¹Adriana M.Mitchell,²Vishnu Reddy,²Benjamin N.L.Sharkey,³Juan A.Sanchez,⁴Thomas H.Burbine,³Lucille Le Corre,⁵Cristina A.Thomas
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113426]
¹College of Optical Sciences, University of Arizona, 1630 E University Blvd, Tucson, AZ 85719, United States of America
²Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ 85719, United States of America
³Planetary Science Institute, 1700 E Fort Lowell, Suite 106, Tucson, AZ 85719, United States of America
⁴Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075, United States of America
⁵Northern Arizona University, PO Box 6010, Flagstaff, AZ 86011, United States of America
Copyright Elsevier

Ordinary chondrites comprise a significant fraction (~75%) of meteorites that fall on the Earth. A key goal in small body science is to link meteorites to their parent bodies in order to understand their formation conditions early in our Solar System history. Dunn et al. (2010a) provided a robust set of equations for deriving olivine/pyroxene chemistry and abundance ratio from visible and near-infrared (0.35–2.5 μm) spectra of silicate-rich asteroids. These equations were calibrated to X-ray diffraction (XRD) and electron microprobe measurements as ground truth for the spectrally derived values. The small body community employs a range of methods to extract spectral band parameters from telescopic spectra of S-/Q-type asteroids and use the Dunn et al. (2010a) equations to constrain mineral chemistry and abundance. The goal of this work is to understand how the changing of polynomial order and method of extracting spectral band parameters from spectra of ordinary chondrite meteorites affects the precision of derived olivine and pyroxene chemistry and abundance compared with laboratory XRD and microprobe values. Based on our analysis, we find that 2nd order polynomials provide good agreement with the linear relationship found by Dunn et al. (2010a), but with a systematic offset. We also find that Band I center values derived from differing polynomial orders cannot be used for extracting mineral chemistry with Dunn et al. (2010a) equations. We find that the Band Area Ratio (BAR) values are independent of polynomial order and the olivine to pyroxene abundance ratio extracted from BAR is immune to changing polynomial order. Of the four published methods for extracting spectral band parameters (Sanchez et al. 2015, Dunn et al. 2010a, Spectral Analysis Routine for Asteroids, or SARA, Modeling for Asteroids, or M4AST), Dunn et al. (2010a)’s method most successfully reproduces both olivine and pyroxene chemistry, followed by Sanchez et al. (2015). SARA most successfully reproduces the olivine to pyroxene abundance ratio, very closely followed by the other three methods. We find systematic underestimation of ordinary chondrite Band I centers compared to Dunn et al. (2010a) and the resulting chemistry derived from them. To account for this underestimation, we have developed a correction factor for band parameters extracted using 4th order polynomial from the Sanchez et al. (2015) method that must be added to Band I centers for asteroids that fall in H, L, LL chondrite zones when using Dunn et al. (2010a) equations.

¹Minami Yasui,¹Masahiko Arakawa,¹ Yusaku Yoshida,¹Kazuma Matsue,¹ Shota Takano
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113414]
¹Graduate School of Science, Kobe University 1-1, Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan
Copyright Elsevier

We conducted oblique impact experiments for porous gypsum spheres and glass spheres simulating primitive and consolidated rocky planetesimals, respectively, and we determined the effects of the impact angle on the impact strength of these rocky planetesimals. The targets were a porous gypsum sphere with a porosity of 50% and a glass sphere without porosity. A spherical polycarbonate projectile impacted the target at 2–7 km s⁻¹ at an impact angle, θ, ranging from 90° (head-on collision) to 10° (grazing collision) by using a two-stage light-gas gun at Kobe University, Japan. The impact strength obtained at a head-on collision was 1330 J kg⁻¹ for the porous gypsum target and 1090 J kg⁻¹ for the glass target, and these values increased markedly with the decrease of the impact angle when the impact angle was smaller than a critical angle, θ_c; the obtained θ_c values were 30° for the porous gypsum target and 55° for the glass target. The normalized largest fragment mass (m_l/M_t) showed a good correlation with an effective specific energy (Q_eff = Qsin²θ); the subsequent empirical equation was ml/Mt=102.02Qeff0.76 for the porous gypsum target and ml/Mt=104.66Qeff1.68 at m_l/M_t <0.75 and ml/Mt=100.12Qeff0.08 at m_l/M_t >0.75 for the glass target. Based on our experimental results, we successfully introduce the effects of an oblique impact on the degree of disruption for primitive and consolidated rocky planetesimals. Our findings demonstrate that in a strength-dominated regime, the catastrophic disruption can occur over a wide range of impact angles (30°–90°) irrespective of the target materials, when the specific energy at the collision is about four times larger than the impact strength.

