¹de Vet, S. J.
¹Earth Surface Science, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, the Netherlands

Two meteorites impacted in 1925 around the town of Serooskerke on the isle of Schouwen, the Netherlands. The largest mass is widely known as the “Ellemeet” diogenite, while a second mass, heavily weathered due to environmental exposure, also survived until the present day. This work aims to reconstruct the history of the 1925 fall and for the first time documents the second mass, known as the “Serooskerke,” by integrating a historical and experimental approach. The study of historical news archives and cadastral records redefined the 1925 impact site at N 51°42.086′ E 3°49.789′. Environmental exposure experiments reproducing the effects of rainfall and frost weathering identified the latter as the main cause for the second mass’ reported disintegration in the field sometime during the 1925–1926 winter. The bulk mineralogy of the second mass was established using XRD powder diffraction for a 2θ range of 3–70° and was found to be identical to an Ellemeet reference sample. UV/VIS/nIR spectroscopy (300–2500 nm) was subsequently used to broadly compare the second mass to HED clan meteorites Bouvante, EET87503, Johnstown and asteroid 4 Vesta in order to corroborate its vestan origin. The historical and geographic relationship of the two masses and the comparable bulk mineralogy supported the pairing of these two meteorites. This makes the Serooskerke a valuable legacy of the 1925 fall, especially as the location of ~50% of the remaining Ellemeet mass is presently unknown.

Reference
de Vet SJ (2015) The 1925 meteorite fall near Ellemeet and Serooskerke, the Netherlands. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12554]
Published by arrangement with John Wiles & Sons

¹David T. Blewett et al. (>10)*
¹Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
*Find the extensive, full author and affiliation list on the publishers website

Exposure to ion and micrometeoroid bombardment in the space environment causes physical and chemical changes in the surface of an airless planetary body. These changes, called space weathering, can strongly influence a surface’s optical characteristics, and hence complicate interpretation of composition from reflectance spectroscopy. Prior work using data from the Dawn spacecraft ( Pieters et al., 2012) found that accumulation of nanophase metallic iron (npFe0), which is a key space-weathering product on the Moon, does not appear to be important on Vesta, and instead regolith evolution is dominated by mixing with carbonaceous chondrite (CC) material delivered by impacts.
In order to gain further insight into the nature of space weathering on Vesta, we constructed model reflectance spectra using Hapke’s radiative-transfer theory and used them as an aid to understanding multispectral observations obtained by Dawn’s Framing Cameras (FC). The model spectra, for a howardite mineral assemblage, include both the effects of npFe0 and that of a mixed CC component. We found that a plot of the 438-nm/555-nm ratio vs. the 555-nm reflectance for the model spectra helps to separate the effects of lunar-style space weathering (LSSW) from those of CC-mixing. We then constructed ratio-reflectance pixel scatterplots using FC images for four areas of contrasting composition: a eucritic area at Vibidia crater, a diogenitic area near Antonia crater, olivine-bearing material within Bellicia crater, and a light mantle unit (referred to as an “orange patch” in some previous studies, based on steep spectral slope in the visible) northeast of Oppia crater. In these four cases the observed spectral trends are those expected from CC-mixing, with no evidence for weathering dominated by production of npFe0. In order to survey a wider range of surfaces, we also defined a spectral parameter that is a function of the change in 438-nm/555-nm ratio and the 555-nm reflectance between fresh and mature surfaces, permitting the spectral change to be classified as LSSW-like or CC-mixing-like. When applied to 21 fresh and mature FC spectral pairs, it was found that none have changes consistent with LSSW.
We discuss Vesta’s lack of LSSW in relation to the possible agents of space weathering, the effects of physical and compositional differences among asteroid surfaces, and the possible role of magnetic shielding from the solar wind.

