¹J. Brian Balta, ^2,3Matthew E. Sanborn, ^1,4Arya Udry, ²Meenakshi Wadhwa, ¹Harry Y. McSween Jr.
¹Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA
²Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
³Department of Earth and Planetary Sciences, UC Davis, Davis, California, USA
⁴Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA

The fall and recovery of the Tissint meteorite in 2011 created a rare opportunity to examine a Martian sample with a known, short residence time on Earth. Tissint is an olivine-phyric shergottite that accumulated olivine antecrysts within a single magmatic system. Coarse olivine grains with nearly homogeneous cores of Mg# >80 suggest slow re-equilibration. Many macroscopic features of this sample resemble those of LAR 06319, including the olivine crystal size distribution and the presence of evolved oxide and olivine compositions. Unlike LAR 06319, however, no magmatic hydrous phases were found in the analyzed samples of Tissint. Minor and trace element compositions indicate that the meteorite is the product of closed-system crystallization from a parent melt derived from a depleted source, with no obvious addition of a LREE-rich (crustal?) component prior to or during crystallization. The whole-rock REE pattern is similar to that of intermediate olivine-phyric shergottite EETA 79001 lithology A, and could also be approximated by a more olivine-rich version of depleted basaltic shergottite QUE 94201. Magmatic oxygen fugacities are at the low end of the shergottite range, with log fO2 of QFM-3.5 to -4.0 estimated based on early-crystallized minerals and QFM-2.4 estimated based on the Eu in pyroxene oxybarometer. These values are similarly comparable to other depleted shergottites, including SaU 005 and QUE 94201. Tissint occupies a previously unsampled niche in shergottite chemistry: containing olivines with Mg# >80, resembling the enriched olivine-phyric shergottite LAR 06319 in its crystallization path, and comparable to intermediate olivine-phyric shergottite EETA 79001A, depleted olivine-phyric shergottite DaG 476, and depleted basaltic shergottite QUE 94201 in its trace element abundances and oxygen fugacity. The apparent absence of evidence for terrestrial alteration in Tissint (particularly in trace element abundances in the whole-rock and individual minerals) confirms that exposure to the arid desert environment results in only minimal weathering of samples, provided the exposure times are brief.

Reference
Balta JB, Sanborn ME, Udry A, Wadhwa M, McSween Jr. HY (2014) Petrology and trace element geochemistry of Tissint, the newest shergottite fall. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12403]

Published by arrangement with John Wiley&Sons

¹Winfried H. Schwarz, ^2,3Martin Schmieder, ^4,5Elmar Buchner, ¹Mario Trieloff, ⁶Jarmo Moilanen, ⁷Teemu Öhman
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²School of Earth and Environment, University of Western Australia, Crawley, WA, Australia
³Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, Perth, WA, Australia
⁴HNU Neu-Ulm University, Neu-Ulm, Germany
⁵Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Stuttgart, Germany
⁶Oulu, Finland
⁷Arctic Planetary Science Institute, Rovaniemi, Finland

A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km Paasselkä impact structure in southeast Finland was dated via 40Ar/39Ar step-heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) (MSWD = 0.32; p = 0.93) is equal to the previously published pseudoplateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling.

Reference
Schwarz WH, Schmieder M, Buchner E, Trieloff M, Moilanen J, Öhman T (2014) A Carnian 40Ar/39Ar age for the Paasselkä impact structure (SE Finland)—An update. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12407]

Published by arrangement with John Wiley & Sons