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

— 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 Fe3+ 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 maghémite 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.