry: The oxygen isotope composition of NWA 5790 was determined by laser fluorination [5]. Two distinct fractions were hand picked from the me-teorite, one olivine-rich and the other a “grey” mesostasis-rich fraction. The oxygen isotope composi-tion of the olivine-rich fraction is: δ17O = 2.55‰; δ180 = 4.32‰; Δ17O = 0.28‰ and the “grey” mesostasis-rich fraction: δ17O = 3.22‰; δ180 = 5.61‰; Δ17O = 0.28‰ (all Δ17O values are linearised [6]).  The Δ17O values of both fractions are within error of the Mars Mass Fractionation Line [7] (Fig. 4), thus confirming the Martian origin of NWA 5790. The relatively large difference in δ180 values between the two fractions is consistent with their mineralogy i.e. one being olivine-rich and the other mesostasis-rich.  Fig. 4. Oxygen isotope composition of NWA 5790  Bulk major-element chemistry reflects the abundance of mesostasis: MgO= 8.0 %, Al2O3= 5.3 %, instead of 11.8% and 1.6% for Nakhla. Element ratios however confirm unambiguously the Martian origin (FeO*/MnO = 38, Na2O/Al2O3 =0.39, K/La = 476, Ga/Al = 3.5 10-4). NWA 5790 displays the highest Th, U and rare earth elements (REE) abundances ever reported for a nakhlite ( e.g. , Th = 0.85 ppm); the REE pattern (figure 5) is characterized by a strong light   Fig. 5: REE patterns of nakhlites compared [3,4,8]. REE enrichment: (La/Yb)n =5.5, and Eu/Eu* = 0.86. Trace element compositions (24 elements) of augite, olivine and mesostasis were obtained by laser ablation ICP-MS (figure 6).   Fig. 6. REE pattern of separate phases compared to MIL 03346 (full lines) [3]. Discussion: According to its texture, mineralogy, chemical and isotopic composition NWA 5790 is a Martian meteorite of the nakhlite group. Augite phe-nocrysts do preserve complex primary zoning not ob-served so far in any of the other nakhlites. This indi-cates a fast cooling rate precluding chemical homog-enization. The presence of Cl-rich amphibole suggests localized contamination by small debris of Cl-rich sediments, as previously reported for MIL 03346 [10]. Compared to other nakhlites its characteristic is the abundance of mesostasis which is reflected in the overall chemical composition. In addition to its abun-dance, the mesostasis is also more evolved, as indi-cated by the trace element abundances (fig.5-6, table 1) [2-4,8,9]; the composition of the whole rock indicates a large fraction of intercumulus liquid. This latter is out of equilibrium with the phenocrysts, and augite and olivine are not in equilibrium either. We can therefore suggest that NWA 5790 represents the topmost section of a lava pile [11] with a higher quenching rate and less compaction. The more evolved composition how-ever suggests a zoned magma chamber.  References: [1] Shih C.-Y. et al. (2009) this vol. [2] Treiman A. H. (2005) Chem. Erde, 65, 203-270. [3] Day J.M.D. et al. (2006) MPS 41, 581-606. [4] Sautter V. et al. (2002) EPSL 195, 223-238. [5] Miller M.F. et al. (1999) Rapid Commun. Mass Spec-trom. 13,1211-1277. [6] Miller M.F. (2002) GCA, 66, 1881-1889. [7] Franchi I.A. et al. (1999) MAPS 34, 657-661. [8] Nakamura N. et al. (1982) GCA 46, 1555-1573. [9] Wadhwa, et al., (2004) Antarct. Meteorite Res. 17, 97-116. [10] Sautter V. et al. (2006) EPSL 252,45-55 [11] Mikouchi T. et al. (2005) LPS XXXVI, CD-ROM #1944. 1696.pdf41st Lunar and Planetary Science Conference (2010)

Northwest Africa 5790. Top Sequence of the Nakhlite Pile

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Northwest Africa 5790. Top Sequence of the Nakhlite Pile

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