J. H. Jones

KR, NASA/JSC, Houston, Texas, USA

Several controversies are associated with the age and origin of the shergottite meteorites, a suite of basaltic samples from Mars. Here, it will be argued that (1) the shergottites have a young igneous age, ≤600 Myr, (2) their parent magmas were relatively dry, (3) the range of initial isotopic compositions in shergottites is most likely due to assimilation of crustal materials by mantle-derived basaltic magmas, and (4) the intercumulus liquid compositions of shergottites such as Shergotty and Zagami are relatively well constrained.

Reference
Jones JH (2015) Various aspects of the petrogenesis of the Martian shergottite meteorites. Meteoritics & Planetary Sciences (in Press)
Link to Article [doi:10.1111/maps.12421]

Published by arrangement with John Wiley & Sons

Annelies Van Hoesel^1,2, Wim Z. Hoek², Gillian M. Pennock¹, Knut Kaiser³, Oliver Plümper¹, Michal Jankowski⁴, Maartje F. Hamers¹, Norbert Schlaak⁵, Mathias Küster⁶, Alexander V. Andronikov⁷ and Martyn R. Drury¹

¹Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands
²Department of Physical Geography, Utrecht University, Utrecht, The Netherlands
³GFZ German Research Centre for Geosciences, Potsdam, Germany
⁴Department of Soil Science and Landscape Management, Nicolaus Copernicus University, Toruń, Poland
⁵State Agency for Mining, Geology and Resources Brandenburg (LBGR), Cottbus, Germany
⁶Institute of Geography and Geology, University of Greifswald, Greifswald, Germany
⁷Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

The Younger Dryas impact hypothesis suggests that multiple airbursts or extraterrestrial impacts occurring at the end of the Allerød interstadial resulted in the Younger Dryas cold period. So far, no reproducible, diagnostic evidence has, however, been reported. Quartz grains containing planar deformation features (known as shocked quartz grains), are considered a reliable indicator for the occurrence of an extraterrestrial impact when found in a geological setting. Although alleged shocked quartz grains have been reported at a possible Allerød-Younger Dryas boundary layer in Venezuela, the identification of shocked quartz in this layer is ambiguous. To test whether shocked quartz is indeed present in the proposed impact layer, we investigated the quartz fraction of multiple Allerød-Younger Dryas boundary layers from Europe and North America, where proposed impact markers have been reported. Grains were analyzed using a combination of light and electron microscopy techniques. All samples contained a variable amount of quartz grains with (sub)planar microstructures, often tectonic deformation lamellae. A total of one quartz grain containing planar deformation features was found in our samples. This shocked quartz grain comes from the Usselo palaeosol at Geldrop Aalsterhut, the Netherlands. Scanning electron microscopy cathodoluminescence imaging and transmission electron microscopy imaging, however, show that the planar deformation features in this grain are healed and thus likely to be older than the Allerød-Younger Dryas boundary. We suggest that this grain was possibly eroded from an older crater or distal ejecta layer and later redeposited in the European sandbelt. The single shocked quartz grain at this moment thus cannot be used to support the Younger Dryas impact hypothesis.

Reference
Van Hoesel A, Hoek WZ, Pennock GM, Kaiser K, Plümper O, Jankowski M, Hamers MF, Schlaak N, Küster M, Andronikov AV and Drury MR (2015) A search for shocked quartz grains in the Allerød-Younger Dryas boundary layer. Meteoritics & Planetary Sciences (in Press)
Link to Article [doi:10.1111/maps.12435]

Published by arrangement with John Wiley & Sons

Luigi Folco¹, Massimo D’Orazio¹, Agnese Fazio¹, Carole Cordier^2,3, Antonio Zeoli⁴, Matthias van Ginneken⁵ and Ahmed El-Barkooky⁶

