¹Dennis Harries, ^1,2Falko Langenhorst
Geochimica et Cosmochimica Acta 222, 53-73, Link to Article [https://doi.org/10.1016/j.gca.2017.10.019]
¹Institute of Geoscience, Friedrich Schiller University Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
²Hawai’i Institute of Geophysics and Planetology, School of Ocean and EarthScience and Technology, University of Hawai’i at Manoa, HI 96822, USA
Copyright Elsevier

We found that the particle RA-QD02-0115 returned by the Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa contains the iron carbide haxonite (Fe_21.9-22.7Co_0.2-0.3Ni_0.2-0.8)C₆ and several Fe,Ni alloys, including multi-domain tetrataenite and spinodally decomposed taenite. Ellipsoidal to nearly spherical voids occur throughout the particle and suggest the presence of a fluid phase during textural and chemical equilibration of the host rock within the parent asteroid of 25143 Itokawa. The calculated solubility of carbon in Fe,Ni metal indicates that the carbide formed at temperatures larger than 600 °C during thermal metamorphism of the LL-chondritic mineral assemblage. Haxonite formed metastably with respect to graphite and cohenite, probably due to its high degree of lattice match with neighboring taenite, a low cooling rate at peak metamorphic temperatures, and the hindered nucleation of graphite. Thermodynamic equilibrium calculations indicate that the fluid present was dry (H₂O-poor) and dominated by methane. The reactive fluid most plausibly had an atomic H/C ratio of 4–5 and was derived from the reduction of macromolecular, insoluble organic matter (IOM) that initially co-accreted with water ice. The initial presence of water is a necessary assumption to provide sufficient hydrogen for the formation of methane from hydrolyzed IOM. Metallic iron was in turn partially oxidized and incorporated into the ferromagnesian silicates during the high-temperature stage of metamorphism. An exemplary bulk reaction from unequilibrated material on the left to an equilibrated assemblage on the right may be written as:

330 CH_0.8O_0.2(IOM) + 500 H₂O_(ice/g) + 681 Fe_{(in alloy)} + 566 FeSiO_{3(in Opx)} → 300 CH_4(g) + 32 H_2(g) + 5 Fe₂₃C_{6(in Hx)} + 566 Fe₂SiO_{4(in Ol)}

(Opx = orthopyroxene, Hx = haxonite, Ol = olivine, g = fluid species).

The best estimate of the fluid/rock ratio in the region of the LL parent body where RA-QD02-0115 formed is about 3 × 10⁻³ and corresponds to an initial ice/rock ratio of about 7 × 10⁻³(both by mass).

¹Dimitri D. Badyukov,^2,3Natalia S. Bezaeva,⁴Pierre Rochette,⁴Jérôme Gattacceca,⁵Joshua M. Feinberg,⁶Myriam Kars,⁷Ramon Egli,⁸Jouko Raitala,⁹Dilyara M. Kuzina
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13006]
¹V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia
²Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia
³Institute of Geology and Petroleum Technologies, Kazan Federal University, Kazan, Russia
⁴Aix-Marseille Université, CNRS, IRD, Coll. France, CEREGE, Aix en Provence, France
⁵Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota, USA
⁶Center for Advanced Marine Core Research, Kochi University, Nankoku, Japan
⁷Central Institute for Meteorology and Geodynamics, Vienna, Austria
⁸Astronomy Department, University of Oulu, Oulu, Finland
⁹Institute of Geology and Petroleum Technologies, Kazan Federal University, Kazan, Russia
Published by arrangement with John Wiley & Sons

Hypervelocity impacts occur on bodies throughout our solar system, and play an important role in altering the mineralogy, texture, and magnetic properties in target rocks at nanometer to planetary scales. Here we present the results of hypervelocity impact experiments conducted using a two-stage light-gas gun with 5 mm spherical copper projectiles accelerated toward basalt targets with ~6 km s⁻¹ impact velocities. Four different types of magnetite- and titanomagnetite-bearing basalts were used as targets for seven independent experiments. These laboratory impacts resulted in the formation of agglutinate-like particles similar in texture to lunar agglutinates, which are an important fraction of lunar soil. Materials recovered from the impacts were examined using a suite of complementary techniques, including optical and scanning electron microscopy, micro-Raman spectroscopy, and high- and low-temperature magnetometry, to investigate the texture, chemistry, and magnetic properties of newly formed agglutinate-like particles and were compared to unshocked basaltic parent materials. The use of Cu-projectiles, rather than Fe- and Ni-projectiles, avoids magnetic contamination in the final shock products and enables a clearer view of the magnetic properties of impact-generated agglutinates. Agglutinate-like particles show shock features, such as melting and planar deformation features, and demonstrate shock-induced magnetic hardening (two- to seven-fold increases in the coercivity of remanence B_cr compared to the initial target materials) and decreases in low-field magnetic susceptibility and saturation magnetization.