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The New Polymict Eucrite Dar Al Gani 983: Petrography, Chemical Composition, Noble Gas Record, and Evolution

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The New Polymict Eucrite Dar Al Gani 983: Petrography, Chemical Composition, Noble Gas Record, and Evolution Meteoritics & Planetary Science 40, Nr 6, 869–879 (2005) Abstract available online at http://meteoritics.org The new polymict eucrite Dar al Gani 983: Petrography, chemical composition, noble gas record, and evolution Andrea PATZER1*, Jochen SCHL‹TER2, Ludolf SCHULTZ3, Dolores H. HILL1, and William V. BOYNTON1 1Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA 2Mineralogisches Museum der Universit‰t Hamburg, D-20146 Hamburg, Germany 3Max-Planck-Institut f¸r Chemie, D-55020 Mainz, Germany *Corresponding author. E-mail: [email protected] (Received 24 September 2004; revision accepted 07 April 2005) Abstract–Mineralogical and chemical studies of Dar al Gani 983 show that this meteorite is a eucrite. Its texture is that of an impact breccia. It contains cumulate pyroxene and feldspar megacrysts, a variety of recrystallized melt clasts, clasts of subophitic basalt, and mesostasis. These components are embedded in a matrix of fragmental pyroxene and plagioclase. In addition, the entire rock is penetrated by glassy melt veins and patches, and displays features of strong shock. The mineralogical and chemical evidence obtained for DaG 983 indicates that this meteorite experienced a complex evolutionary history. The presence of cumulate silicate crystals implies substantial, large scale cratering events on the HED asteroid. As a result of these impacts, rocks from different intrusive bodies to extrusive surface layers were laterally and vertically transported to form a thoroughly mixed megaregolith. DaG 983 represents a sample of this megabreccia. INTRODUCTION provide an outline of the geological settings involved in its formation. Our constraints on the evolutionary path of this Eucrites are differentiated achondrites and share a meteorite were derived from electron microprobe, common provenance with howardites and diogenites (e.g., instrumental neutron activation analysis (INAA), and noble Tschermak 1885; Stolper 1977; Clayton and Mayeda 1996). gas data (for noble gas data see Patzer et al. 2003a). Eucrite mineralogical composition is basaltic. The howardite- eucrite-diogenite (HED) suite is unique among meteorite SAMPLE AND ANALYTICAL TECHNIQUES chemical associations in that it is almost certainly linked to a specific asteroid, 4 Vesta (see Drake 2001, and references Sample therein). Eucrites can be subdivided into noncumulate, cumulate, DaG 983 was recovered during a German-Libyan and polymict types. The noncumulate eucrites are further expedition (J. Schl¸ter and F. Thiedig, Hamburg; L. Schultz, subdivided into three different compositional series: main Mainz; Abd Elfatah M. Abu-Aghreb, Tripoli) from the Dar al group; Stannern trend; and Nuevo Laredo trend. These Gani Plateau in Libya in March 2002 (Schl¸ter et al. 2002). It subtypes are classified by their mg# and Ti contents (BVSP was found as a single stone of 933 g partly covered with black 1981). fusion crust. The interior of the rock is light grey in color, Considerable work has addressed the metamorphism and displaying open cracks and some melt veins with microscopic impact heating of eucrites (e.g., Nyquist et al. 1986, 1997; vesicles. Occasionally, the cracks and vesicles contain white Takeda and Graham 1991; Metzler et al. 1995; Takeda 1997; aggregates of gypsum or calcite. The meteorite’s texture is Yamaguchi et al. 1997a, 2001). Almost all eucritic samples that of a breccia with relatively large light and dark clasts show signs of thermal overprinting and many breccias formed being embedded in a fine-grained fragmental matrix (Fig. 1; by impact mixing. It has been demonstrated that the see Russell et al. 2003). bombardment history of the HED parent body equals the Moon’s in complexity (Bogard 1995). Petrography, Bulk, and Mineral Analyses Our work focuses on the petrographic description of a new eucrite from Libya, Dar al Gani (DaG) 983. We also Two thin sections of DaG 983 (adjacent faces within 869 © The Meteoritical Society, 2005. Printed in USA. 870 A. Patzer et al. Fig. 1. The interior of the DaG 983 polymict eucrite appears unaltered. The influence of weathering, however, is obvious from a few cracks and vesicles that are filled with terrestrial gypsum and calcite microcrystals. The brecciated character of the sample is readily noticeable. Some clasts measure up to 10 mm (the slab has a size of about 50 × 70 mm; photo taken by K. C. Lyncker). ∼2 mm) were prepared for optical microscope studies and synthetic standards and standard rocks. Details on the electron microprobe measurements. The