Reference
Blewett DT et al. (2015) Optical Space Weathering on Vesta: Radiative-transfer Models and Dawn Observations. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.10.012]
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¹Vladimir V. Svetsov, ¹Valery V. Shuvalov
¹Institute for Dynamics of Geospheres, Russian Academy of Sciences, Leninskiy Prospekt 38-1, Moscow, 119334, Russia

In this study we numerically simulated the impacts of asteroids and comets on the Moon in order to calculate the amount of condensate that can be formed after the impacts and compare the results with data for lunar samples. Using available equations of state for quartz and dunite, we have determined pressure and density behind shock waves in these materials for particle velocities behind the shock from 4 to 20 km/s and obtained release adiabats from various points on the Hugoniot curves to very low pressures. For shock waves with particle velocities behind the front below 8 km/s the release adiabats intersect the liquid branch of the two-phase curve and, during the following expansion, the liquid material vaporizes and does not condense, forming a two-phase mixture of melt and vapor. The condensate can appear during expansion of material compressed by a shock with higher (>8 km/s) velocities. Using our hydrocode SOVA, we have conducted numerical simulations of the impacts of spherical quartz, dunite, and water-ice projectiles into targets of the same materials. Impact velocities were 15-25 km/s for stony projectiles and 20-70 km/s for icy impactors, and impact angles were 45°and 90° to the target surface. Along with the masses of condensates we calculated the masses of vaporized and melted material. Upon the impact of a projectile consisting of dunite into a target of quartz at a speed of 20 km/s at an angle of 45°, vaporized and melted masses of the target are equal to 1.6 and 11 in units of projectile mass, respectively, and the mass of condensate is 0.19. Vaporized and condensed masses of the projectile are 0.16 and 0.02, the rest mass of the projectile is melted. The calculated ratio of vaporized to melted mass proved to be on the order of 0.1. However, we calculated that, at impact velocities below 20 km/s, the condensate mass is only a small fraction of the vaporized and melted masses and, consequently, the major part of vapor disperses in vacuum in the form of separate molecules or molecular clusters. At an impact velocity of 15 km/s, the abundance of silicate condensates relative to melt is 0.001 – 0.0001, in agreement with data from lunar samples. Should the observed condensate abundances be representative, the velocities of major asteroid impacts on the Moon could not substantially exceed 20 km/s. Comet impacts at the same velocities produce much smaller amounts of vapor condensate because the low densities of cometary material induce lower shock pressures in the target.

Reference
Svetsov VV, Shuvalov VV (2015) Silicate impact-vapor condensate on the Moon: Theoretical estimates versus geochemical data. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.10.019]
Copyright Elsevier

¹T.S. Peretyazhko, ¹B. Sutter, ²R.V. Morris, ³D.G. Agresti, ¹L. Le, ²D.W. Ming
¹Jacobs, NASA Johnson Space Center, Houston, TX 77058
²NASA Johnson Space Center, Houston, TX 77058
³University of Alabama, Birmingham, AL 35294

Phyllosilicates of the smectite group detected in Noachian and early Hesperian terrains on Mars have been hypothesized to form under neutral to alkaline conditions. These pH conditions would also be favorable for formation of widespread carbonate deposits which have not been detected on Mars. We propose that smectite deposits on Mars formed under moderately acidic conditions inhibiting carbonate formation. We report here the first synthesis of Fe/Mg smectite in an acidic hydrothermal system [200 °C, pHRT ∼4 (pH measured at room temperature) buffered with acetic acid] from Mars-analogue, glass-rich, basalt simulant with and without aqueous Mg or Fe(II) addition under N2-purged anoxic and ambient oxic redox conditions. Synthesized Fe/Mg smectite was examined by X-ray-diffraction, Mössbauer spectroscopy, visible and near-infrared reflectance spectroscopy, scanning electron microscopy and electron microprobe to characterize mineralogy, morphology and chemical composition. Alteration of the glass phase of basalt simulant resulted in formation of the Fe/Mg smectite mineral saponite with some mineralogical and chemical properties similar to the properties reported for Fe/Mg smectite on Mars. Our experiments are evidence that neutral to alkaline conditions on early Mars are not necessary for Fe/Mg smectite formation as previously inferred. Phyllosilicate minerals could instead have formed under mildly acidic pH conditions. Volcanic SO2 emanation and sulfuric acid formation is proposed as the major source of acidity for the alteration of basaltic materials and subsequent formation of Fe/Mg smectite.

