F. Jourdan^a, G. Benedix^b, E. Eroglu^c, P.A. Bland^b and A. Bouvier^d

^aWestern Australian Argon Isotope Facility, Department of Applied Geology and JdL-CMS, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
^bDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845
^cSchool of Chemistry and Biochemistry, The University of Western Australia, Crawley, WA 6009, Australia
^dDepartment of Earth Sciences, University of Western Ontario, London, ON, N6A 3K7, Canada

The Bunburra Rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse-, medium- and fine-grained lithologies. Petrographic observations constrain the limited shock pressure to between ca. 10 GPa and 20 GPa. In this study, we carried out nine ⁴⁰Ar/³⁹Ar step-heating experiments on distinct single-grain fragments extracted from the coarse and fine lithologies. We obtained six plateau ages and three mini-plateau ages. These ages fall into two internally concordant populations with mean ages of 3640 ± 21 Ma (n=7; P=0.53) and 3544 ± 26 Ma (n=2; P=0.54), respectively. Based on these results, additional⁴⁰Ar/³⁹Ar data of fusion crust fragments, argon diffusion modeling, and petrographic observations, we conclude that the principal components of the Bunburra Rockhole basaltic achondrite are from a melt rock formed at ~3.64 Ga by a medium to large impact event. The data imply this impact generated high enough energy to completely melt the basaltic target rock and reset the Ar systematics, but only partially reset the Pb-Pb age. We also conclude that a complete ⁴⁰Ar∗ resetting of pyroxene and plagioclase at this time could not have been achieved at solid-state conditions. Comparison with a terrestrial analogue (Lonar crater) shows that the time-temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve. Ar data also suggest that a second medium-size impact event occurred on a neighboring part of the same target rock at ~3.54 Ga. Concordant low-temperature step ages of the nine aliquots suggest that, at ~3.42 Ga, a third smaller impact excavated parts of the ~3.64 Ga and ~3.54 Ga melt rocks and brought the fragments together. The lack of significant impact activity after 3.5 Ga, as recorded by the Bunburra Rockhole suggest that (1) either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts, or (2) it was covered by a porous heat-absorbing regolith blanket which, when combined with the diminishing frequency of large impacts in the solar system, protected Bunburra from subsequent major heating events. Finally we note that the total (K/Ar) resetting impact event history recorded by some of the brecciated eucrites (peak at 3.8-3.5 Ga) is similar to the large impact history recorded by the Bunburra Rockhole parent body (ca. 3.64-3.54 Ga; this study) and could indicate a similar position in the asteroid belt at that time.

Reference
Jourdan F, Benedix G, Eroglu E, Bland PA and Bouvier A (in press) 40Ar/39Ar impact ages and time-temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.039]
Copyright Elsevier

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E.L. Walton^a,b, T.G. Sharp^c, J. Hu^c and J. Filiberto^d

^aMacEwan University, Department of Physical Sciences, 10700-104 Ave, City Centre Campus, Edmonton, AB, T5J 4S2, Canada
^bUniversity of Alberta, Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB, T6G 2E3, Canada
^cArizona State University, School of Earth and Space Exploration, Tempe, AZ, 85287-1404, U.S.A
^dSouthern Illinois University, Department of Geology, Carbondale, IL

The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understand shock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene + ringwoodite ± stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well as entrained olivine fragments, are now amorphosed silicate perovskite + magnesiowüstite or clinopyroxene + magnesiowüstite. The pressure stabilities of these mineral assemblages are ~15 GPa and >19 GPa, respectively. The ~200-μm-wide margin of thicker, mm-size (up to 1.4 mm) shock melt vein contains clinopyroxene + olivine, with central regions comprising glass + vesicles + Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tissintite + glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as 14 GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after release from the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by Tissint ⩾29−30 GPa. These pressure estimates support our assessment that the peak shock pressure in Tissint was significantly higher than the minimum 19 GPa required to transform olivine to silicate perovskite plus magnesiowüstite
Small volumes of shock melt (<100 μm) quench rapidly (0.01 s), whereas thermal equilibration will occur within 1.2 seconds in larger volumes of melt (1 mm²). The apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in Tissint is consistent with a shock pulse on the order of 10−20 ms combined with a longer duration of post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at ~1 Ma was not exceptionally large

Reference
Walton EL, Sharp TG, Hu J and Filiberto J (in press) Heterogeneous mineral assemblages in Martian meteorite Tissint as a result of a recent small impact event on Mars. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.023]
Copyright Elsevier

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Hurwitz D. M. and D. A. Kring

Center for Lunar Science and Exploration, Lunar and Planetary Institute, Houston, Texas, USA

We modeled the differentiation of the South Pole–Aitken (SPA) impact melt sheet to determine whether noritic lithologies observed within SPA formed as a result of the impact. Results indicate differentiation of SPA impact melt can produce noritic layers that may accommodate observed surface compositions but only in specific scenarios. One of nine modeled impact melt compositions yielded layers of noritic materials that account for observations of noritic lithologies at depths of ~6 km. In this scenario, impact occurred before a hypothesized lunar magma ocean cumulate overturn. The 50 km deep melt sheet would have formed an insulating quenched layer at the surface before differentiating. The uppermost differentiated layers in this scenario have FeO and TiO₂ contents consistent with orbital observations if they were subsequently mixed with the uppermost quenched melt layer and with less FeO- and TiO₂-enriched materials such as ejecta emplaced during younger impacts. These results verify that noritic lithologies observed within SPA could have formed as a direct result of the impact. Therefore, locations within SPA that contain noritic materials represent potential destinations for collecting samples that can be analyzed to determine the age of the SPA impact. Potential destinations include central peaks of Bhabha, Bose, Finsen, and Antoniadi craters, as well as walls of Leibnitz and Schrödinger basins. Additionally, potential remnants of the uppermost quenched melt may be preserved in gabbroic material exposed in “Mafic Mound.” Exploring and sampling these locations can constrain the absolute age of SPA, a task that ranks among the highest priorities in lunar science.

Reference
Hurwitz DM and Kring DA (in press) Differentiation of the South Pole–Aitken basin impact melt sheet: Implications for lunar exploration. Journal of Geophysical Research: Planets 
[doi:10.1002/2013JE004530]
Published by arrangement with John Wiley & Sons

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Vasavada¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

The Mars Science Laboratory mission reached Bradbury Landing in August 2012. In its first 500 sols, the rover Curiosity was commissioned and began its investigation of the habitability of past and present environments within Gale Crater. Curiosity traversed eastward toward Glenelg, investigating a boulder with a highly alkaline basaltic composition, encountering numerous exposures of outcropping pebble conglomerate, and sampling aeolian sediment at Rocknest and lacustrine mudstones at Yellowknife Bay. On sol 324, the mission turned its focus southwest, beginning a year-long journey to the lower reaches of Mt. Sharp, with brief stops at the Darwin and Cooperstown waypoints. The unprecedented complexity of the rover and payload systems posed challenges to science operations, as did a number of anomalies. Operational processes were revised to include additional opportunities for advance planning by the science and engineering teams.

Reference
Vasavada et al. (in press) Overview of the Mars Science Laboratory mission: Bradbury Landing to Yellowknife Bay and beyond. Journal of Geophysical Research: Planets
[doi:10.1002/2014JE004622]
Published by arrangement with John Wiley & Sons

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