Cosmic Dust Flux During the Quaternary: The Record of Large Scoriaceous and Unmelted Micrometeorites from the Transantarctic Mountains Collection

Micrometeorites dominate the flux of extraterrestrial material that reaches Earth [1]. They are classified according to the degree of melting experienced during atmospheric entry heating into three groups: melted micrometeorites or cosmic spherules, partially melted or scoriaceous micrometeorites and unmelted micrometeorites [2]. About 10% of cosmic dust particles collected at the Earth’s surface are unmelted or scoriaceous, thus preserving, at least in part, primary features of their parent bodies [3]. A significant number of large unmelted and scoriaceous micrometeorites were recovered from 1–2 Myr-old sediment traps in the Transantarctic Mountains (TAM) [4,5]. Based on the petrographic characterization of 207 micrometeorites, we report a statistically significant study of the size-frequency distribution of unmelted and scoriaceous micrometeorites in the ~170–1650 μm size range. This study complements previous assessments of micrometeorite size distributions based on totally melted TAM cosmic spherules [5], providing additional constraints on the micrometeorite flux reaching Earth during the Quaternary.

Micrometeorites were identified based on their morphological and petrographic features using a stereomicroscope. To determine mass and size, particles were weighed using a microbalance and imaged under a Field Emission Gun – Scanning Electron Microscope at CISUP (Pisa, Italy). Backscattered electron images provided textural criteria for the identification of unmelted and scoriaceous micrometeorites, including magnetite rims, dehydration cracks, scoriaceous textures, angular to sub-rounded morphologies and characteristic weathering products. Subsequently, all particles were characterised using high-resolution X-ray computed microtomography at the ZAG institute (Ljubljana, Slovenia), allowing non-destructive 3D textural classification into fine-grained, coarse-grained and composite micrometeorites. Twenty-two representative particles were sectioned and examined using backscattered electron imaging to validate the textural classification.

The size-frequency distribution of the TAM unmelted and scoriaceous micrometeorites is bimodal, with two main peaks centred at ~305 and 470 μm. A similar bimodal distribution was previously observed in TAM cosmic spherules, although shifted toward smaller sizes, with peaks at ~145 and 250 μm [5]. The discrepancy between the distributions of unmelted-scoriaceous micrometeorites and melted cosmic spherules is consistent with size-mass reduction produced by atmospheric entry heating and evaporative mass loss acting on a single population of extraterrestrial dust particles. Using the empirical mass–diameter relationship established in this study, we estimate a first-order average mass loss of ~87% associated with complete melting during atmospheric entry. The analysis of fine- and coarse-grained micrometeorites also reveals robust bimodality. The two distinct peaks in the unmelted plus scoriaceous micrometeorite population are centred at ~315 µm and 550 µm for fine- and coarse-grained micrometeorites, respectively. Fine-grained particles are dominated by matrix-rich, hydrous materials typical of primitive carbonaceous precursors, whereas coarse-grained particles are characterised by chondrule-bearing, comparatively dry or weakly hydrated materials associated with more evolved precursors. The identification of chondrules in several coarse-grained and composite particles further supports this interpretation. Composite micrometeorites provide direct evidence for the coexistence of fine- and coarse-grained lithologies within heterogeneous precursor materials. The observed bimodality therefore likely reflects contrasting physical properties, fragmentation behaviour and atmospheric survivability of end-member materials within the near-Earth interplanetary dust complex [6,7]. Based on current micrometeorite mass flux estimates derived from TAM cosmic spherules [5], and accounting for the inferred ~87% atmospheric mass loss, we estimate a time-averaged pre-atmospheric cosmic dust flux of ~12,000 (±6,000) t/yr over the Quaternary. This value is broadly consistent with estimates derived from the LDEF experiment [1] and recent CABMOD-ZoDy models [8], suggesting that the flux of large micrometeorites reaching Earth has remained relatively stable over the last few million years.

Acknowledgments: PRIN2022 Cosmic Dust II, ID# 2022S5A2N7. ASI-MUR SpaceitUP! ID# 2024-5-E.0 – CUP n. I53D24000060005

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