Reference
Peretyazhko TS, Sutter B, Morris RV, Agresti DG, Le L, Ming DW (2015) Fe/Mg smectite formation under acidic conditions on early Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.10.012]
Copyright Elsevier

¹Yasuhito Sekine et al. (>10*)
¹Department of Earth and Planetary Science, University of Tokyo, Bunkyo 113-0033, Japan
*Find the extensive, full author and affiliation list on the publishers website

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Sekine Y et al. (2015) High-temperature water–rock interactions and hydrothermal environments in the chondrite-like core of Enceladus. Nature Communications 6, 8604 Link to Article [doi:10.1038/ncomms9604]

^1,2Kathryn H. McDermott, ¹Richard C. Greenwood, ³Edward R.D. Scott, ¹Ian A. Franchi, ⁴Mahesh Anand
¹Planetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA
²School of Physical Sciences, University of Kent, Canterbury, CT2 7NH
³Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA
⁴Department of Earth Sciences, The Natural History Museum, London, SW7 5BD

The origin of silicate-bearing irons, especially those in groups IAB, IIICD, and IIE, is poorly understood as silicate should have separated rapidly from molten metal. Here we report the results of high precision oxygen isotope analysis of silicate inclusions in eleven group IIE meteorites and a petrological study of silicate inclusions in ten IIE irons including those in Garhi Yasin and Tarahumara, which have not been described in detail before. Oxygen isotopes have also been analysed in 20 H chondrites to investigate their possible relationship with the IIE irons.

Based on petrographic observations and mineral analysis, the silicate-bearing IIE meteorites have been divided into four types according to the nature of their silicate inclusions: 1) primitive chondritic, 2) evolved chondritic, 3) differentiated with >10 vol.% orthopyroxene, and 4) differentiated with

Our data suggest that the IIE meteorites formed on an internally heated H/HH chondrite-like body that experienced the initial stages of differentiation in response to radiogenic heating. However, prior to full differentiation the IIE parent body experienced a major hit-and-run style collision that resulted in silicate-metal mixing. The initial stages of this event involved a phase of rapid cooling that prevented unmixing of metal and silicates. Reassembly of the IIE parent body produced a large regolith blanket that facilitated subsequent slow cooling. The IIE parent body has probably experienced numerous subsequent less catastrophic collisions. The development of alkali glass textures in some differentiated inclusions is probably the result of one of these later events.

Reference
McDermott KH, Greenwood RC, Scott ERD, Franchi IA, Anand M (2015) Oxygen isotope and petrological study of silicate inclusions in IIE iron meteorites and their relationship with H chondrites. Geochimica et Cosmochimica Acta (in Presss)
Link to Article [doi:10.1016/j.gca.2015.10.014]
Copyright Elsevier

¹Ashcroft, H. O.
¹Wood, B. J.
¹Department of Earth Sciences, University of Oxford, Oxford, UK

We have performed an experimental and modeling study of the partial melting behavior of the HED parent body and of the fractional crystallization of liquids derived from its mantle. We estimated the mantle composition by assuming chondritic ratios of refractory lithophile elements, adjusting the Mg# and core size to match the density and moment of inertia of Vesta, and the compositions of Mg-rich olivines found in diogenites. The liquidus of a mantle with Mg# (=100*[Mg/(Mg+Fe)]) 80 is ~1625 °C and, under equilibrium conditions, the melt crystallizes olivine alone until it is joined by orthopyroxene at 1350 °C. We synthesized the melt from our 1350 °C experiment and simulated its fractional crystallization path. Orthopyroxene crystallizes until it is replaced by pigeonite at 1200 °C. Liquids become eucritic and crystal assemblages resemble diogenites below 1250 °C. MELTS correctly predicts the olivine liquidus but overestimates the orthopyroxene liquidus by ~70 °C. Predicted melt compositions are in reasonable agreement with those generated experimentally. We used MELTS to determine that the range of mantle compositions that can produce eucritic liquids and diogenitic solids in a magma ocean model is Mg# 75–80 (with chondritic ratios of refractory elements). A mantle with Mg# ~ 70 can produce eucrites and diogenites through sequential partial melting.

