Spectral and mineralogical alteration process of naturally-heated CM and CY chondrites

1M.Matsuoka,2T.Nakamura,3N.Miyajima,4T.Hiroi,5N.Imae,5A.Yamaguchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.08.042]
1ISAS/JAXA, Sagamihara, Kanagawa 252-5210, Japan
2Tohoku University, Sendai, Miyagi 980-8578, Japan
3Bayerisches Geoinstitut, University of Bayreuth, Bayreuth 95440, Germany
4Brown University, Providence, RI 02912, USA
5Research National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
Copyight Elsevier

Spectral and mineralogical analyses were performed using nine naturally hydrated and dehydrated carbonaceous chondrite samples which were classified into heating stages (HS) from I to IV based on previous X-ray diffraction results. In-situ heating of samples at 120–400 °C was performed during spectral measurements and successfully removed absorbed water and part of rehydrated water from chondrite samples. Reflectance spectra of HS-I samples show the positive slope in visible (Vis)-infrared (IR) range and the significant 0.7- and 3-μm absorption bands. The 0.7-μm band appears in only HS-I sample spectra. With increasing temperature of heating, (1) Vis-IR slope decreases, (2) the 3-μm band becomes shallower, and (3) Christiansen feature (CF) and Reststrahlen bands (RB) shift toward longer wavelength. TEM-EDX analyses showed that the matrix of strongly-heated chondrites consists of tiny olivine, low-Ca pyroxene, and FeNi metallic particles mostly smaller than 100 nm in diameter, instead of Fe-rich serpentines and tochilinite observed in the HS-I chondrite. Therefore, in proportion to the heating degree, amorphization and dehydration of serpentine and tochilinite from HS-I to HS-II may cause the 0.7- and 3-μm band weakening, spectral bluing and darkening of chondrite spectra. In addition, formation of secondary anhydrous silicates and FeNi-rich metal grains at HS-IV would be responsible for the 3-μm band depth decrease, spectral reddening and brightening, CF peak shift, and RB changes of chondrite spectra. Those spectral changes in response to mineralogical alteration processes will be useful to interpret planetary surface composition by remote-sensing observations using ground-based or airborne/space telescopes or spacecraft missions.

A new method for dating impact events – thermal dependency on nanoscale Pb mobility in monazite shock twins

1,2Denis Fougerouse,1,3Aaron J.Cavosie,1,4Timmons Erickson,1,2Steven M.Reddy,1,3Morgan A.Cox,2David Saxey,2William Rickard,5Michael T.D.Wingate
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.08.025]
1School of Earth and Planetary Sciences, Curtin University, Perth, Australia
2Geoscience Atom Probe Facility, John de Laeter Centre, Curtin University, Perth, Australia
3Space Science and Technology Centre, Curtin University, Perth, Australia
4Jacobs – JETS, Astromaterials Research and Exploration Science division, NASA Johnson Space Center, Houston, USA
5Geological Survey of Western Australia, Department of Mines, Industry Regulation and Safety, Perth, Australia
Copyright Elsevier

