Cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica)

1Steven Goderis1Bastien Soens,1,2Matthew S.Huber,1,3,4Seann McKibbin,5Matthias van Ginneken,1Flore Van Maldeghem,6Vinciane Debaille,7Richard C.Greenwood,7Ian A.Franchi,8 Veerle Cnudde,10Stijn Van Malderen,10 Frank Vanhaecke,11,12Christian Koeberl,12Dan Topal,1Philippe Claeys
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.11.016]
1Analytical-, Environmental-, and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
2Department of Geology, University of the Free State, 205 Nelson Mandela Dr., Bloemfontein 9300, South Africa1
3Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Haus 27, Karl-Liebknecht-Straße 24-25, Potsdam-Golm 14476, Germany1
4Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universität Göttingen, Goldschmidtstraße 1, Göttingen 37073, Germany1
5Royal Belgian Institute of Natural Sciences, 29 Rue Vautier, B-1000 Brussels, Belgium
6Laboratoire G-Time, Université Libre de Bruxelles 50, Av. F.D. Roosevelt CP 160/02, B-1050 Brussels, Belgium
7Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
8Department of Geology, Ghent University, Campus Sterre, Krijgslaan 281 – S8, B-9000 Ghent, Belgium
9Department of Earth Sciences, Utrecht University, Princetonlaan 8a, 3584CB Utrecht, the Netherlands
10Department of Chemistry, Ghent University, Krijgslaan, 281 – S12, B-9000 Ghent, Belgium
11Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
12Natural History Museum, Burgring 7, A-1010 Vienna, Austria

A newly discovered sedimentary accumulation of micrometeorites in the Sør Rondane Mountains of East Antarctica, close to the Widerøefjellet summit at ∼2750 meter above sea level, is characterized in this work. The focus here lies on 2099 melted cosmic spherules larger than 200 μm, extracted from 3.2 kg of sampled sediment. Although the Widerøefjellet deposit shares similarities to the micrometeorite traps encountered in the Transantarctic Mountains, both subtle and more distinct differences in the physicochemical properties of the retrieved extraterrestrial particles and sedimentary host deposits are discernable (e.g., types of bedrock, degree of wind exposure, abundance of metal-rich particles). Unlike the Frontier Mountain and Miller Butte sedimentary traps, the size fraction below 240 μm indicates some degree of sorting at Widerøefjellet, potentially through the redistribution by wind, preferential alteration of smaller particles, or processing biases. However, the cosmic spherules larger than 300 μm appear largely unbiased following their size distribution, frequency by textural type, and bulk chemical compositions. Based on the available bedrock exposure ages for the Sør Rondane Mountains, extraterrestrial dust is estimated to have accumulated over a time span of ∼1 to 3 Ma at Widerøefjellet. Consequently, the Widerøefjellet collection reflects a substantial reservoir to sample the micrometeorite influx over this time interval. Petrographic observations and 3D microscopic CT imaging are combined with chemical and triple-oxygen isotopic analyses of silicate-rich cosmic spherules larger than 325 μm. The major element composition of 49 cosmic spherules confirms their principally chondritic parentage. For 18 glassy, 15 barred olivine, and 11 cryptocrystalline cosmic spherules, trace element concentrations are also reported on. Based on comparison with evaporation experiments reported in literature and accounting for siderophile and chalcophile element losses during high-density phase segregation and ejection, the observed compositional sequence largely reflects progressive heating and evaporation during atmospheric passage accompanied by significant redox shifts, although the influence of (refractory) chondrite mineral constituents and terrestrial alteration cannot be excluded in all cases. Twenty-eight cosmic spherules larger than 325 μm analyzed for triple-oxygen isotope ratios confirm inheritance from mostly carbonaceous chondritic precursor materials (∼55% of the particles). Yet, ∼30% of the measured cosmic spherules and ∼50% of all glassy cosmic spherules are characterized by oxygen isotope ratios above the terrestrial fractionation line, implying genetic links to ordinary chondrites and parent bodies currently unsampled by meteorites. The structural, textural, chemical, and isotopic characteristics of the cosmic spherules from the Sør Rondane Mountains, and particularly the high proportion of Mg-rich glass particles contained therein, imply a well-preserved and representative new sedimentary micrometeorite collection from a previously unstudied region in East Antarctica characterized by distinct geological and exposure histories.

 

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