Reference
Ashcroft HO and Wood BJ (2015) An experimental study of partial melting and fractional crystallization on the HED parent Body. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12556]
Published by arrangement with John Wiley&Sons

¹Lidia Pittarello, ²Julia Roszjar, ³Dieter Mader, ⁴Vinciane Debaille, ¹Philippe Claeys,²Christian Koeberl
¹Analytical, Environmental and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Brussels, Belgium
²Natural History Museum Vienna, Vienna, Austria
³Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁴Laboratoire G-Time (Géochimie: Traçage isotopique, minéralogique et élémentaire), Université Libre de Bruxelles, Brussels, Belgium

El’gygytgyn (Chukotka, Arctic Russia) is a well-preserved impact structure, mostly excavated in siliceous volcanic rocks. For this reason, the El’gygytgyn structure has been investigated in recent years and drilled in 2009 in the framework of an ICDP (International Continental Scientific Drilling Program) project. The target rocks mostly consist of rhyodacitic ignimbrites and tuffs, which make it difficult to distinguish impact melt clasts from fragments of unshocked target rock within the impact breccia. Several chemical and petrologic attempts, other than dating individual clasts, have been considered to distinguish impact melt from unshocked volcanic rock of the targets, but none has proven reliable. Here, we propose to use cathodoluminescence (imaging and spectrometry), whose intensity is inversely correlated with the degree of shock metamorphism experienced by the investigated lithology, to aid in such a distinction. Specifically, impact melt rocks display low cathodoluminescence intensity, whereas unshocked volcanic rocks from the area typically show high luminescence. This high luminescence decreases with the degree of shock experienced by the individual clasts in the impact breccia, down to almost undetectable when the groundmass is completely molten. This might apply only to El’gygytgyn, because the luminescence in volcanic rocks might be due to devitrification and recrystallization processes of the relatively old (Cretaceous) target rock with respect to the young impactites (3.58 Ma). The alteration that affects most samples from the drill core does not have a significant effect on the cathodoluminescence response. In conclusion, cathodoluminescence imaging and spectra, supported by Raman spectroscopy, potentially provide a useful tool for in situ characterization of siliceous impactites formed in volcanic target.

Reference
Pittarello L, Roszjar J, Mader D, Debaille V, Claeys P, Koeberl C, (2015) Cathodoluminescence as a tool to discriminate impact melt, shocked and unshocked volcanics: A case study of samples from the El’gygytgyn impact structure. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12559]
Published by arangement with John Wiley & Sons

¹Thomas Kenkmann, ²Abdulkader M. Afifi, ²Simon A. Stewart, ¹Michael H. Poelchau, ²Douglas J. Cook,²Allen S. Neville2
¹Institute of Earth and Environmental Sciences, Geology, Albert-Ludwigs University Freiburg, Freiburg, Germany
²Area Exploration Department, Saudi Aramco, Dhahran, Saudi Arabia

Here we present the first proof of an impact origin for the Saqqar circular structure in northwestern Saudi Arabia (Neville et al. 2014), with an apparent diameter of 34 km, centered at 29°35′N, 38°42′E. The structure is formed in Cambrian–Devonian siliciclastics and is unconformably overlain by undeformed Cretaceous and Paleogene sediments. The age of impact is not well constrained and lies somewhere between 410 and 70 Ma. The subsurface structure is constrained by 2-D reflection seismic profiles and six drilled wells. First-order structural features are a central uplift that rises approximately 2 km above regional datums, surrounded by a ring syncline. The crater rim is defined by circumferential normal faults. The central uplift and ring syncline correspond to a Bouguer gravity high and an annular ring-like low, respectively. The wells were drilled within the central uplift, the deepest among them exceeded 2 km depth. Sandstone core samples from these wells show abundant indicators of a shock metamorphic overprint. Planar deformation features (PDFs) were measured with orientations along (0001), {10inline image3}, and less frequently along {10inline image1} and {10inline image4}. Planar fractures (PFs) predominantly occur along (0001) and {10inline image1}, and are locally associated with feather features (FFs). In addition, some shocked feldspar grains and strongly deformed mica flakes were found. The recorded shock pressure ranges between 5 and 15 GPa. The preserved level of shock and the absence of an allochthonous crater fill suggest that Saqqar was eroded by 1–2 km between the Devonian and Maastrichtian. The documentation of unequivocal shock features proves the formation of the Saqqar structure by a hypervelocity impact event.

