^1,2A. Losiak,³A. Jõeleht,³J. Plado,⁴M. Szyszka,³K. Kirsimäe,⁵E. M. Wild,⁵P. Steier,²C. M. Belcher,¹A. M. Jazwa,³R. Helde
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13431]
¹Planetary Geology Lab, Institute of Geological Sciences, Polish Academy of Sciences, Warsaw, Poland
²wildFIRE Lab, Hatherly Laboratories, University of Exeter, Exeter, UK
³Department of Geology, University of Tartu, Tartu, Estonia
⁴Institute of Geology, Adam Mickiewicz University in Poznań, Poznań, Poland
⁵VERA Laboratory, Faculty of Physics—Isotope Physics, University of Vienna, Vienna, Austria
Published by arrangement with John Wiley & Sons

The Ilumetsa site, in Estonia, consists of two round, rimmed structures that are 725 m apart. The structures are listed as proven impact craters in the Impact Earth database, despite lack of commonly accepted, unequivocal proof of extraterrestrial collision identified at this location. We excavated trenches though the Ilumetsa Large and Ilumetsa Small structures and found small pieces of charcoal within the putative proximal ejecta in both structures, in a similar geological setting as previously identified charcoal in Kaali (Losiak et al. 2016) and Morasko craters (see Szokaluk et al. 2019). Our ¹⁴C dating of charcoal allowed us to conclude that these crater‐like features formed simultaneously between 7170 and 7000 cal. years bp, about 7 ka after deglaciation of this area. A ground penetrating radar survey of the nearby bog shows that no additional Ilumetsa structures bigger than 40 m exist. Geochemical studies of the ejecta and a search using a metal detector did not reveal any clear indication of extraterrestrial material. This suggests Ilumetsa may have been formed by an impact of stony‐iron or stony body, which got significantly weathered in a wet‐temperate climate. The mystery of the formation of the structures at Ilumetsa remains; however, due to significant circumstantial evidence discussed herein, we are confident to call it a “probable” impact site.

¹Billy P. Glass,²Luigi Folco,²Matteo Masotta,^2,3Fabrizio Campanale
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13433]
¹Department of Geological Sciences, University of Delaware, Newark, Delaware, 19716 USA
²Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
³Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia (IIT), Piazza San Silvestro 12, 56127 Pisa, Italy
Published by arrangement with John Wiley & Sons

We examined 16 white opaque inclusions exposed on two polished slices of a Muong Nong‐type Australasian tektite from Muong Phin, Laos. The inclusions usually consist of a core, surrounded by a froth layer, and a quartz neoblast layer. The cores are composed primarily of a mixture of silica glass, coesite, and quartz in varying proportions. A thin (up to ~4 μm) layer of SiO₂‐poor glass enriched in FeO, MgO, CaO, Al₂O₃, and TiO₂ is observed as a bright halo in backscattered electron images around the quartz neoblasts and in places contains μm‐sized crystals, which may be Fe,Mg‐rich spinel. The distribution and textural relationships between the coesite‐bearing inclusions and the tektite matrix point to an in situ formation of the coesite due to an impact, rather than to infall, from a nearby impact, into tektite melt produced by the aerial burst of a bolide. The quartz neoblasts probably formed by crystallization of silica melt squeezed out of the inclusion core during the development of the froth layer. The bright halo may be the result of silica diffusing from the adjacent tektite melt into the growing quartz neoblasts. We propose that the survival of coesite was possible due to the froth layer that acted as a heat sink during bubble expansion and then as a thermal insulator.

