The textures and compositions of fine-grained Antarctic micrometeorites: Implications for comparisons with meteorites

Micrometeorites recovered from the Earth's surface constitute the most abundant interplanetary dust now falling to Earth. We studied eighty-nine fine-grained Antarctic micrometeorites (fg-AMMs) to evaluate their state of alteration and to identify the nature of their precursor materials. Fine-grained AMMs are divided into melted and unmelted groups and subdivided on the basis of textures related to atmospheric entry heating. The textures of melted particles reflect crystallisation after entry heating and those of unmelted AMMs broadly resemble Cl, CM, and CR chondrite matrix. The matrix compositions of the fg-AMMs closely resemble those of CM2 chondrites. Matrices provide a more reliable comparison to meteorites than bulk compositions because components such as chondrules and CAIs are not present in representative proportions within individual AMMs. Divergences from CM-like matrix compositions are generally minor and probably arose during entry heating by the dissociation of volatile-bearing phases at subsolidus temperatures and evaporation and loss of immiscible metallic liquids at higher temperatures. Depletions in Ni and Mg relative to CM matrix are tentatively attributed to terrestrial weathering. No conclusive evidence for contamination during atmospheric residence was observed. Contrary to theory, textural evidence suggests that bow shocks and high thermal gradients existed during deceleration of micrometeoroids in the atmosphere.