¹Ulrich Mansfeld, ¹Falko Langenhorst, ^2,3Matthias Ebert, ²Astrid Kowitz, ²Ralf Thomas Schmitt
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12867]
¹Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany
²Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany
³Institut für Geo- und Umweltnaturwissenschaften, Geologie, Albert-Ludwigs-Universität, Freiburg, Germany
Published by arrangement with John Wiley & Sons

It has been almost exactly half a century since the first synthesis of stishovite in shock experiments on quartz was reported, but its formation conditions during shock is still under debate. Here, we present direct transmission electron microscopic observation of stishovite within material recovered from high-explosive shock experiments on porous sandstone shocked at 7.5 and 12.5 GPa. Our observations allow for new conclusions on the genesis of stishovite in a close-to-nature environment. The formation of stishovite in short-time shock experiments proves that its crystallization is ultrafast <!–(<1 μs). Crystals were found only embedded in amorphous veins indicating homogeneous nucleation. Crystallization from melt rather than from glass can be concluded from the observation of roundish, defect-free crystals up to 150 nm in diameter embedded in nondensified glass. The formation of stishovite at 7.5 GPa is in accordance with the phase diagram of silica, if rapid undercooling is present that becomes only possible by the existence of small hot spots in an otherwise cold material, which is supported by transient heat calculation. The absence of coesite at 7.5 GPa suggests kinetic hindrance of its crystallization from melt and, thus, smaller critical cooling rates compared to stishovite where critical cooling rates are estimated to be as large as 10¹¹ K s⁻¹. While the amorphous veins containing stishovite represent unambiguously hot spots, no associated pressure amplification could be verified within these veins. The rapid liquidus crystallization of stishovite only in hot spots generated in porous material is an alternative formation mechanism to the widely accepted theory of solid–solid transition from quartz to stishovite and might represent the more general mechanism occurring in nature for low shock pressure events.

¹Nancy L. Chabot, ¹E. Alex Wollack, ²William F. McDonough, ²Richard D. Ash, ²Sarah A. Saslow
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12864]
¹Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
²University of Maryland, College Park, Maryland, USA
Published by arrangement with John Wiley & Sons

Experimental trace element partitioning values are often used to model the chemical evolution of metallic phases in meteorites, but limited experimental data were previously available to constrain the partitioning behavior in the basic Fe-Ni system. In this study, we conducted experiments that produced equilibrium solid metal and liquid metal phases in the Fe-Ni system and measured the partition coefficients of 25 elements. The results are in good agreement with values modeled from IVB iron meteorites and with the limited previous experimental data. Additional experiments with low levels of S and P were also conducted to help constrain the partitioning behaviors of elements as a function of these light elements. The new experimental results were used to derive a set of parameterization values for element solid metal–liquid metal partitioning behavior in the Fe-Ni-S, Fe-Ni-P, and Fe-Ni-C ternary systems at 0.1 MPa. The new parameterizations require that the partitioning behaviors in the light-element–free Fe-Ni system are those determined experimentally by this study, in contrast to previous parameterizations that allowed this value to be determined as a best-fit parameter. These new parameterizations, with self-consistent values for partitioning in the endmember Fe-Ni system, provide a valuable resource for future studies that model the chemical evolution of metallic phases in meteorites.