To test the potential of deformation twins to record the age of impact events, micrometre-scale size mechanical twins in shocked monazite grains from three impact structures were analyzed by atom probe tomography (APT). Shocked monazite from Vredefort (South Africa; ∼300 km crater diameter), Araguainha (Brazil; ∼40 km diameter), and Woodleigh (Australia; 60 to 120 km diameter) were studied, all from rocks which experienced pressures of ∼30 GPa or higher, but each with a different post-impact thermal history. The Vredefort sample is a thermally recrystallised foliated felsic gneiss and the Araguainha sample is an impact melt-bearing bedrock. Both Vredefort and Araguainha samples record temperatures > 900 °C, whereas the Woodleigh sample is a paragneiss that experienced lower temperature conditions (350 – 500 °C). A combined 208Pb/232Th age for common {1} twins and shock-specific (01) twins in Vredefort monazite was defined at 1979 ± 150 Ma, consistent with the accepted impact age of ∼2020 Ma. Irrational η1 [0] shock-specific twins in Araguainha monazite yielded a 260 ± 48 Ma age, also consistent with the accepted 250-260 Ma impact age. However, the age of a common (001) twin in Araguainha monazite is 510 ± 87 Ma, the pre-impact age of igneous crystallisation. These results are explained by the occurrence of common (001) twins in tectonic deformation settings, in contrast to the (01) and irrational η1 [0] twins, which have only been documented in shock-deformed rocks. In Woodleigh monazite, APT age data for all monazite twins [(001), (01), newly identified 102°/<23> twin], and host monazite are within uncertainty at 1048 ± 91 Ma, which is interpreted as a pre-impact age of regional metamorphism. We therefore are able to further constrain the poorly known age of the Woodleigh impact to < 1048 ± 91 Ma. These results provide evidence that Pb is expelled from monazite during shock twin formation at high temperature (Vredefort and Araguainha), and also that Pb is not mobilised during twinning at lower temperature (Woodleigh). Our study suggests that twins formed during shock metamorphism have the potential to record the age of the impact event in target rocks that are sufficiently heated during the cratering process.

Survey of Impact Glasses in Shergottites Searching for Martian Sulfate Using X-ray Absorption Near-Edge Structure

1Masashi Shidare,bRyoichi Nakada,3,4Tomohiro Usui,1Minato Tobita,2Kenji Shimizu,5Yoshio Takahashi,1Tetsuya Yokoyama
Geochimica et Cosmochinica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.08.026]
1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Tokyo 152-8551, Japan
2Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe, Nankoku, Kochi 783-8501, Japan
3Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Tokyo 152-8551, Japan
4Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 252-5210, Japan
5Department of Earth and Planetary Science, the University of Tokyo, 7-3-1 Hongo, Tokyo 113-8654, Japan
Copyright Elsevier

The surface of Mars has experienced progressive oxidation, resulting in the formation of sulfate minerals as evidenced from surface exploration missions. However, no clear evidence for the presence of sulfate minerals has been reported within Martian meteorites. This study examined sulfur speciation in impact glasses of three basaltic shergottites, Elephant Moraine (EETA) 79001, Larkman Nunatak (LAR) 06319, and Dhofar 019, using X-ray absorption near-edge structure (XANES) spectroscopy. The measured XANES spectra were classified into four types: (1) sulfide, (2) highly reduced sulfide glass (∼IW+1), (3) mixture of sulfide and sulfate, and (4) sulfate. The sulfate spectra observed from EETA79001 and LAR 06319 were mixed with sulfide from the reduced igneous host rock as impact glasses were formed by shock on the surface of Mars, both sulfide and sulfate would have possibly originated on Mars. Besides, highly reduced sulfide present in the same impact glasses is inconsistent with secondary alteration on the oxic Earth’s environment. In contrast to EETA79001 and LAR 06319, all of the XANES spectra from Dhofar 019 showed the only sulfate, whose origin is most likely from terrestrial alteration. Combining with the geochemical signatures of volatile elements (e.g., D/H, C, and halogens) in impact glasses of EETA79001 and LAR 06319, we propose two possible scenarios for the formation of sulfate species to the shergottite host-rocks: (i) oxidation of sulfide minerals by subsurface oxic water in Mars, or (ii) precipitation of sulfate mineral derived from Martian subsurface water. The difference between the two models is the source of S(VI) species, whether it originated from (i) magmatic sulfide in shergottite or (ii) sulfate ion in the subsurface water/ice. Both models indicate that the ancient (∼4 Ga) water reservoir might have already been oxic, and it requires post-magmatic water–rock interaction that formed sulfate minerals whose oxidized signatures were incorporated into impact glass.

