Micrometeorite dynamic pyrometamorphism: Observation of a thermal gradient in iron-nickel sulfide

Dynamic pyrometamorphism describes the mineralogical changes in a micrometeorite that occur in response to flash-heating during atmospheric entry. Mineral reactions during this event will be endothermic and act as local sinks for thermal energy which delays a uniform temperature distribution in decelerating micrometeorites. The most common pyrometamorphic reaction is formation of magnetite nanocrystal decorations on Mg,Fe-silicate grains and magnetite or (partial) maghemite rims. Constituent silicates also respond to this thermal event and show formation of olivine-maghemite symplectites and growth of garnet and partial laihunitisation. In continued effort to constrain dynamic pyrometamorphism of unmelted interplanetary dust particles (IDP's), I determined the mineralogical composition, and Fe,Ni-sulfide chemistry, in the sulfide-rich particles L2005C39, L2005E40, and L2006A28 using a JEOL 2000FX analytical electron microscope equipped with a TN5500 energy dispersive spectrometer for in situ microanalysis. Sulfide compositions in these three IDP's are distinctly different from those in silicate-rich particle L2005T13 which might suggest a delicate balance of the fO2/fS2 ratios during dynamic pyrometamorphism.