Reference
Kenkmann T, Afifi AM, Stewart SA, Poelchau MH, Douglas J. Cook DJ, Neville AS (2015)
Saqqar: A 34 km diameter impact structure in Saudi Arabia. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12555]
Published by arrangement with John Wiley & Sons

Maike Becker^a,b, Dominik C. Hezel^a,c, Toni Schulz^d, Bo-Magnus Elfers^a, Carsten Münker^a
aInstitut für Geologie und Mineralogie, Universität zu Köln, Zülpicherstrasse 49b, D-50674 Köln, Germany
^bInstitut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 51147 Köln, Germany
^cDepartment of Mineralogy, Natural History Museum, Cromwell Road, SW7 5BD London, UK
^dDepartment of Lithospheric Research, University of Vienna, Althanstraße 14, 1090 Vienna, Austria

Carbonaceous chondrites are an important meteorite group that closely resembles the bulk composition of the solar system. We report the first elemental and isotope dataset for Hf–W in carbonaceous chondrites that includes chondrules, matrix, magnetic fractions as well as bulk compositions. Our study focuses on the three CV3 chondrites, Allende, Vigarano and Bali. Compared to bulk chondrites, matrix splits have low Hf/W ratios and ε¹⁸²W compositions, whereas chondrule splits are characterized by high, but more variable, Hf/W ratios and ε¹⁸²W compositions. Thus, Hf/W ratios behave complementary between chondrules and matrix in the analysed CV chondrites, supporting the view that both components formed from the same parental reservoir. Strong nucleosynthetic effects were observed in most of the analysed CV3 components, especially in matrices and chondrule splits that were found to have large ε¹⁸³W anomalies of several ε-units. All separates define a rough correlation between initial ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W ratios, in agreement with theoretical model trends based on calculations for stellar nucleosynthesis. Our results, therefore, indicate a heterogeneous distribution of s- and r-process W isotopes among the different CV3 chondrite components, arguing for selective thermal processing of early solar system matter during chondrule formation. After correcting for nucleosynthetic anomalies, chondrules and matrix splits of reduced (Vigarano) as well as oxidised (Allende) CV3 chondrites define a linear correlation in ε¹⁸²W vs. ¹⁸⁰Hf/¹⁸⁴W space, which is interpreted as an isochron, covering an age interval within the first ~2.6 Ma after solar system formation. As peak metamorphic temperatures for CV3 chondrites were well below the ¹⁸²Hf–¹⁸²W closure temperature, the resulting isochron within its error most likely defines a common formation interval for all components. The calculated age interval is for the first time based on a combined chondrule-matrix isochron, a marked difference compared to previous studies where only chondrules were analysed. Notably, our formation age interval covers previously reported chondrule formation ages determined using ²⁶Al and Pb–Pb chronometry, illustrating that chondrule and matrix formation started contemporaneously with CAI formation and lasted over a time interval of about 2–3 Ma. Our results also corroborate previous models from ordinary chondrites, in that chondrite parent bodies were not the first planetesimals to have formed in the early solar system.

Reference
Becker M, Hezel DC, Schulz T, Elfers B-M, Münker C (2015) Formation timescales of CV chondrites from component specific Hf–W systematics. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.09.049]
Copyright Elsevier