¹Morgano Maxime,¹Le Guillou Corentin,¹Leroux Hugues,¹Marinova Maya,²Dohmen Ralf
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115669]
¹Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 – UMET – Unité Matériaux et Transformations, F-59000 Lille, France
²Ruhr-Universitaet Bochum, RUB, Institute of Geology, Geophysics and Mineralogy, 44780 Bochum, Germany
Copyright Elsevier

Amorphous silicates are abundant in extraterrestrial objects such as interplanetary dust particles and primitive chondrites. They are thought to be formed through condensation and possibly later exposed to thermal processes in the nebula before being accreted within an asteroid and/or comet.

We aim to constrain the conditions that prevailed during thermal events in the nebula, through experimental work on the chemical and structural evolution of condensed amorphous silicate.

We conducted coupled condensation and heating experiments of Fe-Mg-silicate thin films using the pulsed laser deposition technique. We compared samples condensed at room temperature and annealed in a second step with samples directly condensed on heated substrate, at 450 °C and 700 °C.

For both processes, at temperature as low as 450 °C, iron-rich nanoparticles and Mg-rich domains form, evidencing the high reactivity of the condensed amorphous silicate. This reactivity was found to be even higher for the process of condensation on heated substrate. We also evidence the persistence of amorphous silicate up to 700 °C, in spite of the chemical evolution and the demixion into MgO and SiO2 domains.

These results imply that amorphous silicates condensed from a plasma (and possibly from any process producing atoms in an excited state) are more reactive than quenched glasses of similar composition. In complement to high temperature events that occurred at the time of solar system formation and that formed chondrules for instance, this work emphasizes the importance of mild heating on dust evolution before accretion within parent(s) body(ies). It helps to place chemical and structural constraints on the thermal evolution of amorphous silicate found in primitive chondrites: i) iron segregation as metallic nanoparticles can be generated within a silicate groundmass at temperature as low as 450 °C (and possibly even below) ii) iron-rich chondritic amorphous silicate can persist up to 700 °C.

¹Flore Van Maldeghem,²Matthias van Ginneken,¹Bastien Soens,³Felix Kaufmann,⁴Seppe Lampe,¹Lisa Krämer Ruggiu,³Lutz Hecht,¹Philippe Claeys,¹Steven Goderis
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.06.002]
¹Analytical-, Environmental-, and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
²Centre for Astrophysics and Planetary Science, School of Physical Sciences, Ingram Building, University of Kent, Canterbury CT2 7NH, UK
³Museum für Naturkunde Berlin, Invalidenstrasse 43, Berlin 10115, Germany
⁴Hydrology and Hydraulic Engineering, Faculty of Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
Copyright Elsevier

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.