Collisional and alteration history of the CM parent body

1Lionel G.Vacher, 1Yves Marrocchi, 1Johan Villeneuve, 2Maximilien J.Verdier-Paoletti, 3,4Matthieu Gounelle
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.006]
1CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, F-54501, France
2Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, USA
3IMPMC, MNHN, Sorbonne Universités, UMR CNRS 7590, 57 rue Cuvier, 75005 Paris, France
4Institut Universitaire de France, Maison des Universités, 103 boulevard Saint-Michel, 75005 Paris, France
Copyright Elsevier

Boriskino is a little studied CM2 chondrite composed of millimeter-sized clasts of different lithologies and degrees of alteration. Boriskino thus offers a good opportunity to better understand the preaccretionary alteration history and collisional evolution that took place on the CM parent body. The least altered lithology displays 16O-poor Type 1a calcite and aragonite grains (δ18O ≈ 30-37‰, δ17O ≈ 15-18‰ and Δ17O ≈ -2 to 0‰, SMOW) that precipitated early, before the establishment of the petrofabric, from a fluid whose isotopic composition was established by isotopic exchange between a 16O-poor water and 16O-rich anhydrous silicates. In contrast, the more altered lithologies exhibit 16O-rich Type 2a and veins of calcite (δ18O ≈ 17-23‰, δ17O ≈ 6-9‰ and Δ17O ≈ -4 to -1‰, SMOW) that precipitated after establishment of the deformation, from transported 16O-rich fluid in preexisting fractures. From our petrographic and X-ray tomographic results, we propose that the more altered lithologies of Boriskino were subjected to high intensity impact(s) (10-30 GPa) that produced a petrofabric, fractures and chondrule flattening. Taking all our results together, we propose a scenario for the deformation and alteration history of Boriskino, in which the petrographic and isotopic differences between the lithologies are explained by their separate locations into a single CM parent body. Based on the δ13C-δ18O values of the Boriskino Type 2a calcite (δ13C ≈ 30-71‰, PDB), we propose an alternative δ13C-δ18O model where the precipitation of Type 2a calcite can occurred in an open system environment with the escape of 13C-depleted CH4 produced from the reduction of C-bearing species by H2 released during serpentinization or kamacite corrosion. Assuming a mean precipitation temperature of 110°C, the observed δ13C variability in T2a calcite can be reproduced by the escape of ≈ 15-50% of dissolved carbon into CH4 by Rayleigh distillation.

Analysis of the MESSENGER MASCS Photometric Targets Part II: Photometric Variability Between Geomorphological Units

1Deborah L.Domingue, 2MarioD’Amore, 2Sabrina Ferrari,2Jörn Helbert,3Noam R.Izenberg
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.07.018]
1Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA
2Institute for Planetary Research, DLR, Rutherfordstrasse 2, Berlin, Germany
3The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Copyright Elsevier

This study examines the level of structural uniformity within Mercury’s regolith as a function of geomorphological unit. Using two categories of photometric models (Hapke versus Kaasalainen-Shkuratov), the variation between and within similar geomorphological units are examined with the Mercury Atmosphere and Surface Composition Spectrometer (MASCS) photometry sequence data sets. The results show evidence for variations in the spectral and photometric scattering properties both within similar geomorphological units and between different geomorphological units. The ejecta and cratering units show the largest differences between the modeling results, each indicating variations in different properties. The results for the intercrater materials, smooth materials, and dark materials show consistent results between both models. The variations include possible differences in grain structures, regolith compaction, and surface roughness on micron to millimeter scales.

Chondrule Porosity in the L4 Chondrite Saratov: Dissolution, Chemical Transport, and Fluid Flow

1Jonathan A.Lewis, 1,2Rhian H.Jones, 3Serafina C.Garcea
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.002]
1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131
2School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
3Henry Moseley X-ray Imaging Facility, School of Materials, University of Manchester, Manchester, M13 9PL, UK
Copyright Elsevier

