^1,2Alan E. Rubin
¹Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
²Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA

Maskelynite is a diaplectic glass that forms from plagioclase at shock pressures of ∼20-30 GPa, depending on the Ca concentration. The proportion of maskelynite-rich samples in a basaltic meteorite group correlates with the parent-body escape velocity and serves as a shock indicator of launching conditions. For eucrites (basalts widely presumed to be from Vesta; vesc = 0.36 km s-1), ∼5% of the samples are maskelynite rich. For the Moon (vesc = 2.38 km s-1), ∼30% of basaltic meteorites are maskelynite rich. For Mars (vesc = 5.03 km s-1), ∼93% of basaltic meteorites are maskelynite rich. In contrast, literature data show that maskelynite is rare (∼1%) among mare basalts and basaltic fragments in Apollo 11, 12, 15 and 17 soils (which were never ejected from the Moon). Angrites are unbrecciated basaltic meteorites that are maskelynite free; they were ejected at low-to-moderate shock pressures from an asteroid smaller than Vesta.

Because most impacts that eject materials from a large (⩾100 km) parent body are barely energetic enough to do that, a collision that has little more than the threshold energy required to eject a sample from Vesta will not be able to eject identical samples from the Moon or Mars. There must have been relatively few impacts, if any, that launched eucrites off their parent body that also imparted shock pressures of ∼20-30 GPa in the ejected rocks. More-energetic impacts were required to launch basalts off the Moon and Mars. On average, Vesta ejecta were subjected to lower shock pressures than lunar ejecta, and lunar ejecta were subjected to lower shock pressures than martian ejecta.

H and LL ordinary chondrites have low percentages of shock-stage S5 maskelynite-bearing samples (∼1% and ∼4%, respectively), probably reflecting shock processes experienced by these rocks on their parent asteroids. In contrast, L chondrites have a relatively high proportion of samples containing maskelynite (∼11%), most likely a result of catastrophic parent-body disruption 470 Ma ago.

Reference
Rubin AE (2015) Maskelynite in asteroidal, lunar and planetary basaltic meteorites: An indicator of shock pressure during impact ejection from their parent bodies. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.05.010]
Copyright Elsevier

¹Kazushige Tomeoka,²Ichiro Ohnishi
¹Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan
²EM Business Unit, JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan

Carbonaceous chondrites mainly consist of chondrules and inclusions embedded in a fine-grained matrix. This texture is widely believed to have formed primarily by direct accretion of solar nebular materials, although it may have been modified to various extents by subsequent parent-body processes.

Recently, we studied all chondrules and inclusions larger than 400 μm in diameter and their rims (referred to as chondrules/rims) in the Mokoia CV3 carbonaceous chondrite using a scanning electron microscope, and found that the chondrules/rims experienced various degrees of aqueous alteration and that some also exhibit evidence of thermal metamorphism. The mineralogical and petrographic characteristics of the chondrules/rims suggest that the alteration and metamorphism occurred within the meteorite parent body. In contrast, however, the surrounding matrix does not show evidence of such alteration and metamorphism. These findings indicate that the alteration and metamorphism of the chondrules/rims did not occur in situ. Based on these results, we proposed a model that the chondrules/rims are actually clasts transported from regions in the parent body different from the location where the host meteorite was finally lithified.

If it can be assumed that the chondrules and inclusions studied are representative of all chondrules and inclusions in Mokoia, the results and interpretation pose a fundamental challenge regarding the formation of the whole Mokoia lithology; that is, it cannot be explained by either direct accretion of the solar nebula or conventional parent-body brecciation. We propose a model for the development of the Mokoia lithology through formation of chondrules/rims and fine matrix grains by fragmentation in different regions in the parent body, followed by transportation, mixing, and accumulation in a fluid state, and finally lithification of those objects. These processes may have been repeated, cyclically, within the parent body.

Reference
Tomeoka K, Ohnishi I (2015) Redistribution of chondrules in a carbonaceous chondrite parent body: A model. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.05.012]

Copyright Elsevier

¹Alexei V. Fedkin, ^1,2Lawrence Grossman, ³Munir Humayun, ¹Steven B. Simon, ¹Andrew J. Campbell
¹Dept. of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637
²Also Enrico Fermi Institute, The University of Chicago
³National High Magnetic Field Laboratory and, Dept. of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida 32310

The impact hypothesis for the origin of CB chondrites was tested by performing equilibrium condensation calculations in systems composed of vaporized mixtures of projectile and target materials. When one of the impacting bodies is composed of the metal from CR chondrites and the other is an H chondrite, good agreement can be found between calculated and observed compositions of unzoned metal grains in CB chondrites but the path of composition variation of the silicate condensate computed for the same conditions that reproduce the metal grain compositions does not pass through the measured compositions of barred olivine (BO) or cryptocrystalline (CC) chondrules in the CBs. The discrepancy between measured chondrule compositions and those of calculated silicates is not reduced when diogenite, eucrite or howardite compositions are substituted for H chondrite as the silicate-rich impacting body. If, however, a CR chondrite body is differentiated into core, a relatively CaO-, Al2O3-poor mantle and a CaO-, Al2O3-rich crust, and later accretes significant amounts of water, a collision between it and an identical body can produce the necessary chemical conditions for condensation of CB chondrules. If the resulting impact plume is spatially heterogeneous in its proportions of crust and mantle components, the composition paths calculated for silicate condensates at the same Ptot, Ni/H and Si/H ratios and water abundance that produce good matches to the unzoned metal grain compositions pass through the fields of BO and CC chondrules, especially if high-temperature condensates are fractionated in the case of the CCs. While equilibrium evaporation of an alloy containing solar proportions of siderophiles into a dense impact plume is an equally plausible hypothesis for explaining the compositions of the unzoned metal grains, equilibrium evaporation can explain CB chondrule compositions only if an implausibly large number of starting compositions is postulated. Kinetic models applied to co-condensing metal grains and silicate droplets in a region of the plume with very similar composition, but with high cooling rate and sharply declining Ptot during condensation, produce very good matches to the zoning profiles of Ir, Ni, Co and Cr concentrations and Fe and Ni isotopic compositions observed in the zoned metal grains in CB chondrites but produce very large positive δ56Fe in the cogenetic silicate, which are not found in the chondrules.

Reference
Fedkin AV, Grossman L, Humayun M, Simon SB, Campbell AJ (2015) Condensates from vapor made by impacts between metal-, silicate-rich bodies: Comparison with metal and chondrules in CB chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.05.022]

Copyright Elsevier