chemical analyses of irradiation procedure and used standards have been reported the meteorite’s mineral components were, in part, carried out by Patzer et al. (2004). Elemental concentrations of the with a Cameca Camebax SX 100 Microbeam wavelength- samples were determined by the comparator method. The dispersive electron microprobe at the Department for samples experienced a neutron flux of 6.3 × 1013 n/cm2 in the Mineralogy and Petrology of the Universtit‰t Hamburg. rabbit and 7.56 × 1015 n/cm2 in the lazy susan run. Table 2 Operation conditions were set to 20 kV and 25 nA. For all shows the bulk elemental concentrations obtained by INAA. elements, the kα X-ray line was applied. Pure elements (Ni and Co), andradite (for Ca and Si), apatite (for P), vanadinite RESULTS AND DISCUSSION (for V), as well as synthetic MgO (for Mg), Al2O3 (for Al), ZnS (for Zn and S), MnTiO3 (for Ti and Mn), Cr2O3 (for Cr), Weathering Degree, Shock Stage, and Petrographic and FeS2 (for Fe) were used as standards. Additional Description comprehensive analyses were conducted with a Cameca SX- 50 electron microprobe at the Lunar and Planetary The occurrence of secondary gypsum and carbonate Laboratory, Tucson, Arizona. This microprobe study included microcrystals in cracks and vesicles of DaG 983 indicates that major element quantification, backscattered electron imaging, the meteorite is slightly altered by terrestrial weathering (the and X-ray mapping. An accelerating voltage of 15 kV, a Dar al Gani meteorite field in Libya is situated on Tertiary sample current of 20 nA, and 20 sec peak count times were dolomites and limestone) (Schl¸ter et al. 2002). applied for all analyses. Natural and synthetic standards were The presence of heavily fractured mineral phases and used (see Kring et al. 1996). All data (Hamburg and Tucson) strong mosaicism suggest that DaG 983 is severely shocked. were corrected for absorption, fluorescence, and atomic Most of the plagioclase has been converted into maskelynite. number effects based on the correction method introduced by Abundant glassy melt veins crosscut the rock. In addition, Pouchon and Pichoir (1991). The elemental compositions of some pyroxenes appear to show signs of incipient the major and minor mineral phases are listed in Tables 1a and decomposition or melting. According to the criteria of 1b. Stˆffler et al. (1988), the shock features observed in DaG 983 For the INAA, three chips of 0.174, 0.147, and 0.185 g are stage 2b. were broken off of a subsample. The 0.185 g-aliquant Petrographically, DaG 983 belongs to the polymict contained a macroscopically notable dark clast. The breccias. It is composed of four different types of clasts that irradiation took place at the reactor facility (TRIGA) of the are embedded in a fragmental groundmass of variably coarse- University of Arizona. The specimens were first exposed to a grained pyroxene and plagioclase. Clast types include high flux (“rabbit”) irradiation for 60 seconds followed by a predominant monomineralic pyroxene and feldspar 3-hour (lazy susan) irradiation. Irradiations included both megacrysts, recrystallized melt pockets, and basaltic Table 1a. Chemical composition of selected silicate phases in DaG 983 (av. ox%). Plagioclase Plagioclase Plagioclase Pigeonite Pigeonite Augite Augite Silica Shock melt megaclasts melt pockets spinifex melt megaclasts clast 10 lamellae grains polymorph veins ± ± ± ± ± ± ± ± ± SiO2 45.1 1.3 45.9 1.9 49.0 2.8 49.7 0.9 51.4 0.3 50.3 1.2 51.3 0.9 98.6 1.5 48.0 0.7 ± ± ± ± ± ± ± ± ± Al2O3 34.9 0.8 34.0 1.5 32.9 2.1 0.39 0.23 0.62 0.13 0.75 0.18 0.38 0.03 0.52 0.64 19.3 2.3 CaO 18.1 ± 0.7 17.8 ± 0.5 16.3 ± 1.2 1.71 ± 1.80 2.10 ± 0.38 19.2 ± 1.8 18.7 ± 4.0 0.30 ± 0.37 13.0 ± 0.9 ± ± ± ± ± Na2O 1.10 0.25 0.88 0.12 1.63 0.21 b.d. b.d. 0.05 0.02 b.d. b.d. 0.93 0.15 MgO b.d. 0.19 ± 0.20 b.d. 13.9 ± 0.5 18.4 ± 0.5 11.5 ± 0.2 11.6 ± 0.3 b.d. 5.32 ± 1.03 FeO 0.15 ± 0.14 1.01 ± 0.23 0.59 ± 0.08 32.1 ± 1.9 25.7 ± 0.7 16.4 ± 2.1 16.4 ± 4.1 0.52 ± 0.53 11.6 ± 1.8 ± ± ± ± K2O 0.05 0.02 0.05 0.02 0.12 0.04 b.d. b.d. b.d. b.d. b.d. 0.06 0.02 ± ± ± ± ± Cr2O3 b.d. b.d. b.d. 0.48 0.80 0.28 0.20 0.83 0.66 0.42 0.26 b.d. 0.17 0.05 MnO b.d. b.d. b.d. 1.03 ± 0.08 0.83 ± 0.04 0.53 ± 0.07 0.56 ± 0.11 b.d. 0.38 ± 0.06 ± ± ± ± ± ± TiO2 b.d. b.d. b.d. 0.19 0.10 0.13 0.03 0.44 0.42 0.15 0.05 0.17 0.39 0.76 0.33 Total 99.5 ± 0.4 99.9 ± 0.8 100.6 ± 0.5 99.5 ± 0.6 99.5 ± 0.5 99.9 ± 1.0 99.6 ± 0.5 100.2 ± 0.7 99.5 ± 0.6 n 67 13 3 57 7 7 5 20 19 The newpolymicteucriteDaralGani983 871 An89.8 ± 2.5 An91.5 ± 1.1 An84.1 ± 3.0 En42.2 ± 0.9 En53.6 ± 1.4 En33.2 ± 0.4 En33.9 ± 1.2 Ab9.85 ± 2.40 Ab8.19 ± 1.04 Ab15.2 ± 2.7 Fs55.5 ± 1.4 Fs42.0 ± 1.0 Wo42.4 ± 1.0 Wo39.2 ± 8.0 Or∼0.3 Or∼0.3 Or∼0.7 Wo2.38 ± 1.32 Wo4.3 ± 9 0.80 Fs24.4 ± 0.9 Fs26.9 ± 6.9 b.d.
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