Franchi, I. A.

Greenwood, R. C.

Moggi-Cecchi, V.

Pratesi, G.

English

Open Research Online (OU)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsNWA 4418: A new mesosiderite from Northwest AfricaConference or Workshop ItemHow to cite:Pratesi, G.; Moggi-Cecchi, V.; Franchi, I. A. and Greenwood, R. C. (2009). NWA 4418: A new mesosideritefrom Northwest Africa. In: 40th Lunar and Planetary Science Conference, 23-27 Mar 2009, Houston, Texas.For guidance on citations see FAQs.c© 2009 The AuthorsVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2009/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.ukNWA 4418: A NEW MESOSIDERITE FROM NORTHWEST AFRICA.  G.Pratesi1, V.Moggi-Cecchi2, I.A.Franchi3, R.C.Greenwood3, 1Dipartimento di Scienze della Terra dell'Università degli Studi di Firenze, Via G.La Pira 4, I-50123 Firenze, Italy, e-mail: g.pratesi@unifi.it, 2Museo di Scienze Planetarie, Via Galcianese 20/h, I-59100 Prato, Italy, e-mail: v.moggi@pratoricerche.it, 3Planetary and Space Sciences Research Institute, Open University, Walton Hall, Milton Keynes, GB-MK7 6AA United Kingdom   Introduction A single stone weighing 103,8 g was purchased in 2006 at the Erfoud market. The outer surface of the main mass is partially covered with a black fusion crust. A cut surface reveals a complex texture, with large metal areas set in a black, rocky matrix. Matteo Chinellato owns the main mass, while the type specimen, weighing 20,1 g, as well as a polished thin section [1] are on deposit at the Museum of Planetary Sciences (MSP) of Prato, Italy (inventory number MSP 5044).   Instruments and methods SEM images and EDS analyses have been performed at the MEMA center of the Earth Sciences Department of the University of Florence by means of a Zeiss EVO-MA15 SEM. EMPA-WDS analyses have been performed at the Padova laboratories of the IGG – CNR (National Council of Research) with a Cameca Camebax Microbeam microprobe. Oxygen isotope measurements have been perfomed at the Planetary and Space Sciences Research Institute Laboratories of the Open University by Richard Greenwood and Ian Franchi.   Experimental results The thin section of NWA 4418 shows a disomogeneous texture with large metal areas embedded in a silicate matrix (Figures 1 and 2). Metal accounts for 30 % of the total surface, with kamacite prevailing over taenite (Figure 3). Schreibersite is accessory. Silicate portion is formed by a microbreccia consisting of minute (not larger than ~ 40 µm) olivine, orthopyroxene and plagioclase grains and various clasts, mainly represented by orthopyroxene olivine (up to 200 µm) and plagioclase (up to 100 µm). Orthopyroxene clasts show very small (1 µm) exsolution lamellae (Figure 4), Clinopyroxene clasts are rather rare. Tiny oxide grains dispersed in the silicate matrix and probably deriving from altered metal or troilite can be occasionally observed (Figure 3 and 5). In the matrix orthopyroxene is prevailing over plagioclase thus indicating class B. Shock features are almost absent, suggesting a low shock stage. The sample displays minor oxidation of metal, pointing to a moderate weathering. Textural features like diffuse melting indicate that NWA 4418 may belong to subgroup 3 [2],[3]. SEM and EMPA analyses revealed that orthopyroxene is pigeonitic (Fs29,6En67,3Wo3,1, with a  FeO/MnO ratio ranging from 26.3 to 37.0). Clinopyroxene (Fs14,0En43,9Wo42,0) and exsolution lamellae (Fs21,6En49,8Wo28,6) are instead diopsidic; Plagioclase is anorthitic (An90,6Or0,6). Oxygen isotope analyses (I.A.Franchi and R.C.Greenwood, OU) confirm textural and compositional data and are consistent with a classification as mesosiderite: δ17O = 2.145 ‰, δ18O = 4.524 ‰, ∆17O = - 0.207 ‰.  Figure 1: polarizing optical microscope image of a thin section of the mesosiderite NWA 4418 (sample MSP 5044); transmitted light, crossed polars. Figure 2: polarizing optical microscope image of a thin polished  section of the martian meteorite NWA 4418 (sample MSP 5044); transmitted light, plane polars. 2430.pdf40th Lunar and Planetary Science Conference (2009) Discussion and conclusions The set of data collected on this stony-iron meteorite point to a classification as mesosiderite. Oxygen isotope data plot in the mesosiderite field [4]. Textural and compositional data, as well as similarities with other mesosiderites from Northwest Africa point to a classification as subgroup 3B.  References: [1] Weisberg, M.K. et al. (2008) MAPS, 43, 9, 1555; [2] Hewins R.H. (1984) JGR, 89, C289-C297; [3] Powell B.N. (1971) GCA, 35, 5-34; [4] Greenwood R.C., et al. (2006) LPSC, 37, abs. 1768.    Figure 3: polarizing optical microscope image of a thin polished  section of the martian meteorite NWA 4418 (sample MSP 5044); reflected light, plane polars.                 Figure 4: polarizing optical microscope image of a thin polished  section of the mesosiderite NWA 4418 (sample MSP 5044). Detail of the silicate matrix with a large pyroxene crystal displaying exsolution lamellae; transmitted light, crossed polars.      Figure 5: SEM-EDS image of a thin polished  section of the mesosiderite NWA 4418 (sample MSP 5044). White areas are metal, tiny pale grey spots in the silicate matrix are iron oxides. 2430.pdf40th Lunar and Planetary Science Conference (2009)

NWA 4418: A new mesosiderite from Northwest Africa

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NWA 4418: A new mesosiderite from Northwest Africa

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