Scoriaceous micrometeorites are highly vesicular extraterrestrial
dust particles that have experienced partial melting during atmo
-
spheric entry. We report the occurrence of clusters of anhedral relict
forsterite crystals within these particles that testify to in situ frag
-
mentation. The absence of similar clusters within unmelted micro
-
meteorites suggests that fragmentation occurs during atmospheric
entry rather than by parent body shock reprocessing. Clusters of
broken forsterite crystals are shown to form as a result of fracturing
owing to thermal stress developed during entry heating and require
thermal gradients of >200 K μm
–1
in order for differential thermal
expansion to exceed the critical shear strength of olivine. Thermal
gradients of this magnitude significantly exceed those resulting from
thermal conduction and require the endothermic decomposition of
phyllosilicates. Fragmented relict forsterite within scoriaceous micro
-
meteorites, therefore, indicate that the precursor grains were similar
to CI and CM2 chondrites and retained phyllosilicate prior to atmo-
spheric entry and thus were not dehydrated on the parent asteroid by
shock or thermal metamorphism. Explosive fragmentation of hydrous
asteroids during collisions, therefore, does not significantly bias the
interplanetary dust population.