^1,2Run-Lia Pang,²Dennis Harries,¹Kilian Pollok,^1,3Ai-Cheng Zhang,^2,4Falko Langenhorst
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125541]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
²Institute of Geosciences, Friedrich Schiller University Jena, D-07745 Jena, Germany
³CAS Center for Excellence in Comparative Planetology, China
⁴Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawai’i 96822, USA
Copyright Elsevier

Shock-induced Ti-rich melt pockets in a basaltic eucrite Northwest Africa (NWA) 8003 were studied using scanning and transmission electron microscopy. Unique mineral assemblages consisting of clinopyroxene, ilmenite, vestaite, corundum, and kyanite are observed. Among them, vestaite and corundum in NWA 8003 are first reported to occur in eucrite meteorites. Petrographic and chemical evidences indicate that the Ti-rich melt pockets have formed by in-situ melting of ilmenite, plagioclase, pyroxene, and possibly minor silica and apatite nearby. The temperature rise and melting were caused by the high shock impedance contrast at interfaces between ilmenite and other phases with a distinctly lower density. Crystallization pressure, temperature and cooling time of the Ti-rich melt pockets in NWA 8003 are constrained to be ∼0.9–∼10 GPa, ∼1300–∼1730 °C, and < 1 ms (5–50 μm in size), respectively.

¹Doreen Schmidt,¹ Kilian Pollok,² Gabor Matthäus,²Stefan Nolte,^1,3FalkoLangenhorst
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125542]
¹Institute of Geoscience, Friedrich Schiller University, Carl-Zeiss-Promenade 10, 07745, Jena, Germany
²Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University, Albert-Einstein-Straße 15, 07745, Jena, Germany
³Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawai’i, 96822, USA
Copyright Elsevier

Space weathering by micrometeoroid bombardment is a cosmic phenomenon on atmosphere-free celestial bodies, a process that is expected to particularly overprint planetesimals and cosmic dust in debris discs. We reproduced micrometeoroid impact craters by femtosecond pulsed laser irradiation on oriented enstatite single crystals (En₉₃Fs₇) to investigate the deformation behavior and its orientation dependence. All microcraters show typical bowl shaped morphologies, a glass surface layer with splash like ejecta material and subsurface layering. Although we could reproduce melting and vaporization as typical space weathering effects in the enstatite experiments, there is no formation of agglutinate particles or metallic nanoparticles (npFe⁰). The shock effects in the deformation layer consist of planar structures like microfractures and cleavages, amorphous lamellae, stacking faults and clinoenstatite lamellae. Their activation and/or orientation depends on the shock direction. In special orientations we observe the activation of glide systems along specific low indexed crystallographic planes. Due to the short timescale and the high strain rates, the most prominent effect is the failure of enstatite by microfracturing along non-rational crystallographic planes. Common deformation mechanisms reported in meteorites like the formation of clinoenstatite lamellae via shearing along [001] (100) occur less frequently. Shear is apparently the dominant mechanism in the formation of the above-mentioned effects and causes also their modification by frictional heating. The wide-spread formation of amorphous lamellae is, for example, interpreted to be the result of this shear heating along planar structures. We interpret this unconventional deformation behavior as a consequence of the small spatial and temporal scale of the experiments, resulting in a short-lived spherical shock wave with high deviatoric stresses in contrast to a long pressure pulse and quasi-hydrostatic compression in large scale impacts that produce typical shock features.

¹M.Gellissen,¹A.Holzheid,¹Ph.Kegler,²H.Palme
Geochemistr< (Chemie der Erde)  (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125540]
¹Institut für Geowissenschaften, Universität Kiel, Germany
²Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt / Main, Germany
Copyright Elsevier

We have studied the evaporation of Na, K and Mn from Al-Na-K- and Mn-rich silicate at various conditions. Total alkali oxide contents ranged from 5 to 20%. The evaporation rate of Na increases with temperature and decreasing oxygen fugacity and decreases with duration of heating. The loss of K is in all cases less pronounced than for Na. Heating in an evacuated vacuum furnace is more effective in removing Na and K from melt droplets than in furnaces with one atm gas flow of air or gas mixtures controlling the oxygen fugacity. The strong pumping required to keep the vacuum removes Na and K atoms very effectively. In all experiments, the rate of evaporation is determined by quasi-equilibrium between a thin layer of Na and K rich gas above the molten silicates. The results of the experiments are in agreement with several other studies.

In experiments with more than one sample in the furnace, equilibration of Na- and K-rich samples with Na- and K-poor samples occurred rapidly, mediated by the ambient gas phase.

The results of experiments with Mn in starting compositions showed much stronger losses of Na than Mn under a variety of conditions.

Thus the nearly chondritic Mn/Na ratios in the Earth cannot be the result of evaporation of Na and Mn in Earth-making materials, as the Mn/Na ratios in evaporation residues would be much higher than chondritic ratios. Such evaporation processes may have occurred in the parent material of Moon, Vesta and Mars.

The data suggest, in agreement with earlier hypotheses, that the high and variable contents of Na and K in chondrules require a gas phase high in Na and K equilibrating with chondrule melts. The volume of nebular gas parental to a certain type of chondrites was heated and Na and K were lost from the chondrule precursors to the gas phase. Subsequently the nebular parcel was compressed leading to higher partial pressures of Na and K. Flash heating then produced chondrule melts which incorporated some of the gaseous Na and K and then cooled rapidly. The large range of Na and K contents in chondrule melts reflects very local enrichments of Na and K in the gas phase. Despite these variations bulk chondritic meteorites have well defined bulk Na and K contents, implying a closed system during formation of chondrule and matrix.