Porosity is an important physical property of meteorites and asteroids that affects density, material strength, and thermal conductivity. Porosity can also promote chemical exchange by facilitating the transport of fluids and dissolved ions. We measured the porosity of individual chondrules from the L4 ordinary chondrite Saratov, using X-ray microtomography (μCT) and scanning electron microscopy, to examine the abundance and distribution of porosity in chondrules, and to understand how porosity relates to chemical exchange during parent body processes. Porosity was 1-2% by volume in the chondrules we measured and maximum pore sizes were ∼300 μm. Porosity distribution and morphology indicate that porosity is a secondary feature and most pores >1 μm were formed from the dissolution of chondrule mesostasis glass. Fe and K are preferentially enriched in phases adjacent to the most porous regions: Fe is enriched in pyroxene, and K is enriched in mesostasis where it is observed as either the silica alteration phase merrihueite, or fine-scale, K-feldspar exsolution in albitic feldspar. Some pores can be described as vugs, as they contain euhedral olivine and chromite, with textures indicating vapor deposition. Knowing the chondrule porosity, we estimate the matrix porosity in Saratov to be very high, 40-60%. We suggest that during prograde metamorphism, an aqueous fluid originating from the matrix dissolved chondrule mesostasis glass, producing the observed porosity, and introducing Fe into the pyroxene phenocrysts. Fluids were less abundant through peak metamorphism, and chondrule mesostasis glass crystallized to fine-grained albite. During retrograde metamorphism, high temperature, short duration bursts of a dry, alkali-bearing fluid from the asteroid interior infiltrated the pore network, formed the vug phases, altered silica to merrihueite, and introduced K to the secondary albite. Fine-scale K-feldspar then exsolved from albite during rapid cooling to the ambient temperature. Overall, development of porosity during metamorphism on the L chondrite parent body contributed to the chemical evolution of ordinary chondrite material, as well as affecting physical properties of the parent asteroid.

Shock metamorphic history of >4 Ga Apollo 14 and 15 zircons

1,2Carolyn A. Crow,3Desmond E. Moser,4Kevin D. McKeegan
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13184]
1Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, California, USA
2Department of Geological Sciences, University of Colorado, Boulder, Colorado, USA
3Zircon and Accessory Phase Laboratory, University of Western Ontario, London, Ontario, Canada
4Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, California, USA
Published by arrangement with John Wiley & Sons

During impact events, zircons develop a wide range of shock metamorphic features that depend on the pressure and temperature conditions experienced by the zircon. These conditions vary with original distance from impact center and whether the zircon grains are incorporated into ejecta or remain within the target crust. We have employed the range of shock metamorphic features preserved in >4 Ga lunar zircons separated from Apollo 14 and 15 breccias and soils in order to gain insights into the impact shock histories of these areas of the Moon. We report microstructural characteristics of 31 zircons analyzed using electron beam methods including electron backscatter pattern (EBSP) and diffraction (EBSD). The major results of this survey are as follows. (1) The abundance of curviplanar features hosting secondary impact melt inclusions suggests that most of the zircons have experienced shock pressures between 3 and 20 GPa; (2) the scarcity of recrystallization or decomposition textures and the absence of the high‐pressure polymorph, reidite, suggests that few grains have been shocked to over 40 GPa or heated above 1000 °C in ejecta settings; (3) one grain exhibits narrow, arc‐shaped bands of twinned zircon, which map out as spherical shells, and represent a novel shock microstructure. Overall, most of the Apollo 14 and 15 zircons exhibit shock features similar to those of terrestrial zircon grains originating from continental crust below large (~200 km) impact craters (e.g., Vredefort impact basin), suggesting derivation from central uplifts or uplifted rims of large basins or craters on the Moon and not high‐temperature and ‐pressure ejecta deposits.

Comparison of GEMS in interplanetary dust particles and GEMS‐like objects in a Stardust impact track in aerogel

1Hope A. Ishii
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13182]
1Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at MānoaHonolulu, Hawai’i, USA
Published by arrangement with John Wiley & Sons

