Antarctica: links to chondritic parent bodies and the effects of alteration
Flore Van Maldeghem, Matthias van Ginneken, Bastien Soens, Felix Kaufmann, Seppe Lampe, Lisa Krämer Ruggiu, Lutz Hecht, Philippe Claeys, Steven Goderis
Geochimica et Cosmochimica Acta
Available online 10 June 2023
“Micrometeorites originate from the interplanetary dust complex and continuously fall to the Earth’s surface in large amounts. About 10 to 20% of micrometeorites are not melted upon reaching the Earth’s surface, preserving the primary features and characteristics of the parent material. Consequently, unmelted micrometeorites, together with scoriaceous micrometeorites, an intermediate form between cosmic spherules and unmelted micrometeorites, are pivotal in documenting the nature and evolution of interplanetary dust as well as the modifications experienced by micrometeorites during atmospheric entry. Based on their petrographic features, here we identified and characterized 64 scoriaceous and unmelted micrometeorites with diameters varying between 90 and 410 μm from fine-grained sediment sampled in the Sør Rondane Mountains of East Antarctica. Based on their size distribution, the micrometeorites from the Sør Rondane Mountains show a clear distinction between unmelted micrometeorites (< 300 μm) and cosmic spherules (> 400 μm) and imply an accumulation mechanism or exposure history distinct from other collections (e.g., Transantarctic Mountains). Different exposure windows, weathering processes and environmental factors (e.g., snow cover) could affect the size and composition of preserved particles.
A selection of the particles (n = 49) was further characterized for geochemical composition and high-precision oxygen isotope ratios to identify potential parent bodies and document their alteration histories. About 63% of the particles, exhibiting both coarse- and fine-grained textures, derive from carbonaceous chondritic precursors. Two particles (∼ 4%) display anomalously 16O-poor isotopic compositions similar to that previously observed for (giant) cosmic spherules and unmelted micrometeorites, classified as “group 4” particles. These particles are thought to originate from an unidentified chondritic parent body located in a specific region of the protoplanetary disk or may have been characterized by a distinct alteration history, with recent studies linking them to CY carbonaceous chondrites. Only a single fine-grained particle (∼ 2%) can be assigned to ordinary chondritic parentage with confidence. The partially hydrated fine-grained matrix suggests this particle might be consistent with a Semarkona-like parent body. Approximately 10% of the studied particles exhibit extensive evidence for secondary terrestrial weathering with formation of (hydr)oxides during residence in the Antarctic environment, preventing detailed parent body identification. Ten particles (∼ 20%) could not be assigned to a specific parent group due to ambiguous oxygen isotope values. Overall, the parent body statistics from this study agree with those reported for different collections of a similar size fraction. Clear associations between textural groups and parent bodies could not be established. Even though unmelted micrometeorites are generally considered pristine and often do not exhibit any obvious petrographic evidence of terrestrial weathering, the chemical and isotopic data obtained here confirm that alteration can occur at the microscale and any data on unmelted particles from Antarctic subaerial collections should be evaluated with caution.”