Tracing the earliest stages of hydrothermal alteration on the CM chondrite parent body

1A. J. King,1,2E. Mason,1,3H. C. Bates,1P. F. Schofield,3,4K. L. Donaldson Hanna,3N. E. Bowles,1S. S. Russell
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13734]
1Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD UK
2Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ UK
3Planetary Spectroscopy Facility, Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU UK
4Department of Physics, University of Central Florida, Orlando, Florida, 32816–2385 USA
Published by arrangement with John Wiley & Sons

The CM carbonaceous chondrites are an important resource in our efforts to understand the role of volatiles in the formation of planetary systems. We report the bulk mineralogy, water abundance, and infrared (IR) reflectance spectra of the CM chondrites LaPaz Icefield (LAP) 04514, LAP 04796, LAP 04565, and LAP 02333. They contain abundant Fe- and Mg-rich serpentines (˜70–80 vol%), and based on their phyllosilicate fractions, we classify LAP 04514, LAP 04796, and LAP 04565 as petrologic subtype 1.6 and LAP 02333 as 1.4. This is consistent with estimated water abundances of 9.9 (±1.1) wt% for LAP 04796, 10.4 (±0.1) wt% for LAP 04565, and 11.5 (±0.5) wt% for LAP 02333. However, LAP 04514 contains less water (8.8 ± 0.3 wt%), has a shallower 3 µm band depth, and lacks tochilinite having experienced posthydration temperatures of ˜300–400 °C. We conclude that LAP 04514, LAP 04796, and LAP 04565 are among the least altered CM chondrites, which retain primitive features from the initial building blocks of the CM parent body. Finally, we use the IR spectral features of LAP 04514, LAP 04796, and LAP 04565 to identify C-complex asteroid surfaces that record mild levels of hydration.

Solar energetic particle tracks in lunar samples: A transmission electron microscope calibration and implications for lunar space weathering

1Lindsay P. Keller,2,1Eve L. Berger,1,3Shouliang Zhang,4Roy Christoffersen
Meteoritics & Planteray Science (in Press) Link to Article [https://doi.org/10.1111/maps.13732]
1NASA Johnson Space Center, Mail Code XI3, Houston, Texas, 77058 USA
2Texas State University − Jacobs JETS − NASA Johnson Space Center, Houston, Texas, 77058 USA
3Samsung Austin Semiconductor, Analysis Engineering, 12100 Samsung Blvd, Austin, Texas, 78754 USA
4Jacobs, NASA Johnson Space Center, Mail Code X13, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

Transmission electron microscope (TEM) imaging techniques combined with focused ion beam sample preparation were used to calibrate the solar energetic particle track production rate in lunar samples. Track density measurements by TEM as a function of depth were obtained from lunar rock 64455 that has a well-constrained exposure age of 2 Myr giving a track production rate of 4.4 ± 0.4 × 104 tracks cm−2 yr−1 for a 2π exposure at 1 AU. The typical space weathering effects in mature lunar soils (both vapor-deposited rims and solar wind-damaged rims) accumulate in ˜106 yr based on the new calibration applied to track densities in individual grains. Solar wind-damaged rim widths in anorthite and olivine follow a power law relationship with track density and achieve steady-state widths in a few Myr. Vapor-deposited rim widths show no correlation with exposure age suggesting that their formation is episodic with the full width of vapor-deposited rims accumulating in a single or a few rare impact events. Solar wind-damaged rim development was modeled using the stopping range of ions in matter code. Modeling shows that the solar wind-damaged rims develop rapidly and approach steady-state values in 105–106 yr. Anorthite and olivine record similar track densities for similar exposure ages, but their structural response to solar wind irradiation differs significantly. Solar wind-damaged rims on olivine are not amorphous in contrast to modeling and high flux laboratory experiments and a model is proposed to account for their different response to solar wind irradiation.

