¹C. K. Shearer, ¹P. V. Burger, ¹J. J. Papike, ¹F. M. McCubbin,¹A. S. Bell
¹Institute of Meteoritics and Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA

Merrillite is a ubiquitous accessory phase in a variety of Martian meteorite lithologies. The Martian merrillites exhibit a positive correlation between Mg# and Na and a negative correlation between Mg# and both Mn and vacancies in the octahedral Na-site. Their REE patterns are varied and range from LREE-depleted to LREE-enriched. The dominant cation substitutions in the Martian merrillites are Fe2+VI Mg-site[]Mg2+VI Mg-site and Ca2+VI Na-site + □VI Na-site[]2Na+VI Na-site. The REE substitution into the 8-fold coordinated Ca-site is accommodated by the coupled substitution CaVIII Ca-site + (Na)VI Na-site [](Y3+ + REE3+)VIII Ca-site + □VI Na-site. The REE substitution is significantly more prevalent in lunar merrillite and can be used as a “fingerprint” to distinguish lunar from Martian meteorites. The substitution of OH− (whitlockite) and/or F− (bobdownsite) for O2− on one of the phosphate tetrahedrons appears to be rather insignificant. The correlations among Na, Mg#, Mn, and Na-site vacancies are linked to the premerrillite crystallization history of the melt and the crystal chemical behavior of the Mg- and Na-sites. The former reflects the sequence and extent of plagioclase and pyroxene crystallization. The differences in REE pattern shapes among the merrillites reflect source regions for the Martian basalts and the shapes are not greatly perturbed by the crystallization history. The occurrence of merrillite does not imply low-volatile component in the Martian magmas. However, the low whitlockite and bobdownsite contents suggest that these samples were not altered by hydrothermal fluids and therefore not reset owing to aqueous fluid interactions. Consequently, the young ages of the shergottites are probably true igneous crystallization ages.

Reference
Shearer CK, Burger PV, Papike JJ, McCubbin FM, Bell AS (2014) Crystal chemistry of merrillite from Martian meteorites: Mineralogical recorders of magmatic processes and planetary differentiation. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12355]

Published by arrangement with John Wiley&Sons

¹Agnese Fazio, ¹Luigi Folco, ¹Massimo D’Orazio, ²Maria Luce Frezzotti,^3,4Carole Cordier
¹Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
²Dipartimento di Scienze dell’ambiente e del Territorio e di Scienze della Terra (DISAT), Sezione di Scienze Geologiche Geotecnologie, Milano, Italy
³Université de Grenoble Alpes, ISTerre, Grenoble CEDEX 9, France
⁴CNRS, ISTerre, Grenoble CEDEX 9, France

Kamil is a 45 m diameter impact crater identified in 2008 in southern Egypt. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite on a sedimentary target, namely layered sandstones with subhorizontal bedding. We have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign (February 2010) in order to investigate shock effects recorded in these rocks. Ejecta samples reveal a wide range of shock features common in quartz-rich target rocks. They have been divided into two categories, as a function of their abundance at thin section scale: (1) pervasive shock features (the most abundant), including fracturing, planar deformation features, and impact melt lapilli and bombs, and (2) localized shock features (the least abundant) including high-pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films in shatter cones. In particular, Kamil crater is the smallest impact crater where shatter cones, coesite, stishovite, diamond, and melt veins have been reported. Based on experimental calibrations reported in the literature, pervasive shock features suggest that the maximum shock pressure was between 30 and 60 GPa. Using the planar impact approximation, we calculate a vertical component of the impact velocity of at least 3.5 km s−1. The wide range of shock features and their freshness make Kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures.

Reference
Fazio A, Folco L, D’Orazio M, Frezzotti ML, Cordier C (2014) Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil crater, Egypt. Meteoritics & Planetary Science (In Press)
Link to Article [doi: 10.1111/maps.12385]

Published by arrangement with John Wiley&Sons

¹S. W. Lehner1, ²W. F. McDonough,³P. Németh
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Department of Geology, University of Maryland, College Park, Maryland, USA
³Institute of Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary

We investigated the matrix mineralogy in primitive EH3 chondrites Sahara 97072, ALH 84170, and LAR 06252 with transmission electron microscopy; measured the trace and major element compositions of Sahara 97072 matrix and ferromagnesian chondrules with laser-ablation, inductively coupled, plasma mass spectrometry (LA-ICPMS); and analyzed the bulk composition of Sahara 97072 with LA-ICPMS, solution ICPMS, and inductively coupled plasma atomic emission spectroscopy. The fine-grained matrix of EH3 chondrites is unlike that in other chondrite groups, consisting primarily of enstatite, cristobalite, troilite, and kamacite with a notable absence of olivine. Matrix and pyroxene-rich chondrule compositions differ from one another and are distinct from the bulk meteorite. Refractory lithophile elements are enriched by a factor of 1.5–3 in chondrules relative to matrix, whereas the matrix is enriched in moderately volatile elements. The compositional relation between the chondrules and matrix is reminiscent of the difference between EH3 pyroxene-rich chondrules and EH3 Si-rich, highly sulfidized chondrules. Similar refractory element ratios between the matrix and the pyroxene-rich chondrules suggest the fine-grained material primarily consists of the shattered, sulfidized remains of the formerly pyroxene-rich chondrules with the minor addition of metal clasts. The matrix, chondrule, and metal-sulfide nodule compositions are probably complementary, suggesting all the components of the EH3 chondrites came from the same nebular reservoir.

Reference
Lehner SW, McDonough WF, Németh P (2014) EH3 matrix mineralogy with major and trace element composition compared to chondrules. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12391]

Published by arrangement with John Wiley&Sons

^1,2Haley M. Sapers, ^1,3Gordon R. Osinski, ¹Neil R. Banerjee, ^1,4Ludovic Ferrière, ⁵Philippe Lambert, ^1,6Matthew R. M. Izawa
¹Department of Earth Sciences and the Centre for Planetary Science and Exploration, Western University, London, ON, Canada
²Department of Natural Resource Science, McGill Université, Québec, Canada
³Department of Physics & Astronomy, University of Western Ontario, London, Ontario, Canada
⁴Natural History Museum, Vienna, Austria
⁵Sciences et Applications, Bordeaux, France
⁶Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada

The Rochechouart impact structure, south-central France (45o50′N, 0o46′E), is a partly eroded, approximately 200 Myr, complex impact structure. The impactite suite at Rochechouart provides an excellent example of gradational boundaries and transitional lithologies that have been historically difficult to classify with standard impactite nomenclature. Here, we present the first detailed scanning electron microscopy-based description of the Rochechouart impactites integrated with hand-sample and petrographic observations with the goal of understanding the clast-matrix relationships of transitional lithologies. Three main impact-generated hydrothermal alteration assemblages are also recognized: (1) argillic-like, (2) carbonate, and (3) oxide. Our results support the existence of a continuum between clast-rich impact melt rocks and glass-rich clastic breccias (suevites) that must be represented in universal classification schemes. This suite of impactites from the Rochechouart impact structure is used as a test case for a recently published classification scheme based on the nature of the groundmass setting a precedent for classification of impactites with limited to no geological context such as deeply eroded terrestrial impact structures and future sample return missions. The re-evaluation of the melt-bearing Rochechouart impactites questions the currently accepted size of the crater, suggesting a much larger original crater diameter.

Reference
Sapers HM, Osinski GR, Banerjee NR, Ferrière L, Lambert P, Izawa MRM (2014) Revisiting the Rochechouart impact structure, France. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12381]

Published by arrangement with John Wiley&Sons