S.M. Chernonozhkin, C. González de Vega, N. Artemieva, B. Soens, J. Belza, E. Bolea-Fernandez, M. Van Ginneken, B.P. Glass, L. Folco, M.J. Genge, P. Claeys, F. Vanhaecke. and S. Goderis (2021) Isotopic evolution of planetary crusts by hypervelocity impacts evidenced by Fe in microtektites. // Nature Communications 2021, Vol 12, 5646.
In chemistry classes it is taught that isotopic compositions of the elements are constant on Earth and in the Solar System, and precise atomic weights are provided for each element in the periodic table. While it is true as a first approximation, there are many natural processes, which results in tiny changes of isotopic compositions of the materials involved into these processes. This phenomenon is used by scientist to enrich certain isotopes by e.g. use of centrifuges. It is less famous, however, that the isotopic compositions can be shifted slightly in nature by geological or physical processes, such as condensation and evaporation. While these variations are tiny, they are measurable with modern high precision mass spectrometers, and once measured they allow geochemists to understand the histories of the processes in which the studied materials took part in.
In a new collaborative research carried out by research groups from Belgium, US, Russia, UK and Italy, it is shown that the isotopic composition of even such tough refractory element like iron can be changed in nature by evaporation during massive hypervelocity asteroid impacts on Earth. During the research, the group studied microscopic sub-millimeter glass spherules, which formed during an impact of a 2 km diameter asteroid in South East Asia 790 thousand years ago. These glass pieces form from the crustal rocks under extreme heat and pressure of the asteroid impact, and after ejection they travel ballistically hundreds and thousands kilometers, quenching in flight into glass beads.
Despite the crater of this impact is yet to be found, as it is buried under young basalt flows and sediments and covered by intense vegetation somewhere in Indochina, the materials ejected by this massive impact are found over a vast area in the southern hemisphere, covering ~30% of the Earth surface. The beads of melted glass are found from Southern China to Australia, and from the South East African cost to Indonesia and Philippines. The microscopic glass beads are collected from the bed sediments in the Pacific and Indian Oceans, and most recently also as far as from Antarctica.
During the study, the group investigated the isotopic composition of iron in the microscopic impact glasses collected during multiple oceanographic and Antarctic expeditions. Use of a cutting edge mass spectrometry facilities available in Belgium allowed to discover that the isotopic composition of iron in these microscopic particles is slightly but significantly different from that of the initial rocks of the crust. The physical model of the asteroid impact further confirms the data and shows that the pressures and temperatures in the crater are high enough for the material to reach supercritical state, evaporate and condense the rocks, which created the observed isotopic compositions.
The finding that asteroid impacts can shift isotopic composition of such a refractory element as iron is different from what was hypothesized in most cosmochemical models before, and finding of evaporated and condensed glasses that deposit back on the surface of Earth provides new insights into the history of the Moon and asteroids in the Solar System, the landscape of which is dominated by the impact craters and their surface is covered by the dust composed of billions of melted glassy beads, analogues of the terrestrial impact glasses studied in the research. The study of the tiny beads of glass formed by the asteroid impact in Asia may as such turn over our understanding of how the isotopic composition of planets evolved through the history of the Solar System.