The behavior of osmium and other siderophile elements during impacts: Insights from the Ries impact structure and central European tektites
Lukáš Ackerman, Tomáš Magna, Karel Žák, Roman Skála, Šárka Jonášová, Jiří Mizera, Zdeněk Řanda
Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 27 April 2017
“Impact processes are natural phenomena that contribute to a variety of physico–chemical mechanisms over an extreme range of shock pressures and temperatures, otherwise seldomly achieved in the Earth’s crust through other processes. Under these extreme conditions with transient temperatures and pressures ≥3,000K and ≥100 GPa, followed by their rapid decrease, the behavior of elements has remained poorly understood. Distal glassy ejecta (tektites) were produced in early phases of contact between the Earth’s surface and an impacting body. Here we provide evidence for a complex behavior of Os and other highly siderophile elements (HSE; Ir, Ru, Pt, Pd, and Re) during tektite production related to a hyper-velocity impact that formed the Ries structure in Germany. Instead of simple mixing between the surface materials, which are thought to form the major source of central European tektites (moldavites), and impactor matter, the patterns of HSE contents and 187Re/188Os – 187Os/188Os ratios in moldavites, target sediments and impact-related breccias (suevites) can be explained by several sequential and/or contemporary processes. These involve (i) evaporative loss of partially oxidized HSE from the overheated tektite melt, (ii) mixing of target-derived and impactor-derived HSE vapor (plasma) phases, and (iii) early (high-temperature) condensation of a part of the mixed vapor phase back to silicate melt droplets. An almost complete loss of terrestrial Os from the tektite melt and its replacement with extra-terrestrial Os are indicated by low 187Os/188Os ratios in tektites (<0.163) relative to precursor materials (>0.69). This is paralleled by a co-variation between Os and Ni contents in tektites but not in suevites formed later in the impact process.”