Elucidation of impact event recorded in the lherzolitic shergottite NWA 7397

1Masashi Yoshida,1Masaaki Miyahara,1,2,3Hiroki Suga,4,5Akira Yamaguchi,6Naotaka Tomioka,7Takeshi Sakai,7,8Hiroaki Ohfuji,8Fumiya Maeda,7,8,9Itaru Ohira,8Eiji Ohtani,10Seiji Kamada,11Takuji Ohigashi,11Yuichi Inagaki,12Yu Kodama,3Naohisa Hirao
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13735]
1Graduate School of Advanced Science and Engineering, Hiroshima University, Hiroshima, Higashi, 739-8526 Japan
2Department of Earth and Planetary, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo, Bunkyo-ku, 113-0033 Japan
3Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto Sayo, Hyogo, 679-5198 Japan
4National Institute of Polar Research, Tokyo, 190-8518 Japan
5Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190-8518 Japan
6Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi, Nankoku, 783-8502 Japan
7Geodynamics Research Center, Ehime University, Matsuyama, 790-8577 Japan
8Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
9Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Tokyo, Toshima-ku, 171-8588 Japan
10Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578 Japan
11UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki, Aichi, 444-8585 Japan
12Marine Works Japan, Kochi, Nankoku, 783-8502 Japan
Published by arrangement with John Wiley & Sons

The (plagioclase) lherzolitic shergottite Northwest Africa (NWA) 7397 consists of poikilitic and non-poikilitic lithologies. Coarse-grained low-Ca pyroxene oikocrysts enclose olivine and chromite grains in the poikilitic lithology. The major constituents of the non-poikilitic lithology are olivine, Ca-pyroxene, and plagioclase. Minor amounts of chromite, ilmenite, alkali feldspar, Ca-phosphate, and iron-sulfide are included in the non-poikilitic lithology. Most plagioclase grains in the non-poikilitic lithology have become maskelynite. A melt pocket occurs in the non-poikilitic lithology. Plagioclase in contact with the melt pocket has dissociated into zagamiite + stishovite. Apatite and merrillite entrained in the melt pocket have transformed into tuite. Olivine in contact with the melt pocket has dissociated into bridgmanite (almost vitrified) + ferroan-periclase. Alteration products, iron oxides and hydroxides, also occur in the dissociated olivine although it is not clear when the aqueous alteration occurred. The dissociation reactions of olivine and plagioclase into the high-pressure polymorphs (bridgmanite, ferroan-periclase, zagamiite, and stishovite) are found from lherzolitic shergottites for the first time. The estimated peak shock-pressure and -temperature conditions recorded in melt pockets of NWA 7397 are ˜23 GPa and 2,000 °C at least, respectively, based on the high-pressure mineral assemblages.

Impact plume-formed and protoplanetary disk high-temperature components in CB and CH metal-rich carbonaceous chondrites

1Alexander N. Krot,2Michail I. Petaev,1Kazuhide Nagashima,1Elena Dobrică,3,4Brandon C. Johnson,3Melissa D. Cashion
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13717]
1Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, 906822 USA
2Department of Earth and Planetary Sciences, Harvard University and Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, 02138 USA
3Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana, 47907 USA
4Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana, 47907 USA
Published by arrangement with John Wiley & Sons

