HEATING EXPERIMENTS SIMULATING ATMOSPHERIC ENTRY HEATING OF MICROMETEORITES - CLUES TO THEIR PARENT BODY SOURCES

Depending on their velocity, entry angle and mass, extraterrestrial du st particles suffer certain degrees of heating during entry into Earth 's atmosphere, and the mineralogy and chemical composition of these du st particles are significantly changed. In the present study, pulse-he ating experiments simulating the atmospheric entry heating of micromet eoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temper ature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orguei l were used as analogue material. The experiments show that the volati le elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 mu m size d CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is con cluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besid es explaining the element abundances in micrometeorites, the experimen tally obtained release patterns can also be used as indicators to esti mate the peak heating of dust particles during entry. Using the abunda nces of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100-1200 degrees C is obtained for previousl y analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unhea ted samples mainly consisted of phyliosilicates, these phases almost c ompletely transformed into olivine and pyroxene in the fragments heate d to greater than or equal to 800 degrees C. Therefore, dust particles that still contain hydrous minerals were probably never heated to tem peratures greater than or equal to 800 degrees C in the atmosphere. Du ring continued heating, the grain size of the newly formed silicates i ncreased and the composition of the olivines equilibrated. Applying th ese results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100-1200 degrees C during atmospheric en try heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these m icrometeorites and points to an asteroidal origin of the particles.