Dynamic pyrometamorphism during atmospheric entry of large (~10 micron) pyrrhotite fragments from cluster IDPs

Abstract

Petrological changes in Ni-free and low-Ni pyrrhotite, and much less in pentlandite, during atmospheric entry flash-heating of the sulfide IDPs L2005E40, L2005C39, and L2006A28 support 1) ferrous sulfide oxidation with vacancy formation and Fe^(3+) ordering; and 2) Fe-oxide formation and sulfur vapor loss through abundant vesicles. Melting of metastable chondritic aggregate materials at the IDP surface has occurred. All changes, e.g., formation of a continuous maghmite rim, proceeded as solid-state reactions at a peak heating temperature of ~700 °C. This temperature in combination with particle size and density suggest a ~10 km/s^(-1) entry velocity. The IDPs probably belonged to cluster IDPs that entered the atmosphere with near-Earth or Earth-crossing asteroid velocities. They could be debris from extinct or dormant comet nuclei, which is consistent with shock comminution of pyrrhotite in these IDPs.