We report on the mineralogy, petrology, and oxygen isotopic compositions of ferroan olivine–pyroxene-normative cryptocrystalline chondrules (Fe-CCs) in CH chondrites and discuss their origin and the origin of other components in the genetically related CH and CB chondrites. There are two kinds of Fe-CCs: (1) compositionally uniform (Fe/[Fe+Mg] = 0.17–0.34) chondrules with euhedral Fe, Ni-metal grains and (2) metal-free chemically zoned (Fe/[Fe+Mg] = 0.05–0.4) chondrules surrounded by ferroan olivine (Fa44−62) rims; the Fe/(Fe+Mg) ratio increases toward the rims. Both types contain low CaO and Al2O3 (<0.04 wt%), but relatively high contents of MnO and Cr2O3 (up to 1 wt%). Compositionally uniform euhedral Fe, Ni-metal grains are Ni-rich (9–20 wt%) and have subsolar Co/Ni ratio. There is a positive correlation between iron content in the metal grains and Fe/(Fe+Mg) ratio in silicate portion of their host chondrules. Some Fe-CCs experienced postcrystallization solid-state reduction of ferroan silicates to metallic iron. Ferroan cryptocrystalline chondrules and olivine rims have similar oxygen isotopic compositions (interchondrule Δ17O ranges from ˜ −2‰ to 2‰), which are slightly 16O-depleted relative to those of magnesian olivine–pyroxene-normative cryptocrystalline chondrules (Mg-CCs; Δ17O ˜ −2‰) commonly observed in CBs and CHs. We suggest that the Fe-CCs and Mg-CCs formed in the impact plume under different redox conditions (˜IW−1 and ˜IW−3, respectively), which may have been controlled by heterogeneous distribution of water-bearing phases (water ice, hydrated materials) in the collided bodies and/or in the disk. We propose the following impact plume scenario for the origin of Fe-CCs: (1) condensation of ferromagnesian silicate melt around Fe, Ni-metal melt droplets from a highly oxidized portion of the plume; (2) crystallization of euhedral metal grains from the supercooled ferromagnesian silicate melt followed by its solidification; (3) condensation of ferroan olivine rims around solidified Fe-CCs; (4) high-temperature annealing of Fe-CCs and their rims in the plume accompanied by Fe-Mg interdiffusion between ferroan olivine rims and their host chondrules. Subsequently, some Fe-CC experienced solid-state reduction to various degrees, possibly in the reduced portions of gaseous plume. The impact plume-produced or reprocessed components in CBs and CHs include Ca,Al-poor magnesian and ferroan cryptocrystalline chondrules; Ca,Al-rich skeletal olivine chondrules; isotopically uniform, 26Al-poor 16O-depleted (Δ17O ˜ −15 to −5‰) igneous CAIs surrounded by igneous forsterite rims; chemically zoned and unzoned Fe,Ni-metal grains; and metal-sulfide nodules. These objects are dominant in CBs and abundant in CHs. The CH chondrites also contain other high-temperature chondritic components, which avoided processing in the plume and most likely predate the plume event: 26Al-poor and 26Al-rich, mostly 16O-rich CAIs (Δ17O ˜ −40 to −10‰) surrounded by Wark–Lovering rims, and porphyritic chondrules (magnesian [type I], ferroan [type II], and Al-rich) showing a range of Δ17O (from ˜ −10 to ˜ +5‰). Some of these components appear to have been melted in the plume. We conclude that CH and CB chondrites contain multiple generations of chondrules and refractory inclusions formed by different mechanisms at different times and different regions of the protoplanetary disk, consistent with the hypothesis of Wasson and Kallemeyn (1990).

Formation of chondrule analogs aboard the International Space Station

1Tamara E. Koch,1Dominik Spahr,1Beverley J. Tkalcec,1Miles Lindner,1David Merges,2Fabian Wilde,1Björn Winkler,1,3Frank E. Brenker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13731]
1Insitute of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany
2Helmholtz-Zentrum Hereon, Max-Planck Strasse 1, 21502 Geesthacht, Germany
3Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, 1680 East-West Road, Honolulu, Hawai‘i, 96822 USA
Published by arrangement with John Wiley & Sons

Chondrules are thought to play a crucial role in planet formation, but the mechanisms leading to their formation are still a matter of unresolved discussion. So far, experiments designed to understand chondrule formation conditions have been carried out only under the influence of terrestrial gravity. In order to introduce more realistic conditions, we developed a chondrule formation experiment, which was carried out at long-term microgravity aboard the International Space Station. In this experiment, freely levitating forsterite (Mg2SiO4) dust particles were exposed to electric arc discharges, thus simulating chondrule formation via nebular lightning. The arc discharges were able to melt single dust particles completely, which then crystallized with very high cooling rates of >105 K h−1. The crystals in the spherules show a crystallographic preferred orientation of the [010] axes perpendicular to the spherule surface, similar to the preferred orientation observed in some natural chondrules. This microstructure is probably the result of crystallization under microgravity conditions. Furthermore, the spherules interacted with the surrounding gas during crystallization. We show that this type of experiment is able to form spherules, which show some similarities with the morphology of chondrules despite very short heating pulses and high cooling rates.

Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon

1Anders Plan,2Gavin G. Kenny,3Timmons M. Erickson,1Paula Lindgren,1Carl Alwmark,1,4,5Sanna Holm-Alwmark,6Philippe Lambert,1Anders Scherstén,1Ulf Söderlund
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13723]
1Department of Geology, Lund University, Sölvegatan 12, Lund, 223 62 Sweden
2Department of Geosciences, Swedish Museum of Natural History, Stockholm, SE-104 05 Sweden
3Jacobs—JETS, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058 USA
4Niels Bohr Institute, University of Copenhagen, Copenhagen, DK-2100 Denmark
5Natural History Museum Denmark, University of Copenhagen, Copenhagen, DK-2100 Denmark
6CIRIR—Center for International Research and Restitution on Impacts and on Rochechouart, Sciences et Applications, 218 Boulevard Albert 1er, Bordeaux, 33800 France
Published by arrangement with John Wiley & Sons

Reidite, the high-pressure zircon (ZrSiO4) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer-sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stage III) from the ˜200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO4 under extreme P–T conditions. The shocked zircon grains have preserved various amounts of reidite ranging from 4% up to complete conversion. The grains contain various variants of reidite, including the common habits: lamellae and granular reidite. In addition, three novel variants have been identified: blade, wedge, and massive domains. Several of these crosscut and offset each other, revealing that reidite can form at multiple stages during an impact event. Our data provide evidence that reidite can be preserved in impactites to a much greater extent than previously documented. We have further characterized reversion products of reidite in the form of fully recrystallized granular zircon grains and minute domains of granular zircon in reidite-bearing grains that occur in close relationship to reidite. Neoblasts in these grains have a distinct crystallography that is the result of systematic inheritance of reidite. We interpret that the fully granular grains have formed from prolonged exposure of temperatures in excess of 1200 °C. Reidite-bearing grains with granular domains might signify swift quenching from temperatures close to 1200 °C. Grains subjected to these specific conditions therefore underwent partial zircon-to-reidite reversion, instead of full grain recrystallization. Based on our ZrSiO4 microstructural constraints, we decipher the grains evolution at specific P–T conditions related to different impact stages, offering further understanding of the behavior of ZrSiO4 during shock.

Salt grains in hypervelocity impacts in the laboratory: Methods to sample plumes from the ice worlds Enceladus and Europa

1C. R. Fisher,1M. C. Price,1M. J. Burchell
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13729]
1Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, Kent, CT2 7NH UK
Published ba arrangement with John Wiley & Sons

The plumes naturally erupting from the icy satellite Enceladus were sampled by the Cassini spacecraft in high-speed fly-bys, which gave evidence of salt. This raises the question of how salt behaves under high-speed impact, and how it can best be sampled in future missions to such plumes. We present the results of 35 impacts onto aluminum targets by a variety of salts (NaCl, NaHCO3, MgSO4, and MgSO4·7H2O) at speeds from 0.26 to 7.3 km s−1. Using SEM-EDX, identifiable projectile residue was found in craters at all speeds. It was possible to distinguish NaCl and NaHCO3 from each other, and from the magnesium sulfates, but not to separate the hydrous from anhydrous magnesium sulfates. Raman spectroscopy on the magnesium sulfates and NaHCO3 residues failed to find a signal at low impact speeds (<0.5 km s−1) where there was insufficient projectile material deposited at the impact sites. At intermediate speeds (0.5 to 2–3 km s−1), identifiable Raman spectra were found in the impact craters, but not at higher impact speeds, indicating a loss of structure during the high speed impacts. Thus, intact capture of identifiable salt residues on solid metal surfaces requires impact speeds between 0.75 and 2 km s−1.