Petrogenesis of a Vitrophyre in the Martian Meteorite Breccia Nwa 7034
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45th Lunar and Planetary Science Conference (2014) 1948.pdf PETROGENESIS OF A VITROPHYRE IN THE MARTIAN METEORITE BRECCIA NWA 7034. A. Udry1, N. G. Lunning1, and H. Y. McSween Jr.1, 1Planetary Geosciences Institute, Dept. of Earth and Planetary Sci- ences, University of Tennessee, Knoxville, TN 37996, USA. Introduction: The martian meteorite breccia of Fo55-63. The olivines are too Fe-rich to be in equilib- Northwest Africa (NWA) 7034 and its paired meteor- rium with the pyroxenes. The groundmass is generally ites NWA 7533 and NWA 7475 are the first samples composed of glass, most of which has a nearly homo- of martian regolith available for study on Earth. These geneous basaltic composition. Different localized are likely polymict breccias because separate clasts compositions of glass likely represent silica and alkali have chemical compositions [2-4] that are not con- feldspar in the protolith that were not fully homoge- sistent with a single source. NWA 7034 contains a nized during melting. The pyroxene in the vitrophyre is variety of clasts, similar to clasts also described in enriched in light REE (LREE) compared to pyroxenes NWA 7533: gabbroic clasts consisting of pyroxene, in other martian meteorites. feldspar, phosphates, and oxides, phosphate-rich clasts We calculated the bulk-vitrophyre major element containing the same phases as the gabbroic clasts, but composition using modally weighted EMP analyses of with larger apatites and oxides, a mugearite clast, a pyroxene, olivine, and the common basaltic glass (Fig. trachyandesite clast, and two types of “melt” clasts [1- 2). The vitrophyre plots in the basaltic field in the TAS 3]. [1] found small melt spherules, and a large vitro- diagram and has an alkali content similar to Gusev phyric clast containing skeletal pyroxene and olivine in basalts, whereas the bulk-breccia composition [1] a glassy matrix. shows enrichment in alkalis compared to the vitro- We focused our study on this single large vitro- phyre and martian meteorites (Fig. 2b). Si, Al, and Mg phyric clast in NWA 7034 (Fig. 1). This melt clast has contents of the vitrophyre are closer to Gusev basalt a texture not previously observed in martian meteorites compositions than to any martian meteorites. The and may represent a new martian composition. It is NWA 7034 whole-breccia composition [1] is lower in likely to be compositionally representative of the pro- Fe (10 wt %) than the vitrophyre (15.5 wt%), Gusev tolith rock based on its large size (5 x 4.5 mm) and basalts, and most of the martian meteorites (Fig. 2a). relatively fine-grained texture. Some clasts observed in NWA 7533 by [5] defined as clast-laden impact melt rocks (CLIMR) have similar major element compositions to the NWA 7034 vitro- phyre and more closely resemble the latter than the NWA 7034 host breccia. In addition, the vitrophyre clast composition falls in the Gusev soils field [6] ex- cept it is more enriched in Fe than Gusev soils. The vitrophyric clast pattern is parallel to the NWA 7034 host breccia REE pattern [1]. The skeletal textures of both olivine and pyroxene suggest a high degree of undercooling. According to crystal growth experiments, these features correspond to very fast cooling rates, on the order of thousands of degrees Celsius per hour [7-8]. Ni content in the bulk-vitrophyre (1020 ppm) is significantly higher than in other martian basalts, in- Fig. 1 X-Ray Mg map of the whole NWA 7034 vitrophyre cluding the whole-rock NWA 7034 breccia (392 ppm) clast. [1] but is similar to the Gusev soils. Additionally, the Results and Discussion: Modal abundances in the Ni content of the vitrophyre is higher than in NWA clast are 75% glass, 18% pyroxene, and 7% olivine. 