NORTHWEST AFRICA 12774 – A NEW BASALTIC ANGRITE

V.H. Hoffmann1,2, K. Wimmer3, T. Mikouchi4, M. Kaliwoda5, R. Hochleitner5, M. Funaki6, M. Torii7, M. Jost8 1Faculty of Geosciences, Dep. Geo- and Environmental Sciences, Univ. Munich; 2Dep. Geosciences, Univ. Tübingen; 3Nördlingen, Germany; 4The University Museum, The Univ. of Tokyo, Japan; 5Mineralogical State Collection Munich, Germany; 6Tokyo; 7Okayama, Japan; 8Brügg, Switzerland.

The New Basaltic Angrite Northwest Africa 12774

Introduction Angrites and NWA 12774 1a A new angrite, NWA 12774, was reported in the Meteoritical There are several specific characteristics of angrites, most Bulletin 108 [1]. The , a single stone with a weight of important: (a) very low or no shock, (b) presence of a large variety 454 gr, represents the 18th unpaired angrite, quite a rare group of silico-phosphate phases and (c) significant contents of of [1-5]. A very limited number of finds (20) and partly large-sized spinels of various compositions (see also only one reported fall, Angra dos Reis (1869, Brasil) are known [1]. magnetic signature) [1-5]. The latter concerns hercynite (Fe Four groups of angrites can be discriminated presently in terms of and Al rich), chromite and Ti-Al-Mg substituted magnetite, but petrogenesis: (1) basaltic-quenched (fine-grained), (2) plutonic- also stoichiometric magnetite which is quite rare in . subvolcanic (coarse-grained), (3) NWA 2999 and pairs – possibly a The following phases have been identified by Raman regolith - and (4) NWA 8535, the first dunitic angrite (deep Spectroscopy in NWA 12774 (see also figs. 3,4): mantle) [1-5]. (Fo-Fa), Kirschsteinite - Laihunite, fassaite (CPX), and other Fe-sulfides, feldspar (anorthite), silico-phosphates, In 2019 we have started with pilot studies on a slice of NWA 12774 amorphous / graphitic carbon, , numerous inclusions in [2]. NWA 12774 is characterized by a very fresh-looking interior 1b olivine and CPX, a range of opaque phases. No indications without fusion crust and is classified as a quenched-basaltic olivine- for shock or terrestrial alterations have been found so far. phyric angrite (so belonging to group 1) with large xenocrysts of olivine and Al-Ti augite arranged in a dark-black, fine-grained 2a 2b groundmass [2-4]. Additional results on mineralogy, petrology and chemical composition are reported by [3,4] in this conference.

In our poster contribution we will present results of investigations by mineralogical and magnetic means, and by Raman spectroscopy (phase composition). The data will be compiled with results of our systematic studies performed on a Fig. 1 (a): Main mass of NWA 12774 [© K. Wimmer], (b) full slice Fig. 2: Optical microscopy on NWA 12774: (a) Olivine and fassaite larger set of angrites (Angra dos Reis, d’Orbigny, Sahara 99555, of the main mass, cut and polished. Upper right – up to 5mm with opaque inclusions (eg Fe-oxides, Fe-sulfides, Fe-Ni metal), NWA 1670, 2999, 4590 and 7203). sized olivine megacrysts [© www.saharagems.com]. magn (500x). (b) Fe-Ni metal droplets in matrix olivine (200x). Mineralogy and Phase Composition of NWA 12774 Launch Pairing with Asuka 881371/12209?

SP Silico-phosphate (SP) Olivine (Fo-Fa, Ol) Troilite (T) C Carbon (C) phases Ol

C T

SP

T

Feldspar (anorthite) An K-L Anorthite (An) (minor other phases) Kirschsteinite-Larnite (K-L) An Fig. 5: Asuka 12209 main mass (paired with Asuka 881371, inset [4]) [© Japan Polar Research Assoc.]. An There are significant similarities of NWA 12774 to Asuka 12209, both stones are probably launch paired and crystallized within the same lava flow. The stones are characterized by a porphyritic texture with mm-sized olivine xenocrysts in a dark- grey medium-grained matrix dominated by olivine, clinopyroxene and anorthite [1,5]. Asuka 12209 was found in 2012 in Antarctica in the Nansen icefield near the Japanese Asuka Station by a joint Japan / Belgium meteorite expedtition. The stone is paired with Asuka Fig. 3a,b: Typical Raman spectra of NWA 12774 phases: (a) Silico- Fig. 4a,b: Typical Raman spectra of NWA 12774 matrix : (a) 881371 which was found in the same area already in the year phosphate and amorphous carbon phases; (b) typical spectrum of Fo-rich olivine and troilite, (b) Olivine (Kirschsteinite-Larnite series) 1988 [NIPR-JARE, 4,5,6]. anorthite without any shock indications. and anorthite. Magnetic signature and classification Summary and Conclusions

