Primary Variations in Micrometeorites with Entry Velocity

Introduction: Micrometeorites are that fraction of the extraterrestrial dust flux that survives atmospheric entry to be recovered from the Earth’s surface. The majority of large micrometeorites (>50 μm), recovered from Antarctic ice, experience significant heating during atmospheric entry with the result that volatilebearing phases, such as clay minerals, have thermally decomposed. A significant proportion of particles also have been partially or completely melted [1]. The degree of heating depends on peak temperature and duration of the thermal pulse caused by the hypervelocity collision of air molecules during deceleration and is a function of entry velocity, entry angle and particle size. The dependence on entry angle ensures that even a single velocity population of micrometeoroids will experience a range of heating effects. The overall degree of entry heating, however, will nevertheless increase with velocity [2]. Since dust particles from the same source will have a restricted range of geocentric velocities, changes in the preatmospheric mineralogical and chemical properties of micrometeorites with degree of entry heating may allow identification of discrete populations of particles within the micrometeorite flux. Analyses of petrological variations with degree of heating are complicated by changes in the mineralogy and composition of particles accompanying heating. Identifying the primary features of heated micrometeorites, particularly in particles which have experienced melting, is problematic, however, large (>5 μm) anhydrous silicates are found as relict phases within melted particles and allow comparison with unmelted particles. The current study presents data on variations in the abundance and nature of primary anhydrous silicates to enable entry heating affects to be correlated between different particle types. Primary Anhydrous Silicates: Mg-rich pyroxene and olivine are the most abundant anhydrous silicates within large micrometeorites. In contrast to carbonaceous chondrites, pyroxene is most abundant. Anhydrous silicates are found in two forms in unmelted particles: (1) as isolated grains, often with fragmental outlines, embedded in the matrices of fine-grained MMs (fgMMs), and (2) as grains within coarsegrained MMs (cgMMs), which are often present as phenocrysts in particles with igneous textures. Finegrained MMs have been interpreted as equivalent to the fine-grained matrices of C1-C3 chondrites, although clay minerals have typically decomposed to amorphous dehyroxylates or recrystallised to submicron olivine and pyroxene [3]. The majority of cgMMs have been interpreted as fragments of chondrules from C2-C3 chondrites since the presence of fine-grained matrix attached to some particles indicates an origin as small igneous objects from similar parent bodies to fgMMs [4]. The relative abundance of micrometeorites dominated by large anhydrous silicates (cgMM-derived), of isolated anhydrous silicates within fine-grained matrix/mesostasis (fgMM-derived) and those lacking primary anhydrous silicates thus may be used to evaluate variations with entry heating. Effects of Entry Heating: Anhydrous silicates show few changes due to heating at subsolidus temperatures, however, on melting both reaction with the melt and direct fusion occur. In fine-grained particles heating first results in partial melting of the matrix and