7533 CLIMR (589 ppm) [5], which are unambiguously The edges of the clast show a 300-µm wide rim that is impact-melt rocks based on their Ir concentration. If distinct from the clast interior. This rim contains finer- the vitrophyre formed by impact melting of the Gusev grained acicular pyroxene aligned parallel to the con- basalt Humphrey (Mg/Ni = 382), the indigenous Ni tact, as well as a higher abundance of Fe-oxides and an would be 170 ppm and thus 850 ppm Ni would be absence of olivine. The elongated skeletal pyroxene from a non-martian source. The exogenic Ni is equiva- grains show slight compositional zoning, with an aver- lent to 7.7 wt % CI chondritic contamination. age core composition of En69Wo3Fs28 and average rim Low Zn and S concentrations suggest that the pro- composition of En60Wo5Fs35. The olivine grains have tolith of the vitrophyre did not include martian soil. a hopper-like texture and are zoned with compositions The absence of a soil component differs from both the 45th Lunar and Planetary Science Conference (2014) 1948.pdf host breccia and the observations of the NWA 7533 CLIMR [5]. The vitrophyre bulk major and trace element com- position is different from the NWA 7034 bulk-breccia composition [1]. Using the least-squares method, we compared the bulk composition of the vitrophyre to known martian igneous rock compositions: the vitro- phyre composition most closely resembles Humphrey rock, one of the Gusev basalts (Fig. 2). Thus, it is very likely that this vitrophyre represents an impact melt of a martian basalt with a Humphrey-like composition that was contaminated by a chondrite impactor. Summary and Conclusions: The vitrophyre is an impact melt. The exogenic Ni component (850 ppm) is evidence for its impact origin, as is its occurrence with CLIMR in a regolith breccia. Based on the vitrophyre’s bulk composition, the protolith was not the host breccia but another martian rock. We compared it to all analyzed martian basalts, and the major element composition of the bulk- vitrophyre is most similar to the Gusev crater basaltic rock Humphrey. We infer that the target rock was a Humphrey-like igneous rock, for which we did not have a meteorite analog prior to this study. These martian breccias potentially include samples Fig. 2. Bulk composition diagrams for NWA 7034 bulk vitro- of other igneous lithologies that are not represented by phyre, basaltic, olivine-phyric, and lherzolitic shergottites, currently described martian meteorites. nakhlites, Chassigny, ALH 84001, Bounce Rock, and Gusev References: [1] Agee C. B. et al. (2013) Science, 339, basalts (Adirondack, Humphrey, Mazatzal) and soils from 780-785. [2] Santos A. R. et al. (2013a) LPS XLIV, Abstract [6;9-23]. These various compositions are plotted on: a) Mn- Fe (wt%) diagram with martian meteorite lines [6], b) Total #2533. [3] Santos A. R. et al. (2013b) MetSoc 76th, Abstract alkalis versus silica (wt%) diagram, c) Ca/Si-Mg/Si (atomic #5284 [4] Hewins R. H. et al. (2013) LPS XLIV, Asbtract ratio) diagram used for classification of martian meteorites, #2385. [5] Humayun M. et al. (2013) Nature, 503, 513-516 d) Mg/Si-Al/Si (atomic ratio) diagram, and e) Ni (ppm)-Mg [6] McSween H. Y. al. (2009) Science, 324, 736-739. [7] (wt%) diagram with terrestrial and martian fractionation Faure F. and Schiano P. (2005) EPSL, 236, 882-898. [8] lines [6]. Faure F. et al. (2003) Contrib. Mineral. Petrol., 145, 251- 263. [9] Lodders K. (1998) MAPS, 33, 183-190. [10] Rubin A. E. et al. (2000) Am. Min., 89, 867-872. [11] Dreibus G. et al. (2000) MAPS, 35, A49 [12] Barrat J.-A. et al. (2002) MAPS, 37, 487-499. [13] Jambon A. et al. (2002) MAPS, 37, 1147-1164 [14] Shirai N. and Ebihara M. (2004) Antarct. Metorite Res., 17, 55-67 [15] Gillet P. et al. (2005) MAPS, 40, 1175-1184 [16] Ikeda Y. et al. (2006) Antarct. Meteorite Res., 19, 20-44 [17] Day J. M. D. et al. (2006) MAPS, 41, 581-606. [18] Anand M. et al. (2008) LPS XXXIX, Abstract #2173 [19] Treiman A. H. and Irving A. J. (2008) MAPS, 43, 829-854. [20] Lin Y. et al. (2008) MAPS, 43, 1179-1187. [21] McSween H. Y. et al. (2008) JGR, 113, E06S04 [22] Basu Sarbadhikari A. et al. (2009) GCA, 73, 2190-2214. [23] Zipfel J. et al. (2011) MAPS 46, 1-20. .