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Petrology, Texture, and Impact-Shock Effects of a Low-Ti Mare Basalt

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Petrology, Texture, and Impact-Shock Effects of a Low-Ti Mare Basalt Meteoritics & Planetary Science 44, Nr 1, 87–94 (2009) Abstract available online at http://meteoritics.org Lunar meteorite LaPaz Icefield 04841: Petrology, texture, and impact-shock effects of a low-Ti mare basalt Eddy HILL1, 2*, Lawrence A. TAYLOR1, Christine FLOSS3, and Yang LIU1 1Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA 2Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA 3Laboratory for Space Sciences, Washington University, Saint Louis, Missouri 63130, USA *Corresponding author. E-mail: Eddy Hill <[email protected]> (Received 04 June 2008; revision accepted 09 August 2008) Abstract–Found during the 2004 U.S. Antarctic Search for Meteorites season, LaPaz Icefield (LAP) 04841 represents an addition to the LaPaz lunar basalts suite and brings the total mass collected to 1.93 kg. The presence of FeNi grains, troilite, and the anorthositic composition of plagioclase are evidence for the lunar origin of this meteorite. Pyroxene and olivine Mn/Fe values plot along the trend set for lunar basalts. Analyses of chromite grains provide a V/(Al + Cr) ratio of 1.33 ± 13, translating to an fO2 one log unit below the IW buffer, in accordance with previous fO2 estimates for lunar basalts. Application of the Zr-cooling speedometer, for ilmenite and ulvöspinel pairs, gives a cooling rate of 5.2 °C/day, matching previous estimates of cooling rates for the LaPaz lunar meteorites and Apollo mare basalts. Mineral modes and chemistries, as well as trace-element patterns, provide compelling evidence for pairing of this meteorite to others in the LaPaz lunar basalt suite. INTRODUCTION SX-50 electron microprobe (EMP). Trace-element concentrations were determined with the Cameca IMS 3f The 2004 Antarctic Search for Meteorites resulted in the secondary ion mass spectrometer (SIMS) at Washington addition of a sixth LaPaz lunar basalt to the collection. This University. Analytical methods are as those described by Day find of LaPaz Icefield (LAP) 04841 measured 5 × 2.5 × 2.5 cm et al. (2006). and weighed 0.06 kg, bringing the total mass of LaPaz lunar basalts to 1.93 kg. Preliminary reports suggest that this new Petrography and Textures find may be paired to the other five LaPaz meteorites (Antarctic Meteorite Newsletter 2006) and is most similar to LAP 04841 is a coarse-grained, holocrystalline, low-Ti LAP 02205. Pairing of LAP 04841 would increase the volume lunar basalt with ophitic texture (Fig. 1a). The major of material available for study of this one lunar basaltic flow. mineralogy consists of pyroxene and plagioclase, with We present a detailed study of petrography, mineral interstitial ilmenite. Minor phases include olivine, Ti- chemistry, and shock deformation features of LAP 04841. chromite, ulvöspinel, and chromian ulvöspinel. The Two polished thin sections (LAP 04841,5 & 14), and a thick relationship between the latter three can be seen in Fig. 3. section (LAP 04841, 18), with respective surface areas of Mesostasis consists of fayalite, SiO2, K-rich glass, and trace approximately 0.5 cm2, 0.7 cm2, and 1.3 cm2, formed the amounts of FeNi, troilite, ilmenite, ulvöspinel, phosphates, basis for this study. Sections 04841,5 and 14 originate from and baddeleyite. The order of crystallization, inferred from LAP 04841,1; a tetrahedron shaped piece of LAP 04841 mineral associations, was determined as: First, Ti-chromite + approximately 1.2 × 1.5 cm. Section 04841,18 comes from olivine; pyroxene + ulvöspinel; pyroxene + plagioclase + LAP 04841,16; a portion approximately 1 cm away from LAP ilmenite; and late-stage mesostasis. Representative EMP 04841,1 (K. Righter, personal communication). analyses of significant phases are presented in Table 1. All slides contain evidence of impact-induced shock RESULTS effects. These include imbricated melt veins, micro-faults and step-faults, undulatory to mosaic extinction and shock Major-element compositions of glass and mineral phases, induced lamellae of pyroxene, and undulatory extinction and as well as modal proportions, were obtained using a Cameca maskelynization of plagioclase. 