¹Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
²Université de Grenoble Alpes, Grenoble, CEDEX 9, France
³CNRS, Istitute des Sciences de la Terre (ISTerre), Grenoble, CEDEX 9, France
⁴Museo Nazionale dell’Antartide, Università di Siena, Siena, Italy
⁵Department of Earth Science and Engineering, Imperial College, London, UK
⁶Department of Geology, Faculty of Sciences, Cairo University, Giza, Egypt

We report on the microscopic impactor debris around Kamil crater (45 m in diameter, Egypt) collected during our 2010 geophysical expedition. The hypervelocity impact of Gebel Kamil (Ni-rich ataxite) on a sandstone target produced a downrange ejecta curtain of microscopic impactor debris due SE–SW of the crater (extending ~300,000 m2, up to ~400 m from the crater), in agreement with previous determination of the impactor trajectory. The microscopic impactor debris include vesicular masses, spherules, and coatings of dark impact melt glass which is a mixture of impactor and target materials (Si-, Fe-, and Al-rich glass), plus Fe-Ni oxide spherules and mini shrapnel, documenting that these products can be found in craters as small as few tens of meters in diameter. The estimated mass of the microscopic impactor debris (20 t, likely 50–60 t).

Reference
Luigi Folco L, D’Orazio M, Fazio A, Cordier C, Zeoli A, van Ginneken M and El-Barkooky A (2015) Microscopic impactor debris in the soil around Kamil crater (Egypt): Inventory, distribution, total mass, and implications for the impact scenario. Meteoritics & Planetary Sciences (in Press)
Link to Article [doi:10.1111/maps.12427]

Published by arrangement with John Wiley & Sons

M. Alvaro1^1,†, M. C. Domeneghetti², A. M. Fioretti³, F. Cámara^4,5 and L. Marinangeli⁶

¹Dipartimento di Geoscienze, Università degli Studi di Padova, Padova, Italy
²Dipartimento di Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia, Pavia, Italy
³Instituto di Geoscienze e Georisorse CNR, UOS di Padova, Padova, Italy
⁴Dipartimento di Scienze della Terra, Università di Torino, Torino, Italy
⁵CrisDi, Interdepartmental Centre for the Research and Development of Crystallography, Torino, Italy
⁶International Research School of Planetary Sciences, Università G. d’Annunzio, Chieti, Italy
^†Dipartimento di Scienze della Terra e dell’Ambiebnte, Università degli Studi di Pavia, Pavia, Italy

Recently it has been shown that the relatively low closure temperature (Tc) of 500 (100)°C calculated for augite from Miller Range nakhlite (MIL 03346,13) using the available geothermometers would correspond to a slow cooling rate inconsistent with the petrologic evidence for an origin from a fast-cooled lava flow. Moreover, previous annealing experiments combined with HR-SC-XRD on an augite crystal from MIL 03346 clearly showed that at 600 °C, the Fe2+-Mg degree of order remained unchanged, thus suggesting that the actual Tc is close to this temperature. In order to clarify this discrepancy, we undertook an ex situ annealing experimental study at 700, 800, and 900 °C, until the equilibrium in the intracrystalline Fe2+-Mg exchange is reached, using an augite crystal from Miller Range nakhlite (MIL 03346,13) with a composition of about En36Fs24Wo40. These data allowed us to calculate the following new geothermometer calibration for Martian nakhlites:

where The application of this new equation to other Martian nakhlites (NWA 988 and Nakhla) suggests that for augite with composition close to that of MIL 03346, the Tc is up to 170 °C higher with respect to the one calculated using the previous available geothermometer equation, thus suggesting a significantly faster cooling in agreement with petrologic evidence.

Reference
Alvaro M, Domeneghetti MC, Fioretti AM, Cámara F and Marinangeli L (2015) A new calibration to determine the closure temperatures of Fe-Mg ordering in augite from nakhlites. Meteoritics & Planetary Sciences (in Press)
Link to Article [doi:10.1111/maps.12436]

Published by arrangement with John Wiley & Sons