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Northwest Africa 11575: Unique Ungrouped Trachyandesite Achondrite

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Northwest Africa 11575: Unique Ungrouped Trachyandesite Achondrite 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2226.pdf NORTHWEST AFRICA 11575: UNIQUE UNGROUPED TRACHYANDESITE ACHONDRITE. C. B. Agee, M. A. Habermann, K. Ziegler, Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, [email protected]. Introduction: We report here the discovery of a mostly lath-like in shape, and typically 200x50 µm in unique achondritic meteorite, Northwest Africa size (fig. 1b). We also observed ubiquitous potassium (NWA) 11575, which has an estimated trachyandesite feldspar and a silica polymorph, which were common- bulk composition and oxygen isotopes plotting within ly found in contact with each other, and together make the LL-ordinary chondrite field. up ~5% of the modal mineralogy. Minor ubiquitous History and Physical Charateristics: NWA phases make up ~1% of the modal mineralogy; these 11575 was purchased by Darryl Pitt in Mauritania, include ilmenite, iron sulfide phase, iron metal (nickel 2016. The meteorite was reportedly found in June 2016 not detected with EMPA), chromite, phosphate phase, near the border region of Mali and Algeria. The speci- zircon, and iron oxide. men is a single stone, 598 grams, and is ~80% covered by fusion crust. A saw cut surface revealed a light col- 14 NWA 11575 Shock Melt Vein ored surface with an aphanitic texture. A single, ~1 mm wide, dark colored shock melt vein crosscuts this 12 surface. Also present are a few smaller xenoliths which ) 10 Trachyite % t Trachy- are dark in color, angular, and aphanitic; the largest of w ( andesite Rhyolite 8 O which is approximately 1 cm across (see [1] for images 2 Basaltic K trachy- + of the main mass and deposit sample). O 6 Trachy- andesite 2 a basalt NWA Pyroxenes N GRA 06128/9 11575 Di Hd 4 Basaltic Dacite Basalt Andesite Andesite 40 40 2 Picro- basalt NWA 11119 0 35 40 45 50 55 60 65 70 75 20 20 SiO (wt%) 2 RIMS CORES Fig. 2 Total alkalis versus silica (TAS) diagram showing the average En 0 0 Fs composition of a shock melt vein in NWA 11575, which is a proxy 0 20 40 60 80 100 for bulk compositon. Also shown are values for two other magmati- cally evolved achondrites GRA 06128/06129 [2] and NWA 11119 Feldspars [3]. 60 The dark xenolithic clasts consist of pyroxene mi- 70 crophenocrysts set in groundmass of quench crystals 80 and mesostasis, and are described in [1]. The shock Oligoclase melt vein average composition (within the host light 90 K-spar lithology) plots in the trachyandesite field of the TAS 100 diagram with an error elipse that extends into the ande- AB 0 10 20 30 40 50 60 70 80 90 100 OR site and trachyite fields (fig. 2). Fig. 1a (upper) Quadrilateral diagram showing the pyroxene compo- sitions in NWA 11575. The arrows show the approximate zoning Electron microprobe results: Magnesian pigeonite and crystallization trend within individual grains (n=45). Fig. 1b Fs28.3±3.3 Wo7.3±1.6, Fe/Mn=37±4, n=5; augite (lower) Truncated ternary diagram showing the feldspar composi- Fs24.4±4.2 Wo30.0±3.8, Fe/Mn=33±4, n=21; ferropi- tions in NWA 11575 (n=43). geonite Fs57.3±6.2 Wo14.7±3.7, Fe/Mn=48±3, n=19; Petrology: Electron microprobe and SEM exami- plagioclase Ab81.3±1.9 An17.4±2.0 Or1.3±0.2, n=20; nations were performed on a polished probe mount and potassium feldspar Ab4.0±0.9 An0.4±0.3 Or95.6±1.1, on the saw-face of the deposit sample, respectively. n=23; shock melt vein (proxy for light lithology bulk An ophitic texture of pyroxene and plagioclase grains, composition) SiO = 61.0±2.1, Al O = 14.7±2.6, making up approximately 90-95% of the modal miner- 2 2 3 Cr O = 0.11±0.05, MgO = 2.8±1.1, FeO = 8.4±3.1, alogy, was observed. Pyroxenes are typically 300-500 2 3 MnO = 0.17±0.07, CaO = 4.8±0.6, Na O = 6.3±1.1, µm in size and show significant igneous zoning. The 2 K O = 0.6±0.2 (all wt%), n=20. Figure 3 shows the Fe- pyroxene grains have cores of magnesian pigeonite, 2 Mn relationship for NWA 11575 pyroxenes compared outwardly transitioning to a zone of augite, and to planetary pyroxenes. The magensian-rich