Alan E. Rubin

Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA

MIL 11207 (R6) and LAP 04840 (R6) contain hornblende and phlogopite; MIL 07440 (R6) contains accessory titan-phlogopite and no hornblende. All three meteorites have been shocked: MIL 11207 contains extensive sulfide veins, pyroxene that formed from dehydrated hornblende, and an extensive network of plagioclase glass; MIL 07440 contains chromite-plagioclase assemblages, chromite veinlets and blebs, pincer-shaped plagioclase patches, but no sulfide veins; LAP 04840 contains olivine grains with chromite-bleb-laden cores and opaque-free rims, rare grains of pyroxene that formed from dehydrated hornblende, and no sulfide veins. These meteorites appear to have been heated to maximum temperatures of approximately 700–900 °C under conditions of moderately high PH₂O (perhaps 250–500 bars). All three samples underwent postshock annealing. During this process, olivine crystal lattices healed (giving the rocks the appearance of shock-stage S1), and diffusion of Fe and S from thin sulfide veins to coarse sulfide grains caused the veins to disappear in MIL 07440 and LAP 04840. This latter process apparently also occurred in most S1–S2 ordinary chondrites of high petrologic type. The pressure–temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers (as suggested in the literature). A giant impact or a series of smaller impacts would then have been required to excavate the hornblende- and biotite-bearing rocks and bring them closer to the surface. It was in that latter location where the samples were shocked, deposited in a hot ejecta blanket of low thermal diffusivity, and annealed.

Reference
Rubin AE (in press) Shock and annealing in the amphibole- and mica-bearing R chondrites. Meteoritics & Planetary Science
[doi:10.1111/maps.12315]
Published by arrangement with John Wiley & Sons

Link to Article

Andreas Morlok^a,b, Andrew B. Mason^c, Mahesh Anand^a,b, Carey Lisse^d, Emma S. Bullock^e and Monica Grady^a,b

^aDepartment of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
^bPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA UK
^cFinnish Centre for Astronomy with ESO (FINCA), University of Turku, Tuorla Observatory, Väisäläntie 20, FI-21500 Piikkiö, Finland
^dJohns Hopkins University -APL, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
^eDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington DC 20560, USA

In order to link infrared observations of dust formed during planet formation in debris disks to mid-infrared spectroscopic data of planetary materials from differentiated terrestrial and asteroidal bodies, we obtained absorption spectra of a representative suite of terrestrial crustal and mantle materials, and of typical Martian meteorites.
A series of debris disk spectra characterized by a strong feature in the 9.0-9.5 μm range (HD23514, HD15407a, HD172555 and HD165014), is comparable to materials that underwent shock, collision or high temperature events. These are amorphous materials such as tektites, SiO₂-glass, obsidian, and highly shocked shergottites as well as inclusions from mesosiderites (Group A).
A second group (BD+20307, Beta Pictoris, HD145263, ID8, HD113766, HD69830, P1121, and Eta Corvi) have strong pyroxene and olivine bands in the 9-12 μm range and is very similar to ultramafic rocks (e.g. harzburgite, dunite)(Group B).
This could indicate the occurrence of differentiated materials similar to those in our Solar System in these other systems.
However, mixing of projectile and target material, as well as that of crustal and mantle material has to be taken into account in large scale events like hit-and-run and giant collisions or even large-scale planetary impacts. This could explain the olivine-dominated dust of group B.
The crustal-type material of group A would possibly require the stripping of upper layers by grazing-style hit-and run encounters or high energy events like evaporation/condensation in giant collisions. In tidal disruptions or the involvement of predominantly icy/water bodies the resulting mineral dust would originate mainly in one of the involved planetesimals. This could allow attributing the observed composition to a specific body (such as e.g. Eta Corvi).

Reference
Morlok M, Mason AB, Anand M, Lisse C, Bullock ES and Grady M (in press) Dust from collisions: A way to probe the composition of exo-planets?. Icarus
[doi:10.1016/j.icarus.2014.05.024]
Copyright Elsevier

Link to Article