Constraining ordinary chondrite composition via near-infrared spectroscopy

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.

Effects of oblique impacts on the impact strength of porous gypsum and glass spheres: Implications for the collisional disruption of planetesimals in thermal evolution

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, θc; the obtained θc values were 30° for the porous gypsum target and 55° for the glass target. The normalized largest fragment mass (ml/Mt) showed a good correlation with an effective specific energy (Qeff = Qsin2θ); the subsequent empirical equation was ml/Mt=102.02Qeff0.76 for the porous gypsum target and ml/Mt=104.66Qeff1.68 at ml/Mt <0.75 and ml/Mt=100.12Qeff0.08 at ml/Mt >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.

MINERALOGY, PETROLOGY, AND OXYGEN ISOTOPIC COMPOSITION OF NORTHWEST AFRICA 12379, METAL-RICH CHONDRITE WITH AFFINITY TO ORDINARY CHONDRITES

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 (Fa18.1–28.3, average Fa24.9±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 (Fs3.2–18.7Wo0.2–4.5; average Fs14.7±3.7Wo1.4±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.

Linking shock textures revealed by BSE, CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure, Brazil

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.

Mineralogy and petrology of Dominion Range 08006: A very primitive CO3 carbonaceous chondrite

1Jemma Davidson,1Conel M.O’D.Alexander,2Rhonda M.Stroud,3Henner Busemann,1Larry R.Nittler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.032]
1Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington DC, 20015-1305, USA
2Naval Research Laboratory Code 6366, 4555 Overlook Ave. SW, Washington, DC 20375, USA
3Institute of Petrology and Geochemistry, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
Copyright Elsevier

Here we report the relative degrees of thermal metamorphism for five Antarctic Ornans-like carbonaceous (CO) chondrites, including Dominion Range (DOM) 08006, as determined from the Cr-content of their FeO-rich (ferroan) olivine. These five CO3 chondrites complete the previously poorly-defined CO3.00 to 3.2 chondrite metamorphic trend. DOM 08006 appears to be a highly primitive CO chondrite of petrologic type 3.00. We report the detailed mineralogy and petrography of DOM 08006 using a coordinated, multi-technique approach.

The interchondrule matrix in DOM 08006 consists of unequilibrated mixtures of silicate, metal, and sulfide minerals and lacks Fe-rich rims on silicates indicating that DOM 08006 has only experienced minimal, if any, thermal metamorphism. This is also reflected by the Co/Ni ratios of Ni-rich and Ni-poor metal, a sensitive indicator of thermal metamorphism, and the presence of euhedral chrome-spinel grains, which typically become subhedral to anhedral during progressive metamorphism. DOM 08006 matrix shows minor evidence for aqueous alteration and while the presence of magnetite surrounding metal in chondrules indicates that there has been some interaction with fluid, much metal remains and none of the sulfides analyzed show evidence of being formed by aqueous alteration. Furthermore, the plagioclase of ∼50% of chondrules analyzed show resolvable excess silica indicating that these chondrules have experienced minimal, if any, reprocessing in the CO parent body.

Noble gas data for DOM 08006 show that it contains the highest concentrations of trapped 36Ar and 132Xe of all CO chondrites analyzed to date, further indicating that DOM 08006 is the most primitive CO chondrite known. The cosmic ray exposure age of DOM 08006 is estimated to be ∼19 Ma.

The minimally altered nature of DOM 08006 demonstrates that it is an extremely important sample for providing valuable insight into early Solar System conditions. At a total weight of 667 g, a significant amount of material is available for a wide array of future studies.

The water and fluorine content of 4 Vesta

1,2Adam R.Sarafian,1,3Sune G.Nielsen,3,4Horst R.Marschall,3Glenn A.Gaetani,5Kevin Righter,6Eve L.Berger
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.023]
1NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA
2Corning Research and Development Corporation, Corning, NY 14873
3Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA
4Institut für Geowissenschaften, Goethe Universtät Frankfurt, 60438 Frankfurt am Main, Germany
5NASA-JSC, Mailcode XI2, 2101 NASA Pkwy, Houston, TX 77058, USA
6GeoControl Systems Inc. – Jacobs JETS Contract –NASA JSC, USA
Copyright Elsevier

