A large meteoritic event over Antarctica ca. 430 ka ago inferred from chondritic spherules from the Sør Rondane Mountains.

Introduction: Impactors several tens up to 200 m in size are likely to suffer complete disruption and to produce large airbursts, similarly to the Tunguska event over Russia in 1908 [e.g., 1]. Observations and numerical modeling of medium sized impacts producing large airbursts have shown that such impacts represent an important fraction of extraterrestrial matter accretion to Earth, with Tunguska-like events occurring every 100 to 10,000 years, which is notably more frequent than crater-forming impact events. However, little is known about occurrences of such airburst events in the geological record, principally because of the lack of readily identifiable evidences such as impact craters. Finding residues of such events is thus critical for assessing the complete impact history of the Earth. Here we present the discovery of extraterrestrial particles in the Sør Rondane Mountains, Queen Maud Land, Antarctica, which were produced during a “touchdown” impact event ca. 430 ka ago, when a large airburst vapor jet interacted with the Antarctic ice sheet.

Material and Methods: Twenty nine igneous particles were recovered from glacial sediment collected during the 2017-2018 BELAM (Belgian Antarctic Meteorites) expedition that took place in the Sør Rondane Mountains, Queen Maud Land, Antarctica. Glacial sediment was sampled from a flat eroded summit in the Walnumfjellet (WN) area. ¹⁰Be exposure age of nearby summits suggest that the first sampled area has been continuously exposed over the last 870 ka [2]. About half the particles are compound spherules consisting of two or more spherules fused together. The petrography and mineralogy of 18 particles were determined at the Royal Belgian Institute of Natural Sciences of Brussels, Belgium. Their major and trace element compositions were determined at the Museum für Naturkunde of Berlin, Germany, and at Florida State University, USA, respectively. Oxygen isotopic compositions were determined by means of secondary ion mass spectrometry at the CRPG of Nancy, France.

Results: The mineralogy of the particles consists of olivine and spinel, with minor interstitial glass. On the basis on their internal textures and spinel content, we identify four groups of particles: 1/ the spinel-rich particles (SR; N = 9; ≥19% vol. spinel); 2/ Porphyritic olivine (PO; N = 5; <10% vol. spinel); 3/ Barred olivine (BO; N = 3; <10% vol. spinel); and 4/ one cryptocrystalline (CC; ⁓15% vol. spinel).  The bulk major and trace element compositions of the particles are chondritic, pointing to a meteoritic origin. Spinel chemistry in SR particles is characterized by an Fe³⁺/Fetot of 77-89, where in porphyritic olivine particles, Fe³⁺/Fe_tot is lower at 60-62. Bulk spinel chemical compositions suggest highly oxidizing conditions during the formation of SR particles, suggesting that they formed in the lower atmosphere, whereas conditions were much less oxidizing for SP particles [3]. Chemistry and similarities to textural groups in BIT-58 impact particles suggest that all WN particles formed during a single impact event. However, age incompatibly prevents a pairing of WN particles with the impact event recorded in BIT-58. On a petrological and chemical level, WN particles match ⁓430 ka old impact particles found as layers in EPICA Dome C and Dome Fuji (i.e. L1 and DF2641, respectively) [4; 5], suggesting a continental distribution. A likely scenario is the disruption of a large (i.e. at least 100 m in size) chondritic asteroid over Antarctica ~430 ka ago. Oxygen isotopic signatures of WN particles are characterized by a highly negative δ¹⁸O, ranging from -35 to -52‰, and Δ¹⁷O ranging from -0.5 to - 1.2‰, consistent with oxygen isotopic compositions of L1 and DF2621 particles. Highly negative δ¹⁸O values are also consistent with interaction with the Antarctic icesheet during formation of the particles. This suggests that WN, L1 and DF2621 particles were produced during a touchdown impact, which occurs when the jet of melted and vaporized meteoritic material resulting from a large airburst reaches the surface, in this case the Antarctic icesheet, at high velocity. Numerical simulations of a projectile with a diameter of 100 to 150 m entering Earth’s atmosphere at a velocity of 20 km s⁻¹ and an impact angle from 15° to 90° support such a touchdown scenario and, in particular, explain the complex chemical and isotopic conditions leading to the formation of the four observed textural groups SR, SP, BO and CC.

Conclusion: We report the discovery of meteoritic ablation spheres from the Sør Rondane Mountains. Their chondritic chemistry, coupled with characteristic spinel chemical compositions and oxygen isotopic signatures show that they formed in the lower atmosphere during a large touchdown event over the Antarctic icesheet. Combining chemical and isotopic composition with a numerical model help understanding the complex formation processes occurring during this unique impact event over Antarctic likely ~430 ka ago.

References: [1] Artemieva N. A. and Shuvalov V. V. (2016) Annu. Rev. Earth Planet. Sci. 44: 37–56. [2] Suganuma Y. et al. (2014) Quat. Sc. Rev. 97: 102-120. [3] Van Ginneken M. et al. (2010) Earth Planet. Sci. Lett. 293: 104–113. [4] Narcisi B. et al. (2007) Geophys. Res. Lett. 34: (2007) [5] Misawa K. et al. (2010) Earth Planet. Sci. Lett. 289: 287–297. [5] H. Motoyama (2007) Eos Trans. AGU  88, abs. #C51A-0076.