¹Christian A.Jansen,¹Frank E.Brenker,²Jutta Zipfel,³Andreas Pack,⁴Luc Labenne,⁵Kazuhide Nagashima,^1,5,6Alexander N.Krot,⁶Martin Bizzarro,⁶MartinSchiller
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125537]
¹Geoscience Institute/Mineralogy, Goethe University Frankfurt, Altenhöferallee 1, 60438, Frankfurt am Main, Germany
²Forschungsinstitut und Naturmuseum Senckenberg, Sektion Meteoritenforschung, Senckenberganlage 25, 60325, Frankfurt am Main, Germany
³Georg-August-Universität, Geowissenschaftliches Zentrum, Goldschmidtstraße 1, 37077, Göttingen, Germany
⁴Labenne Meteorites, Paris, France
⁵Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI, 96822, USA
⁶StarPlan – Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, Copenhagen, DK-1350, Denmark
Copyright Elsevier

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa_18.1–28.3, average Fa_24.9±3.2, PMD = 12.8; Cr₂O₃ = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs_3.2–18.7Wo_0.2–4.5; average Fs_14.7±3.7Wo_1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ¹⁷O ∼ 0.2‒1.4 ‰, average ∼ 0.8 ‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa₂₇) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ¹⁷O of the ferroan olivine (∼ 4 ‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ¹⁸O is significantly higher (15―19 ‰, average = 17 ‰ vs. 3―12 ‰, average = 8 ‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.

¹Natalia HAUSER,¹Wolf Uwe REIMOLD,²Aaron J. CAVOSIE,³Alvaro P. CROSTA,⁴Winfried H. SCHWARZ,⁴Mario TRIELOFF,¹Carolinna DA SILVA MAIA DE SOUZA,¹Luciana A. PEREIRA,¹Eduardo N. RODRIGUES,¹Matthews BROWN
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13371]
¹Laboratory of Geochronology, Geosciences Institute, Brasilia University, 70910 900 Brasılia, DF, Brazil²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Bentley, WA 6102, Australia³Institute of Geosciences, University of Campinas, 13083-855, Campinas, SP, Brazil⁴Institute of Earth Sciences, Klaus-Tschira-Labor fur Kosmochemie, Heidelberg University, Im Neuenheimer Feld 234-236,69120 Heidelberg, German
Published by arrangement with John Wiley & Sons

A silicious impact melt rock from polymict impact breccia of the northern part of the alkali granite core of the Araguainha impact structure, central Brazil, has been investigated. The melt rock is thought to represent a large mass of impact‐generated melt in suevite. In particular, a diverse population of zircon grains, with different impact‐induced microstructures, has been analyzed for U‐Pb isotopic systematics. Backscattered electron and cathodoluminescence images reveal heterogeneous intragrain domains with vesicular, granular, vesicular plus granular, and vesicular plus (presumably) baddeleyite textures, among others. The small likely baddeleyite inclusions are not only preferentially located along grain margins but also occur locally within grain interiors. LA‐ICP‐MS U‐Pb data from different domains yield lower intercept ages of 220, 240, and 260 Ma, a result difficult to reconcile with the previous “best age” estimate for the impact event at 254.7 ± 2.7 Ma. SIMS U‐Pb data, too, show a relatively large range of ages from 245 to 262 Ma. A subset of granular grains that yielded concordant SIMS ages were analyzed for crystallographic orientation by EBSD. Orientation mapping shows that this population consists of approximately micrometer‐sized neoblasts that preserve systematic orientation evidence for the former presence of the high‐pressure polymorph reidite. In one partially granular grain (#36), the neoblasts occur in linear arrays that likely represent former reidite lamellae. Such grains are referred to as FRIGN zircon. The best estimate for the age of the Araguainha impact event from our data set from a previously not analyzed type of impact melt rock is based on concordant SIMS data from FRIGN zircon grains. This age is 251.5 ± 2.9 Ma (2σ, MSWD = 0.45, p = 0.50, n = 4 analyses on three grains), indistinguishable from previous estimates based on zircon and monazite from other impact melt lithologies at Araguainha. Our work provides a new example of how FRIGN zircon can be combined with in situ U‐Pb geochronology to extract an accurate age for an impact event.