pigeonite rimmed by a zone of sub-calcic ferroaugite to ferropi- cores and augitic mantles of NWA 11575 pyroxenes geonite (fig. 1a). Plagioclase grains are oligoclase, 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2226.pdf plot between the Earth and Mars, while the ferropi- (fig. 4). However the three data points from different geonite rims plot between the Moon and Mars. fragments of the meteorite, form a slope of 0.53, which is parallel to the mass dependent terrestrial fractiona- PLANETARY PYROXENES .040 tion line (TFL) and suggests igneous differentiation After Papike et al. (2009) [8]. Whether or not NWA 11575 was derived from an .035 LL-ordinary chondrite precursor remains to be tested. .030 On the other hand, the magmatically evolved bulk composition of NWA 11575 is markedly different ) .025 VESTA MARS u f from typical LL-melt rocks which are mafic to ultra- a ( .020 mafic and resemble a chondritic precursor bulk com- n M .015 position. The trachyandesitic composition of the shock MOON melt vein of NWA 11575 plots on the TAS diagram in .010 a broadly similar location to the estimated bulk com- .005 EARTH Pyroxenes NWA 11575 position of ungrouped achondrites GRA 06128/06129 0.000 (fig. 3); and also has similar oligoclase plagioclase 0.0 .2 .4 .6 .8 1.0 1.2 1.4 (fig. 5). However, GRA 06128/06129 are not likely Fe (afu) related to NWA 11575, since they are olivine-bearing, Fig. 3 Fe versus Mn (atomic formula units) diagram for planetary lack K-feldspar, and have oxygen isotopes that plot pyroxenes after [4] showing the range of values for pyroxenes in below the TFL. The recently reported silica-rich NWA 11575 (red dots) (n=45). achondrite NWA 11119 [3] is markedly different from NWA 11575 in that it is strongly depleted in total alka- Oxygen Isotopes: Oxygen isotopes were performed at lis, possesses anorthitic plagioclase, and plots well UNM on 3 acid-washed fragments analyzed by laser fluori- 18 17 below the triple oxygen isotope TFL (fig. 5). nation and gave d O= 4.875, 5.583, 5.349; d O= 3.760, 17 4.137, 4.006; D O= 1.186, 1.189, 1.182 (linearized, all per Solar System Magmatic Rocks mil, TFL slope=0.528), (fig. 4). 1.4 NWA 11575 1.2 4.5 1.0 NWA 11042 .8 NWA 7034 4.0 LL-chondrites .6 ) .4 SNC NWA 11575 ‰ 3.5 ( .2 Aubrites Earth Moon ) O 0.0 W 7 Angrites O -.2 1 M 3.0 Δ GRA 06128/9 HED S - -.4 V ‰ -.6 ( NWA 7034 ' 2.5 O SNC -.8 7 NWA 11119 NWA 7325 1 Lunar δ -1.0 HED 2.0 GRA 06128/29 -1.2 NWA 11042 NWA11119 -1.4 NWA 011 1.5 NWA 7235 -1.6 NWA 6698 NWA 11575 0 10 20 30 40 50 60 70 80 90 100 1.0 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 Plagioclase (AN content) Fig. 5 Plagioclase anorthite content versus D17O diagram showing δ18O' (‰ V-SMOW) Fig. 4 Triple oxygen isotope diagram showing the values for NWA the range of values for NWA 11575 (upper left). Also shown are 11575 (red stars). Also shown are values from other achondrites and value ranges from other achondrites and solar system magmatic solar system magmatic rocks. The field for LL-ordinary chondrites is rocks. shown approximately as a dashed ellipse. References: [1] Habermann M. A. and Agee C. B. Comparison with other Solar System Magmatic (2018) LPS XLIX, this conference. [2] Day J. M. D. et Rocks: Pyroxene compositional core-rim zoning al. (2009) Nature, 457, 179-182. [3] Srinivasan P. et al. trends in NWA 11575, which reflect a change in tem- (2017) 80th Annual MetSoc, Abstract #6129.. perature and composition of the liquid during crystalli- [4] Papike J. J. et al. (2009) Geochimica Cosmo- zation, are similar to that observed in Martian basalt chimica Acta, 73, 7443-7485. [5] McKay G. et al QUE 94201 [4,5] and in Apollo 15 Mare basalts [6]. In (1996) LPS XXVII, 851-852. [6] Kushiro I. (1973) fact, this clinopyroxene differentiation trend toward Carnegie Inst. Yearbook, 72, 647-650. [7] Hargraves extreme enrichment in the ferrosilite component is well R. B. et al. (1970) Science, 167, 631-633. documented in lunar igneous rocks, but is unknown in [8] Greenwood R. C. et al. (2017) Chemie der Erde, terrestrial pyroxenes [7]. Oxygen isotopes of NWA 77, 1-43. 11575 plot within the field defined by LL-chondrites .
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