^1,2Ryota Moriwaki,^3,4Tomohiro Usui,²Minato Tobita,²Tetsuya Yokoyama
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.014]
¹Planetary Exploration Research Center, Chiba Institute of Technology, Address: 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Address: 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
³Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Address: 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
⁴Earth-Life Science Institute, Tokyo Institute of Technology, Address: 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
Copyright Elsevier

Radiogenic isotopic compositions of shergottite meteorites suggest that early planetary differentiation processes, which are related to the crystallization of the Martian Magma Ocean (MMO), resulted in the geochemically heterogeneous Martian mantle. In order to understand the early geochemical evolution of Mars, we investigated the Pb isotope systematics in the depleted Martian mantle on the basis of the analyses of two geochemically depleted shergottites, Dar al Gani (DaG) 476 and Yamato 980459 (Y-980459). Their initial Pb isotopic compositions were estimated from geochemical analyses of highly leached acid residues and age-correction calculations using reference crystallization ages. This yielded μ-values (²³⁸U/²⁰⁴Pb) for the DaG 476 and Y-980459 source reservoirs of 2.33 ± 0.07 and 2.32 ± 0.06, respectively. These μ-values are distinct from those of other depleted shergottite source reservoirs (e.g., 1.4 ± 0.1 for the Tissint meteorite) and show a negative correlation with corresponding ¹⁴⁷Sm/¹⁴⁴Nd, ¹⁷⁶Lu/¹⁷⁷Hf, ɛ¹⁸²W, and ε¹⁴²Nd compositions. Such correlations between long- and short-lived isotopic signatures suggest that a geochemically heterogeneous depleted shergottite source mantle was formed on the early Mars. This geochemical heterogeneity would have been formed by variable mixing of depleted and enriched end-member components that originally formed by fractional crystallization in the MMO. Local remelting in the geochemically depleted Martian mantle after the crystallization of the MMO is another possible explanation for the formation of a geochemically heterogeneous depleted shergottite source mantle.

^1,2F.Jourdan,^1,2T.Kennedy,²G.K.Benedix,³E.Eroglu,¹C.Mayer
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.036]
¹Western Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University, Australia
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Australia
³Discipline of Chemical Engineering, WA School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Perth, WA 6845, Australia
Copyright Elsevier

Eucrites are extraterrestrial basalts and cumulate gabbros formed, and subsequently more or less metamorphosed, at the crustal level of the HED (Howardite-Eucrite-Diogenite) parent body, thought to be the asteroid 4 Vesta. Unbrecciated eucrites offer the best way to understand the igneous, metamorphic and cooling processes occurring in the crust of Vesta following accretion since they were not substantially affected/altered by secondary impact processes. The ⁴⁰Ar/³⁹Ar system of unbrecciated eucrites should be in a relatively pristine state, and thus can inform us on the early volcanic and thermal history of the HED parent body, and, in particular, the cooling history of various crustal parts below the ∼300 °C isotherm, which represents the average closure temperature of the Ar diffusion in plagioclase.

We analysed plagioclase and pyroxene (± groundmass) separates of two cumulate (Moore County and Moama), and five (Caldera, BTN 00300, EET 90020, GRA 98098, QUE 97053) equilibrated basaltic eucrites with the ⁴⁰Ar/³⁹Ar technique using a Thermo© ARGUS VI multi-collection mass spectrometer. The two cumulate unequilibrated gabbros also gave cooling ages of 4531 ± 11 Ma and 4533 ± 12 Ma and combined with a fast cooling rate estimated from lamella thicknesses, suggest that magmatic activity persisted up to 4533 ± 11 and 4535 ± 12 Ma and that the plutons were intruded in a relatively shallow part of the crust, above the metamorphosed regions. Four equilibrated eucrites yielded a well-defined cluster of ages between 4523 ± 8 Ma to 4514 ± 6 Ma. Those ages indicate when the part of the upper crust, where those eucrites probably resided (∼10-15 km deep), cooled below ∼300°C at a rate of 17.3 ± 3.6°C/Ma (2σ). Such a slow cooling rate combined with available global thermal models, supports the hypothesis of a global crustal metamorphism by burial and reheating of lava flows. Finally, an age of 4531 ± 5 Ma was obtained for metamorphosed eucrite EET 90020 and, combined with petrographic observations, indicates the age of a major crustal excavation event by impact. ⁴⁰Ar diffusion models suggest that it is possible to differentiate impact vs crustal cooling provided that a sufficient quantity of pyroxene is measured by ⁴⁰Ar/³⁹Ar.