The magnetic signature and paleomagnetic signal of the Angrite L MS O MS Average Presently 21 unpaired angrites have been published in angrites was topic of only a very few investigations. The 1. Basaltic / Quenched Angrites MS: 2.97 Meteoritical Bulletin (4/2020). NWA 12774 is a typical member general magnetic signature was studied by [12,13]. On several of the basaltic/fine-grained angrite group. The stone is D’Orbigny 3.11 2.99 3.05 investigated angrites a strong and stable magnetic remanence characterized by a very fresh interior, lacking terrestrial was detected which was interpreted as an original thermomagnetic LEW 87051 2.62? --- alterations or shock features. Our investigations brought remanence. The record points to a paleomagnetic field intensity of NWA 1296 2.92 2.92 fascinating insights concerning the complexity of the phase about 10 µT on the angrite (4564-4558 Mill. years ago, NWA 1670 2.98 2.80 2.89 composition and texture. The quite large-sized inclusions of formation of the lava flows) of which the origin is still under debate: silico-phosphates will allow much more detailed structural and NWA 7203 int 3.00 3.00 core-dynamo or solar nebula field? [13]. We found original magnetic -chemical investigations on this very rare components ext 3.17 3.17 remanences in all studied angrites (see tab. 1) but not in NWA 2999 in future. NWA 12774 fits well with the general trend of the / 7203 (details in [14]). Sahara 99555 2.93 3.02 2.98 published magnetic characteristics of basaltic angrites. The dominating magnetic recorders were found to be low-Ti NWA 12774 2.98 +/- 0.03 2.98 titanomagnetite (near magnetite) in most studied angrites, whereby 2. Plutonic / Subvolcanic Angrites MS: 2.96 in NWA 2999/pairs kamacite (Fe-Ni) and magnetite represent the References major magnetic recorders (details in [14]). LEW 86010 2.62? --- Angra dos Reis (fall) 2.61 3.26 2.94 [1] NWA 12774, Angrites: Meteoritical Bulletin 04/2020. [2] Wimmer K. et al., 2019. NWA 12774, Paneth #0050. In this contribution we focus on the compilation of the magnetic NWA 4590 2.99 2.95 2.97 susceptibility (MS) only, which is a very helpful and independent [3] Hayashi H. et al., 2020. NWA 12774, LPSC #2360. NWA 4801 / 8405 2.96 2.96 classification parameter [11-14], see table 1. MS of the new [4] Irving A. et al., 2020. NWA 12774, LPSC #2399. basaltic/quenched angrite NWA 12774 gave a value of 3. Regolith Breccia (Iron-Meteor. Component) [6] [5] www.meteoritestudies.com, 04/2020. NWA 2999 and pairs 4.54 4.43 4.49 [6] Jambon A., 2012. LPSC, #1758. [7] Keil K., 2012. Geochemistry, 72, 191-218. 2.98 +/ 0.03 log MS [10-9 m3/kg] 4. Dunitic Angrite (Deep Mantle) [8] Yanai K., Kojima H., 1995. Catalogue Antarct. Meteor., NIPR. NWA 8535 No Data No Data which represents an average MS value of a set of investigated [9] Mikouchi T. et al., 2016. Antarct. Meteor. NIPR, OA-00190-01. fragments / slices [2]. Generally, only falls are useful for a well Tab. 1: Compilation of angrite MS data (log MS in 10-9 m3/kg). [10] Mikouchi T. et al., 2011. 1st Solids Solar System, #9142. constrained magnetic classification, however in the case of angrites Literature values (L) are from [11,12], (O) are new, original data. [11] Macke R., 2010. PhD Thesis, Univ. Central Florida, 332 pp. we have only one fall. Tab. 1 provides an overview of all angrite MS Surprisingly, the average MS values of the basaltic and plutonic [12] Rochette P. et al., 2009. Meteor. Planet. Sci. 44, 405–427. data. MS of LEW 86010/87051 has to be claryfied because the angrites are indistinguishable and the influence of terrestrial [13] Weiss B.P. et al., 2008. Science 322, 713-716. stones are not paired and belong to two different angrite groups alteration effects are only minor; MS of NWA 12774 fits very well in [14] Hoffmann V. et al., 2020, in preparation. which was not indicated in [12]. NWA 2999/4590 have been this scheme. MS of NWA 2999/pairs is about 50% higher which provided by A. Irving, Univ. Washington, for our investigations. proves the presence of a (impacted) Ni-Fe component.