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 thei r state of alteration and to identify the nature of their precursor ma terials. 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 afte r entry heating and those of unmelted AMMs broadly resemble CI, CM, an d CR chondrite matrix. The matrix compositions of the fg-AMMs closely resemble those of CM2 chondrites. Matrices provide a more reliable com parison to meteorites than bulk compositions because components such a s chondrules and CAIs are not present in representative proportions wi thin individual AMMs. Divergences from CM-like matrix compositions are generally minor and probably arose during entry heating by the dissoc iation of volatile-bearing phases at subsolidus temperatures and evapo ration and loss of immiscible metallic liquids at higher temperatures. Depletions in Ni and Mg relative to CM matrix are tentatively attribu ted to terrestrial weathering. No conclusive evidence for contaminatio n during atmospheric residence was observed. Contrary to theory, textu ral evidence suggests that bow shocks and high thermal gradients exist ed during deceleration of micrometeoroids in the atmosphere. Copyright (C) 1997 Elsevier Science Ltd.