87 © The Meteoritical Society, 2009. Printed in USA. 88 E. Hill et al. Fig. 1. Electron microprobe backscattered images displaying the texture and mineralogy of LAP 04841. a) LAP 04841 is a coarse-grained, holocrystalline, ophitic basalt with pyroxene (Px), plagioclase (Pl) and ilmenite (Ilm) as its main phases. Mesostasis (MS) can be seen between pyroxene and pyroxene-plagioclase contacts. b) Titaniferrous chromite (TiC) and chromian ulvöspinel (CrU) appear in association with Fig. 2. Range of chemical compositions exhibited by the major pyroxene. Olivine (Ol) is fully included within pyroxene. The arrowed silicate phases in LAP 04841 and comparison to data from Day et al. line indicates the approximate analysis path for the data presented in (2006). Pyroxene (a) compositions cover almost the entire spectrum, Table 2. Pyroxene has progressive Fe-enrichment from core to rim. ranging from low-Ca, hi-Mg augites to extreme Fe-enrichment approaching ferrosilite and hedenbergite. The range of values Mineralogy matches those obtained by Day et al. (2006) (gray box) for the other LaPaz lunar basalts. Plagioclase (b) has anorthitic compositions that match the tight range observed in lunar basalts. Olivine (c) Pyroxene is the most abundant of the phases present and compositions range from high-Fo to extreme Fe-enrichment in the accounts for 56.2 vol% of the samples. Grains are anhedral mesostasis, reaching Fa98. and range in size up to ~0.5 mm. Pyroxenes display an ample range of compositions (Fig. 2). Cores are pigeonite and progressive Fe enrichment to high-Fe augite (Fs59Wo39), and compositions grade continuously, from core to rim, with pyroxferroite (Fs86Wo13) (Fig. 1b and Table 2). Ti/Al and Ti/ Lunar meteorite LAP 04841: Petrology, texture, and impact-shock effects of a low-Ti mare basalt 89 Table 1. Representative EMP analyses of silicate and oxide phases in LAP 04841. Pyroxene Pigeonite Fe-pyroxene < 5% enstatite Plagioclase Fayalite Olivine K-glass SiO2 51.2 46.0 51.2 46.4 29.7 36.3 78.6 TiO2 0.52 1.15 0.45 0.14 0.03 Al2O3 0.79 1.18 0.95 33.0 11.5 Cr2O3 0.26 <0.03 0.45 <0.03 0.22 MgO 16.2 1.85 17.2 0.26 0.64 30.0 <0.03 CaO 5.55 10.2 6.64 18.1 0.62 0.37 1.45 MnO 0.42 0.54 0.38 0.72 0.30 FeO 24.7 38.5 22.4 0.61 67.8 33.0 1.84 BaO 0.55 Na2O <0.03 <0.03 <0.03 1.01 0.24 K2O 0.03 6.46 Total 99.7 99.4 99.7 99.4 99.7 100.2 100.6 Mesostasis Ilmenite ilmenite Ti-chromite Ulvöspinel Cr-ulvöspinel SiO2 0.03 0.09 0.21 0.10 0.06 TiO2 52.1 51.5 5.65 29.9 26.6 ZrO2 0.04 0.28 Al2O3 0.14 0.08 11.7 2.58 4.65 V2O3 0.73 0.17 0.31 Cr2O3 0.13 0.07 43.0 6.75 13.6 MgO 0.13 0.11 1.87 0.52 2.18 CaO 0.18 0.04 0.03 0.04 0.10 MnO 0.43 0.41 0.30 0.36 0.23 FeO 46.6 46.9 35.9 59.3 52.6 Total 99.8 99.5 99.5 99.9 100.4 Table 2. Representative EMP transect of pyroxene crystal. Pyroxene transect Core Rim SiO2 52.6 52.1 50.2 48.7 47.7 46.5 46.3 46.2 46.2 46.3 45.5 TiO2 0.48 0.57 1.18 1.34 1.17 1.10 0.83 0.59 0.59 0.59 0.60 Al2O3 1.00 1.20 1.98 1.46 1.24 1.18 0.77 0.53 0.51 0.48 0.62 Cr2O3 0.41 0.43 0.51 0.17 0.11 0.09 0.06 <0.03 <0.03 <0.03 <0.03 MgO 18.7 17.7 13.6 8.43 4.58 1.90 1.36 0.85 0.83 0.48 0.44 CaO 5.31 7.18 10.5 11.7 11.8 10.6 8.17 5.88 5.76 6.36 5.41 MnO 0.40 0.39 0.41 0.42 0.50 0.54 0.50 0.61 0.67 0.60 0.50 FeO 21.9 21.0 22.2 28.7 33.4 38.5 42.5 45.8 46.0 45.5 47.4 Na2O <0.03 <0.03 0.04 0.05 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 Total 100.7 100.6 100.6 100.9 100.4 100.5 100.4 100.5 100.6 100.3 100.4 CPFU Si 1.97 1.96 1.92 1.93 1.94 1.94 1.96 1.97 1.97 1.98 1.96 Ti 0.01 0.02 0.03 0.04 0.04 0.03 0.03 0.02 0.02 0.02 0.02 Al 0.04 0.05 0.09 0.07 0.06 0.06 0.04 0.03 0.03 0.02 0.03 Cr 0.01 0.01 0.02 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 1.04 0.99 0.78 0.50 0.28 0.12 0.09 0.05 0.05 0.03 0.03 Ca 0.21 0.29 0.43 0.50 0.51 0.48 0.37 0.27 0.26 0.29 0.25 Mn 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Fe 0.69 0.66 0.71 0.95 1.14 1.34 1.50 1.63 1.64 1.63 1.70 Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total 3.99 3.99 3.99 4.00 3.99 3.99 4.00 4.00 4.00 3.99 4.01 Wo 11 15 22 26 27 25 19 14 13 15 13 En 54 51 41 26 14 6 4 3 3 2 1 Fs 35 34 37 49 59 69 77 83 84 83 86 CPFU-Cations per formula unit; Wo-Wollastonite; En-Enstatite; Fs-Ferrosilite.
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