^1,2Yang Liu,^2,3Ioannis P. Baziotis,³Paul D. Asimow,⁴Robert J. Bodnar,²Lawrence A. Taylor
Meteoritcs & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12726]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
²Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA
³Department of Natural Resources Management and Agricultural Engineering, Laboratory of Mineralogy and Geology, Agricultural University of Athens, Athens, Greece
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
⁴Department of Geosciences, Virginia Tech, Blacksburg, Virginia, USA
Published by arrangement with John Wiley & Sons

The Tissint meteorite is a geochemically depleted, olivine-phyric shergottite. Olivine megacrysts contain 300–600 μm cores with uniform Mg# (~80 ± 1) followed by concentric zones of Fe-enrichment toward the rims. We applied a number of tests to distinguish the relationship of these megacrysts to the host rock. Major and trace element compositions of the Mg-rich core in olivine are in equilibrium with the bulk rock, within uncertainty, and rare earth element abundances of melt inclusions in Mg-rich olivines reported in the literature are similar to those of the bulk rock. Moreover, the P Kα intensity maps of two large olivine grains show no resorption between the uniform core and the rim. Taken together, these lines of evidence suggest the olivine megacrysts are phenocrysts. Among depleted olivine-phyric shergottites, Tissint is the first one that acts mostly as a closed system with olivine megacrysts being the phenocrysts. The texture and mineral chemistry of Tissint indicate a crystallization sequence of: olivine (Mg# 80 ± 1) → olivine (Mg# 76) + chromite → olivine (Mg# 74) + Ti-chromite → olivine (Mg# 74–63) + pyroxene (Mg# 76–65) + Cr-ulvöspinel → olivine (Mg# 63–35) + pyroxene (Mg# 65–60) + plagioclase, followed by late-stage ilmenite and phosphate. The crystallization of the Tissint meteorite likely occurred in two stages: uniform olivine cores likely crystallized under equilibrium conditions; and a fractional crystallization sequence that formed the rest of the rock. The two-stage crystallization without crystal settling is simulated using MELTS and the Tissint bulk composition, and can broadly reproduce the crystallization sequence and mineral chemistry measured in the Tissint samples. The transition between equilibrium and fractional crystallization is associated with a dramatic increase in cooling rate and might have been driven by an acceleration in the ascent rate or by encounter with a steep thermal gradient in the Martian crust.

^1,2Jörg Fritz,³Roald Tagle,⁴Luisa Ashworth,²Ralf Thomas Schmitt,⁵Axel Hofmann,⁶Béatrice Luais,⁴Phillip D. Harris,^2,7Desirée Hoehnel,⁸Seda Özdemir,^2,9Tanja Mohr-Westheide,^9,10Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12736]
¹Saalbau Weltraum Projekt, Heppenheim, Germany
²Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
³Bruker-Nano GmbH, Berlin, Germany
⁴GeoSpectral Imaging, Johannesburg, South Africa
⁵Department of Geology, University of Johannesburg, Johannesburg, South Africa
⁶Centre de Recherches Pétrographiques et Géochimiques, CRPG UMR 7358 CNRS-UL, France
⁷Institut für Erd und Umweltwissenschaften, Universität Potsdam, Potsdam-Golm, Germany
⁸Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁹Institut für Geologische Wissenschaften, Freie Universität Berlin (FU Berlin), Berlin, Germany
¹⁰Natural History Museum, Vienna, Austria
Published by arrangement with John Wiley & Sons

A Paleoarchean impact spherule-bearing interval of the 763 m long International Continental Scientific Drilling Program (ICDP) drill core BARB5 from the lower Mapepe Formation of the Fig Tree Group, Barberton Mountain Land (South Africa) was investigated using nondestructive analytical techniques. The results of visual observation, infrared (IR) spectroscopic imaging, and micro-X-ray fluorescence (μXRF) of drill cores are presented. Petrographic and sedimentary features, as well as major and trace element compositions of lithologies from the micrometer to kilometer-scale, assisted in the localization and characterization of eight spherule-bearing intervals between 512.6 and 510.5 m depth. The spherule layers occur in a strongly deformed section between 517 and 503 m, and the rocks in the core above and below are clearly less disturbed. The μXRF element maps show that spherule layers have similar petrographic and geochemical characteristics but differences in (1) sorting of two types of spherules and (2) occurrence of primary minerals (Ni-Cr spinel and zircon). We favor a single impact scenario followed by postimpact reworking, and subsequent alteration. The spherule layers are Al2O3-rich and can be distinguished from the Al2O3-poor marine sediments by distinct Al-OH absorption features in the short wave infrared (SWIR) region of the electromagnetic spectrum. Infrared images can cover tens to hundreds of square meters of lithologies and, thus, may be used to search for Al-OH-rich spherule layers in Al2O3-poor sediments, such as Eoarchean metasediments, where the textural characteristics of the spherule layers are obscured by metamorphism.