Comet 81P/Wild 2 dust, the first comet sample of known provenance, was widely expected to resemble anhydrous chondritic porous (CP) interplanetary dust particles (IDPs). GEMS, distinctly characteristic of CP IDPs, have yet to be unambiguously identified in the Stardust mission samples despite claims of likely candidates. One such candidate is Stardust impact track 57 “Febo” in aerogel, which contains fine‐grained objects texturally and compositionally similar to GEMS. Their position adjacent the terminal particle suggests that they may be indigenous, fine‐grained, cometary material, like that in CP IDPs, shielded by the terminal particle from damage during deceleration from hypervelocity. Dark‐field imaging and multidetector energy‐dispersive X‐ray mapping were used to compare GEMS‐like‐objects in the Febo terminal particle with GEMS in an anhydrous, chondritic IDP. GEMS in the IDP are within 3× CI (solar) abundances for major and minor elements. In the Febo GEMS‐like objects, Mg and Ca are systematically and strongly depleted relative to CI; S and Fe are somewhat enriched; and Au, a known aerogel contaminant, is present, consistent with ablation, melting, abrasion, and mixing of the SiOx aerogel with crystalline Fe‐sulfide and minor enstatite, high‐Ni sulfide, and augite identified by elemental mapping in the terminal particle. Thus, GEMS‐like objects in “caches” of fine‐grained debris abutting terminal particles are most likely deceleration debris packed in place during particle transit through the aerogel.

Impactites of the Mistastin Lake impact structure: Insights into impact ejecta emplacement

1,2,3Marianne M. Mader, 1,2,4Gordon R. Osinski
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13173]
1Centre for Planetary Science and Exploration, University of Western OntarioLondon, Ontario, N6A 5B7, Canada
2Department of Earth Sciences, University of Western OntarioLondon, Ontario, Canada
3Centre for Earth & Space, Royal Ontario MuseumToronto, Ontario, Canada
4Department of Physics and Astronomy, University of Western OntarioLondon, Ontario, Canada
Published by arrangement with John Wiley & Sons

The Mistastin Lake impact structure is an intermediate‐size (~28 km apparent crater diameter), complex crater formed ~36 Myr. The original crater has been differentially eroded; however, a subdued terraced rim and distinct central uplift are still observed and impactites are well exposed in three dimensions. The inner portion of the structure is covered by Mistastin Lake and the surrounding area is locally covered by soil/glacial deposits and vegetation. The crystalline target rocks of the Mistastin Lake region are dominated by anorthosite, granodiorite, and quartz monzonite. Previous studies of the Mistastin Lake impactites have primarily focussed on the impact melt rocks. This study further evaluates the entire suite of impactite rocks in terms of their location within the crater structure and emplacement mechanisms. Locally, allochthonous impactite units including impact melt and various types of breccias are distributed around the lake in the terraced rim and are interpreted as proximal ejecta deposits. A multistage model for the origin and emplacement of impact melt rocks and the formation of impact ejecta is proposed for the Mistastin Lake impact structure based on a synthesis of the field and petrographic observations. This model involves the generation of a continuous ballistic ejecta blanket during the excavation stage, followed by the emplacement of melt‐rich, ground‐hugging flows during the terminal stages of crater excavation and the modification stage of crater formation. Impact melt‐bearing breccias—also termed “suevite” at other sites—are present in several distinct settings within the Mistastin Lake structure and likely have more than one formation mechanism.

From the mid‐Ordovician into the Late Silurian: Changes in the micrometeorite flux after the L chondrite parent breakup

1Ellinor Martin, 1Birger Schmitz, 2Hans‐Peter Schönlaub
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13174]
1Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
2Center for Geosciences, Austrian Academy of Sciences, Vienna, Austria
Published by arrangement with John Wiley & Sons

We present the first reconstruction of the micrometeorite flux to Earth in the Silurian Period. We searched 321 kg of condensed, marine limestone from the Late Silurian Cellon section, southern Austria, for refractory chrome‐spinel grains from micrometeorites that fell on the ancient sea floor. A total of 155 extraterrestrial spinel grains (10 grains >63 μm, and 145 in the 32–63 μm fraction) were recovered. For comparison, we searched 102 kg of similar limestone from the mid‐Ordovician Komstad Formation in southern Sweden. This limestone formed within ~1 Ma after the breakup of the L chondrite parent body (LCPB) in the asteroid belt. In the sample we found 444 extraterrestrial spinel grains in the >63 μm fraction, and estimate a content of at least 7000 such grains in the 32–63 μm fraction. Our results show that in the late Silurian, ~40 Ma after the LCPB, the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude, almost down to background levels. Among the ordinary chondrites, the dominance of L‐chondritic micrometeorites has waned off significantly, from >99% in the post‐LCPB mid‐Ordovician to ~60% in the Late Silurian, with ~30% H‐, and ~10% LL‐chondritic grains. In the Late Silurian, primitive achondrite abundances are similar to today’s value, contrasting to the much higher abundances observed in pre‐LCPB mid‐Ordovician sediments.