The processes that controlled accretion of water and volatiles to the inner solar system remain enigmatic, because it is difficult to determine the absolute concentrations of volatile elements in planetary bodies. In this contribution we study rare unequilibrated eucrites derived from the asteroid 4 Vesta, to determine the water and fluorine content of this asteroid by measuring the volatile content of pyroxene. Common thermal metamorphism in most equilibrated eucrites would have diffusively reset magmatic volatile contents. The unequilibrated eucrites are, therefore, the most suitable samples to determine primary magmatic volatile contents of 4 Vesta. We find H2O and F contents in pyroxenes of 4–11 µg/g and 0.12–0.23 µg/g. We also determine a H2O partition coefficient for clinopyroxene and melt equilibrated at 0.1 MPa of DH2O = 0.1, which is higher than values previously reported for higher pressures. The higher compatibility of H2O in this experiment could partially be due to high OH/H2O ratio at the low total water contents in this experimental charge, but only further more detailed experiments will fully delineate the reasons for the more compatible behavior for water at lower pressures. However, given the lack of H2O partitioning data at low pressures we conclude that our 0.1 MPa experiment is the most appropriate to calculate magmatic water contents for melt in equilibrium with eucrite pyroxene. After using appropriate partition coefficients we calculate melt concentrations of 50–70 µg/g H2O and 1.5–2.4 µg/g F. In turn, these are converted into bulk 4 Vesta water and F contents of 10–70 µg/g H2O and 0.3–2 µg/g F by assuming eucrite formation via either mantle partial melting or extraction from a magma ocean. We also measure the D/H of the clinopyroxenes and show that these are identical to the results of previous studies that reported D/H in eucrite apatite. These values match those found in carbonaceous chondrites suggesting that water in 4 Vesta accreted from carbonaceous chondrites and not from cometary material.

Trace Element Partitioning between CAI-Type Melts and Grossite, Melilite, Hibonite, and Olivine

1,2Gokce Ustunisik,2,3Denton S.Ebel,3,2David Walker,4,1Roger L.Nielsen,3,2Marina Gemma
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.038]
1Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, 57701
2Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, 10024-5192
3Department of Earth and Environmental Sciences, Lamont Doherty Earth Observatory of Columbia University, Palisades, NY, 10964-8000
4104 CEOAS Admin, College of Earth Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331
Copyright Elsevier

We determined the mineral-melt partition coefficients (Di’s) and the compositional and/or temperature dependency between grossite, melilite, hibonite, olivine and Ca-, Al-inclusion (CAI)-type liquids for a number of light (LE), high field strength (HFSE), large ion lithophile (LILE), and rare earth (REE) elements including Li, Be, B, Sr, Zr, Nb, Ba, La, Ce, Eu, Dy, Ho, Yb, Hf, Ta, Th. A series of isothermal crystallization experiments was conducted at 5 kbar pressure and IW+1 in graphite capsules. The starting compositions were selected based on the calculated and experimentally confirmed phase relations during condensation in CI dust-enriched systems (Ebel and Grossman, 2000, Ebel, 2006, Ustunisik et al., 2014).

The partition coefficients between melt and gehlenite, hibonite, and grossite show that the trace element budget of igneous CAIs is controlled by these three major Al-bearing phases in addition to pyroxene. In general, LE, LILE, REE, and HFSE partition coefficients (by mass) decrease in the order of Gehlenite-MeltDi > Hibonite-MeltDi > Grossite-MeltDi. The partition coefficients between gehlenitic melilite and CAI melt are approximately Be:0.14, B:0.07, Sr:0.79, Zr:0.02, Nb:0.01-0.02, Ba:0.05, La:1.03-3.44, Ce:1.2-3.86, Eu:1.19-2.88, Dy:1.14-3.23, Ho:1.04-2.91, Yb:0.7-1.70, Hf:0.02, Ta:0.01-0.02, Th:0.31-1.71. These results suggest that Gehlenite-MeltDi vary by a factor of 2-3 in different melt compositions at the same T (∼1500 oC). Hibonite-MeltDi exhibit a range as Be:0.02-0.04, B:0.01, Sr:0.21-0.66, Zr:0.02-0.18, Nb:0.03-0.05, Ba:0.02-0.06, La:0.56-4.38, Ce:0.52-3.54, Eu: 0.33-0.84, Dy: 0.25-0.32, Ho:0.17-0.29, Yb:0.05-0.19, Hf:0.05-0.38, Ta:0.02, Th:0.31-1.71. Increased Al and Ca, relative to earlier work, increases the compatibility of Gehlenite-MeltDi , and also the compatibility of Hibonite-MeltDi, especially for La and Ce.Grossite-MeltDi of individual mineral-melt pairs are Be:0.43, Sr:0.31, Zr:0.09, Nb:0.01, Ba:0.03, La:0.06, Ce:0.07, Eu:0.13, Dy:0.04, Ho:0.04, Yb:0.03, Hf:0.01, Ta:0.01, Th:0.01 for #18 at 1550 oC and as Sr:0.29, Nb:0.03, La:0.07, Ce:0.09, Eu:0.10, Dy:0.05, Ho:0.04, Yb:0.02, Hf:0.003, Ta:0.02, Th:0.02 for #7 at 1490 oC.

Olivine partitioning experiments confirm that olivine contribution to the trace element budget of CAIs is small due to the low Olivine-MeltDi at a range of temperatures while Olivine-MeltDEu, Yb are sensitive to changes in T and fO2. The development of a predictive model for partitioning in CAI-type systems would require more experimental data and use of analytical instruments capable of obtaining single phase analyses for crystals < 5µm.